Consciousness
Overview
 
Consciousness is a term that refers to the relationship between the mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

 and the world with which it interacts. It has been defined as: subjectivity
Subjectivity
Subjectivity refers to the subject and his or her perspective, feelings, beliefs, and desires. In philosophy, the term is usually contrasted with objectivity.-Qualia:...

, awareness
Awareness
Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

, the ability to experience
Experience
Experience as a general concept comprises knowledge of or skill in or observation of some thing or some event gained through involvement in or exposure to that thing or event....

 or to feel
Feeling
Feeling is the nominalization of the verb to feel. The word was first used in the English language to describe the physical sensation of touch through either experience or perception. The word is also used to describe experiences, other than the physical sensation of touch, such as "a feeling of...

, wakefulness, having a sense of self
Self
The self is an individual person as the object of his or her own reflective consciousness. The self has been studied extensively by philosophers and psychologists and is central to many world religions.-Philosophy:...

hood, and the executive control system of the mind. Despite the difficulty in definition, many philosophers believe that there is a broadly shared underlying intuition about what consciousness is.
Encyclopedia
Consciousness is a term that refers to the relationship between the mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

 and the world with which it interacts. It has been defined as: subjectivity
Subjectivity
Subjectivity refers to the subject and his or her perspective, feelings, beliefs, and desires. In philosophy, the term is usually contrasted with objectivity.-Qualia:...

, awareness
Awareness
Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

, the ability to experience
Experience
Experience as a general concept comprises knowledge of or skill in or observation of some thing or some event gained through involvement in or exposure to that thing or event....

 or to feel
Feeling
Feeling is the nominalization of the verb to feel. The word was first used in the English language to describe the physical sensation of touch through either experience or perception. The word is also used to describe experiences, other than the physical sensation of touch, such as "a feeling of...

, wakefulness, having a sense of self
Self
The self is an individual person as the object of his or her own reflective consciousness. The self has been studied extensively by philosophers and psychologists and is central to many world religions.-Philosophy:...

hood, and the executive control system of the mind. Despite the difficulty in definition, many philosophers believe that there is a broadly shared underlying intuition about what consciousness is. As Max Velmans
Max Velmans
Max Velmans is a Professor of Psychology at Goldsmiths, University of London. From 2003 to 2006 he was Chair of the Consciousness and Experiential Psychology Section of the British Psychological Society and has been involved in many other initiatives to encourage studies in consciousness...

 and Susan Schneider wrote in The Blackwell Companion to Consciousness: "Anything that we are aware of at a given moment forms part of our consciousness, making conscious experience at once the most familiar and most mysterious aspect of our lives."

Philosophers
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

 since the time of Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...

 and Locke
John Locke
John Locke FRS , widely known as the Father of Liberalism, was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, he is equally important to social...

 have struggled to comprehend the nature of consciousness and pin down its essential properties. Issues of concern in the philosophy of consciousness include whether the concept is fundamentally valid; whether consciousness can ever be explained mechanistically; whether non-human consciousness exists and if so how it can be recognized; how consciousness relates to language; and whether it may ever be possible for computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s or robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...

s to be conscious. Perhaps the thorniest issue is whether consciousness can be understood in a way that does not require a dualistic
Dualism (philosophy of mind)
In philosophy of mind, dualism is a set of views about the relationship between mind and matter, which begins with the claim that mental phenomena are, in some respects, non-physical....

 distinction between mental and physical states or properties.

At one time consciousness was viewed with skepticism by many scientists, but in recent years it has become a significant topic of research in psychology
Psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

 and neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

. The primary focus is on understanding what it means biologically and psychologically for information
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...

 to be present in consciousness—that is, on determining the neural and psychological correlates of consciousness. The majority of experimental studies assess consciousness by asking human subjects for a verbal report of their experiences (e.g., "tell me if you notice anything when I do this"). Issues of interest include phenomena such as subliminal perception
Subliminal stimuli
Subliminal stimuli , contrary to supraliminal stimuli or "above threshold", are any sensory stimuli below an individual's threshold for conscious perception. The large majority of research has found that subliminal messages do not produce strong or lasting changes in behavior...

, blindsight
Blindsight
Blindsight is a phenomenon in which people who are perceptually blind in a certain area of their visual field demonstrate some response to visual stimuli...

, denial of impairment
Anosognosia
Anosognosia /æˌnɒsɒgˈnəʊsɪə/ is a condition in which a person who suffers disability seems unaware of the existence of his or her disability. Unlike denial, which is a defense mechanism, anosognosia is rooted in physiology...

, and altered states of consciousness
Altered state of consciousness
An altered state of consciousness , also named altered state of mind, is any condition which is significantly different from a normal waking beta wave state. The expression was used as early as 1966 by Arnold M. Ludwig and brought into common usage from 1969 by Charles Tart: it describes induced...

 produced by psychoactive drugs or spiritual or meditative techniques.

In medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

, consciousness is assessed by observing a patient's arousal and responsiveness, and can be seen as a continuum of states ranging from full alertness and comprehension, through disorientation, delirium, loss of meaningful communication, and finally loss of movement in response to painful stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

. Issues of practical concern include how the presence of consciousness can be assessed in severely ill, comatose, or anesthetized people, and how to treat conditions in which consciousness is impaired or disrupted.

Etymology and early history

The origin of the modern concept of consciousness is often attributed to John Locke
John Locke
John Locke FRS , widely known as the Father of Liberalism, was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, he is equally important to social...

's Essay Concerning Human Understanding, published in 1690. Locke defined consciousness as “the perception of what passes in a man’s own mind.” His essay influenced the 18th century view of consciousness, and his definition appeared in Samuel Johnson
Samuel Johnson
Samuel Johnson , often referred to as Dr. Johnson, was an English author who made lasting contributions to English literature as a poet, essayist, moralist, literary critic, biographer, editor and lexicographer...

's celebrated Dictionary
A Dictionary of the English Language
Published on 15 April 1755 and written by Samuel Johnson, A Dictionary of the English Language, sometimes published as Johnson's Dictionary, is among the most influential dictionaries in the history of the English language....

 (1755).

The earliest English language uses of "conscious" and "consciousness" date back, however, to the 1500s. The English word "conscious" originally derived from the Latin conscius (con- "together" + scire "to know"), but the Latin word did not have the same meaning as our word—it meant knowing with, in other words having joint or common knowledge
Knowledge
Knowledge is a familiarity with someone or something unknown, which can include information, facts, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject...

 with another
. There were, however, many occurrences in Latin writings of the phrase conscius sibi, which translates literally as "knowing with oneself", or in other words sharing knowledge with oneself about something. This phrase had the figurative meaning of knowing that one knows, as the modern English word "conscious" does. In its earliest uses in the 1500s, the English word "conscious" retained the meaning of the Latin conscius. For example Thomas Hobbes
Thomas Hobbes
Thomas Hobbes of Malmesbury , in some older texts Thomas Hobbs of Malmsbury, was an English philosopher, best known today for his work on political philosophy...

 in Leviathan
Leviathan (book)
Leviathan or The Matter, Forme and Power of a Common Wealth Ecclesiasticall and Civil — commonly called simply Leviathan — is a book written by Thomas Hobbes and published in 1651. Its name derives from the biblical Leviathan...

wrote: "Where two, or more men, know of one and the same fact, they are said to be Conscious of it one to another." The Latin phrase conscius sibi, whose meaning was more closely related to the current concept of consciousness, was rendered in English as "conscious to oneself" or "conscious unto oneself". For example, Archbishop Ussher wrote in 1613 of "being so conscious unto myself of my great weakness". Locke's definition from 1690 illustrates that a gradual shift in meaning had taken place.

A related word was conscientia, which primarily means moral
Morality
Morality is the differentiation among intentions, decisions, and actions between those that are good and bad . A moral code is a system of morality and a moral is any one practice or teaching within a moral code...

 conscience
Conscience
Conscience is an aptitude, faculty, intuition or judgment of the intellect that distinguishes right from wrong. Moral judgement may derive from values or norms...

. In the literal sense, "conscientia" means knowledge-with, that is, shared knowledge. The word first appears in Latin juridical texts by writers such as Cicero
Cicero
Marcus Tullius Cicero , was a Roman philosopher, statesman, lawyer, political theorist, and Roman constitutionalist. He came from a wealthy municipal family of the equestrian order, and is widely considered one of Rome's greatest orators and prose stylists.He introduced the Romans to the chief...

. Here, conscientia is the knowledge that a witness has of the deed of someone else. René Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...

 (1596–1650) is generally taken to be the first philosopher to use "conscientia" in a way that does not fit this traditional meaning. Descartes used "conscientia" the way modern speakers would use "conscience." In Search after Truth he says "conscience or internal testimony" (conscientia vel interno testimonio).

In philosophy

The philosophy of mind
Philosophy of mind
Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

 has given rise to many stances regarding consciousness. Any attempt to impose an organization on them is bound to be somewhat arbitrary. Stuart Sutherland
Stuart Sutherland
Norman Stuart Sutherland , always known professionally as Stuart Sutherland, was a British psychologist and writer....

 exemplified the difficulty in the entry he wrote for the 1989 version of the Macmillan Dictionary of Psychology:
Most writers on the philosophy of consciousness have been concerned to defend a particular point of view, and have organized their material accordingly. For surveys, the most common approach is to follow a historical path by associating stances with the philosophers who are most most strongly associated with them, for example Descartes, Locke, Kant, etc. The main alternative, followed in the present article, is to organize philosophical stances according to the answers they give to a set of basic questions about the nature and status of consciousness.

Is consciousness a valid concept?

The most compelling argument for the existence of consciousness is that the vast majority of mankind have an overwhelming intuition that there truly is such a thing. Skeptics argue that this intuition, in spite of its compelling quality, is false, either because the concept of consciousness is intrinsically incoherent, or because our intuitions about it are based in illusions. Gilbert Ryle
Gilbert Ryle
Gilbert Ryle , was a British philosopher, a representative of the generation of British ordinary language philosophers that shared Wittgenstein's approach to philosophical problems, and is principally known for his critique of Cartesian dualism, for which he coined the phrase "the ghost in the...

, for example, argued that traditional understanding of consciousness depends on a Cartesian dualist
Dualism (philosophy of mind)
In philosophy of mind, dualism is a set of views about the relationship between mind and matter, which begins with the claim that mental phenomena are, in some respects, non-physical....

 outlook that improperly distinguishes between mind and body, or between mind and world. He proposed that we speak not of minds, bodies, and the world, but of individuals, or persons, acting in the world. Thus, by speaking of 'consciousness' we end up misleading ourselves by thinking that there is any sort of thing as consciousness separated from behavioral and linguistic understandings. More generally, many philosophers and scientists have been unhappy about the difficulty of producing a definition that does not involve circularity or fuzziness.

Is it a single thing?

Many philosophers have argued that consciousness is a unitary concept that is understood intuitively by the majority of people in spite of the difficulty in defining it. Others, though, have argued that the level of disagreement about the meaning of the word indicates that it either means different things to different people, or else is an umbrella term
Umbrella term
An umbrella term is a word that provides a superset or grouping of concepts that all fall under a single common category. Umbrella term is also called a hypernym. For example, cryptology is an umbrella term that encompasses cryptography and cryptanalysis, among other fields...

 encompassing a variety of distinct meanings with no simple element in common.

Ned Block proposed a distinction between two types of consciousness that he called phenomenal (P-consciousness) and access (A-consciousness). P-consciousness, according to Block, is simply raw experience: it is moving, colored forms, sounds, sensations, emotions and feelings with our bodies and responses at the center. These experiences, considered independently of any impact on behavior, are called qualia
Qualia
Qualia , singular "quale" , from a Latin word meaning for "what sort" or "what kind," is a term used in philosophy to refer to subjective conscious experiences as 'raw feels'. Examples of qualia are the pain of a headache, the taste of wine, the experience of taking a recreational drug, or the...

. A-consciousness, on the other hand, is the phenomenon whereby information in our minds is accessible for verbal report, reasoning, and the control of behavior. So, when we perceive
Perception
Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...

, information about what we perceive is access conscious; when we introspect
Introspection
Introspection is the self-observation and reporting of conscious inner thoughts, desires and sensations. It is a conscious and purposive process relying on thinking, reasoning, and examining one's own thoughts, feelings, and, in more spiritual cases, one's soul...

, information about our thoughts is access conscious; when we remember
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

, information about the past is access conscious, and so on. Although some philosophers, such as Daniel Dennett
Daniel Dennett
Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

, have disputed the validity of this distinction, others have broadly accepted it. David Chalmers
David Chalmers
David John Chalmers is an Australian philosopher specializing in the area of philosophy of mind and philosophy of language, whose recent work concerns verbal disputes. He is Professor of Philosophy and Director of the Centre for Consciousness at the Australian National University...

 has argued that A-consciousness can in principle be understood in mechanistic terms, but that understanding P-consciousness is much more challenging: he calls this the hard problem of consciousness
Hard problem of consciousness
The hard problem of consciousness is the problem of explaining how and why we have qualitative phenomenal experiences. David Chalmers contrasts this with the "easy problems" of explaining the ability to discriminate, integrate information, report mental states, focus attention, etc...

.

Some philosophers believe that Block's two types of consciousness are not the end of the story. William Lycan
William Lycan
William G. Lycan is a noted American philosopher teaching at the University of North Carolina at Chapel Hill,where he is the William Rand Kenan, Jr. Distinguished Professor. He won the Class of 2001 Outstanding Faculty Award and a Distinguished Teaching Award for Post-Baccalaureate Instruction in...

, for example, argued in his book Consciousness and Experience that at least eight clearly distinct types of consciousness can be identified (organism consciousness; control consciousness; consciousness of; state/event consciousness; reportability; introspective consciousness; subjective consciousness; self-consciousness)—and that even this list omits several more obscure forms.

How does it relate to the physical world?

The first influential philosopher to discuss this question specifically was Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...

, and the answer he gave is known as Cartesian dualism
Dualism (philosophy of mind)
In philosophy of mind, dualism is a set of views about the relationship between mind and matter, which begins with the claim that mental phenomena are, in some respects, non-physical....

. Descartes proposed that consciousness resides within an immaterial domain he called res cogitans
Mental substance
Mental substance refers to the concept held by dualists and idealists, that minds are made-up of non-physical substance. This substance is often referred to as consciousness....

 (the realm of thought), in contrast to the domain of material things which he called res extensa
Res Extensa
Res extensa is one of the three substances described by René Descartes in his Cartesian ontology, alongside res cogitans and God. Translated from Latin, "res extensa" means "extended thing"...

 (the realm of extension). He suggested that the interaction between these two domains occurs inside the brain, perhaps in a small midline structure called the pineal gland
Pineal gland
The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...

.

Although it is widely accepted that Descartes explained the problem cogently, few later philosophers have been happy with his solution, and his ideas about the pineal gland have especially been ridiculed. Alternative solutions, however, have been very diverse. They can be divided broadly into two categories: dualist
Dualism (philosophy of mind)
In philosophy of mind, dualism is a set of views about the relationship between mind and matter, which begins with the claim that mental phenomena are, in some respects, non-physical....

 solutions that maintain Descartes's rigid distinction between the realm of consciousness and the realm of matter but give different answers for how the two realms relate to each other; and monist
Monism
Monism is any philosophical view which holds that there is unity in a given field of inquiry. Accordingly, some philosophers may hold that the universe is one rather than dualistic or pluralistic...

 solutions that maintain that there is really only one realm of being, of which consciousness and matter are both aspects. Each of these categories itself contains numerous variants. The two main types of dualism are substance dualism (which holds that the mind is formed of a distinct type of substance not governed by the laws of physics) and property dualism
Property dualism
Property dualism describes a category of positions in the philosophy of mind which hold that, although the world is constituted of just one kind of substance - the physical kind - there exist two distinct kinds of properties: physical properties and mental properties...

 (which holds that the laws of physics are universally valid but cannot be used to explain the mind). The three main types of monism are physicalism
Physicalism
Physicalism is a philosophical position holding that everything which exists is no more extensive than its physical properties; that is, that there are no kinds of things other than physical things...

 (which holds that the mind consists of matter organized in a particular way), idealism
Idealism
In philosophy, idealism is the family of views which assert that reality, or reality as we can know it, is fundamentally mental, mentally constructed, or otherwise immaterial. Epistemologically, idealism manifests as a skepticism about the possibility of knowing any mind-independent thing...

 (which holds that only thought truly exists and matter is merely an illusion), and neutral monism
Neutral monism
Neutral monism, in philosophy, is the metaphysical view that the mental and the physical are two ways of organizing or describing the same elements, which are themselves "neutral," that is, neither physical nor mental. This view denies that the mental and the physical are two fundamentally...

 (which holds that both mind and matter are aspects of a distinct essence that is itself identical to neither of them). There are also, however, a large number of idiosyncratic theories that cannot cleanly be assigned to any of these camps.

Since the dawn of Newtonian science with its vision of simple mechanical principles governing the entire universe, some philosophers have been tempted by the idea that consciousness could be explained in purely physical terms. The first influential writer to propose such an idea explicitly was Julien Offray de La Mettrie
Julien Offray de La Mettrie
Julien Offray de La Mettrie was a French physician and philosopher, and one of the earliest of the French materialists of the Enlightenment...

, in his book Man a Machine
Man a Machine
Man a Machine is a work of materialist philosophy by the 18th-century French physician and philosopher Julien Offray de La Mettrie, first published in 1748...

 (L'homme machine). His arguments, however, were very abstract. The most influential modern physical theories of consciousness are based on psychology
Psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

 and neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

. Theories proposed by neuroscientists such as Gerald Edelman
Gerald Edelman
Gerald Maurice Edelman is an American biologist who shared the 1972 Nobel Prize in Physiology or Medicine for work with Rodney Robert Porter on the immune system. Edelman's Nobel Prize-winning research concerned discovery of the structure of antibody molecules...

 and Antonio Damasio
Antonio Damasio
Antonio Damasio is David Dornsife Professor of Neuroscience at the University of Southern California, where he heads USC's Brain and Creativity Institute and Adjunct Professor at the Salk Institute. Prior to taking up his posts at USC, in 2005, Damasio was M.W...

, and by philosophers such as Daniel Dennett
Daniel Dennett
Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

, seek to explain consciousness in terms of neural events occurring within the brain. Many other neuroscientists, such as Christof Koch
Christof Koch
Christof Koch is an American neuroscientist working on the neural basis of consciousness. He is the Lois and Victor Troendle Professor of Cognitive and Behavioral Biology at California Institute of Technology, where he has been since 1986...

, have explored the neural basis of consciousness without attempting to frame all-encompassing global theories. At the same time, computer scientists working in the field of Artificial Intelligence
Artificial intelligence
Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...

 have pursued the goal of creating digital computer programs that can simulate or embody consciousness
Artificial consciousness
Artificial consciousness , also known as machine consciousness or synthetic consciousness, is a field related to artificial intelligence and cognitive robotics whose aim is to define that which would have to be synthesized were consciousness to be found in an engineered artifact .Neuroscience...

.

A few theoretical physicists have argued that classical physics is intrinsically incapable of explaining the holistic aspects of consciousness, but that quantum theory
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

 provides the missing ingredients
Quantum mind
The quantum mind or quantum consciousness hypothesis proposes that classical mechanics cannot explain consciousness, while quantum mechanical phenomena, such as quantum entanglement and superposition, may play an important part in the brain's function, and could form the basis of an explanation of...

. Several theorists have therefore proposed quantum mind
Quantum mind
The quantum mind or quantum consciousness hypothesis proposes that classical mechanics cannot explain consciousness, while quantum mechanical phenomena, such as quantum entanglement and superposition, may play an important part in the brain's function, and could form the basis of an explanation of...

 (QM) theories of consciousness. Notable theories falling into this category include the Holonomic brain theory
Holonomic brain theory
The holonomic brain theory, originated by psychologist Karl Pribram and initially developed in collaboration with physicist David Bohm, is a model for human cognition that is drastically different from conventionally accepted ideas: Pribram and Bohm posit a model of cognitive function as being...

 of Karl Pribram
Karl H. Pribram
Karl H. Pribram is a professor at Georgetown University, in the United States, and an emeritus professor of psychology and psychiatry at Stanford University and Radford University...

 and David Bohm
David Bohm
David Joseph Bohm FRS was an American-born British quantum physicist who contributed to theoretical physics, philosophy, neuropsychology, and the Manhattan Project.-Youth and college:...

, and the Orch-OR theory
Orch-OR
Orch-OR is a theory of consciousness, which is the joint work of theoretical physicist Sir Roger Penrose and anesthesiologist Stuart Hameroff. Mainstream theories assume that consciousness emerges from the brain, and focus particularly on complex computation at synapses that allow communication...

 formulated by Stuart Hameroff
Stuart Hameroff
Stuart Hameroff is an anesthesiologist and professor at the University of Arizona known for his scientific studies of consciousness.-Career:...

 and Roger Penrose
Roger Penrose
Sir Roger Penrose OM FRS is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the Mathematical Institute, University of Oxford and Emeritus Fellow of Wadham College...

. Some of these QM theories offer descriptions of phenomenal consciousness, as well as QM interpretations of access consciousness. None of the quantum mechanical theories has been confirmed by experiment, and many scientists and philosophers consider the arguments for an important role of quantum phenomena to be unconvincing.

Why do people believe that other people are conscious?

Many philosophers consider experience to be the essence of consciousness, and believe that experience can only fully be known from the inside, subjectively. But if consciousness is subjective and not visible from the outside, why do the vast majority of people believe that other people are conscious, but rocks and trees are not? This is called the problem of other minds
Problem of other minds
The problem of other minds has traditionally been regarded as an epistemological challenge raised by the skeptic. The challenge may be expressed as follows: given that I can only observe the behavior of others, how can I know that others have minds? The thought behind the question is that no matter...

. It is particularly acute for people who believe in the possibility of philosophical zombie
Philosophical zombie
A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

s, that is, people who think it is possible in principle to have an entity that is physically indistinguishable from a human being and behaves like a human being in every way but nevertheless lacks consciousness.

The most commonly given answer is that we attribute consciousness to other people because we see that they resemble us in appearance and behavior: we reason that if they look like us and act like us, they must be like us in other ways, including having experiences of the sort that we do. There are, however, a variety of problems with that explanation. For one thing, it seems to violate the principle of parsimony
Occam's razor
Occam's razor, also known as Ockham's razor, and sometimes expressed in Latin as lex parsimoniae , is a principle that generally recommends from among competing hypotheses selecting the one that makes the fewest new assumptions.-Overview:The principle is often summarized as "simpler explanations...

, by postulating an invisible entity that is not necessary to explain what we observe. Some philosophers, such as Daniel Dennett
Daniel Dennett
Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

 in an essay titled The Unimagined Preposterousness of Zombies, argue that people who give this explanation do not really understand what they are saying. More broadly, philosophers who do not accept the possibility of zombies generally believe that consciousness is reflected in behavior (including verbal behavior), and that we attribute consciousness on the basis of behavior. A more straightforward way of saying this is that we attribute experiences to people because of what they can do, including the fact that they can tell us about their experiences.

How can we know whether non-human animals are conscious?

The topic of animal consciousness is beset by a number of difficulties. It poses the problem of other minds in an especially severe form, because animals, lacking language, cannot tell us about their experiences. Also, it is difficult to reason objectively about the question, because a denial that an animal is conscious is often taken to imply that it does not feel, its life has no value, and that harming it is not morally wrong. Descartes, for example, has sometimes been blamed for mistreatment of animals due to the fact that he believed only humans have a non-physical mind. Most people have a strong intuition that some animals, such as dogs, are conscious, while others, such as insects, are not; but the sources of this intuition are not obvious.

Philosophers who consider subjective experience the essence of consciousness also generally believe, as a correlate, that the existence and nature of animal consciousness can never rigorously be known. Thomas Nagel
Thomas Nagel
Thomas Nagel is an American philosopher, currently University Professor of Philosophy and Law at New York University, where he has taught since 1980. His main areas of philosophical interest are philosophy of mind, political philosophy and ethics...

 spelled out this point of view in an influential essay titled What Is it Like to Be a Bat?
What Is it Like to Be a Bat?
"What Is It Like to Be a Bat?" is a well-known paper by the American philosopher Thomas Nagel, first published in The Philosophical Review in October 1974, and later in Nagel's Mortal Questions...

. He said that an organism is conscious "if and only if there is something that it is like to be that organism — something it is like for the organism"; and he argued that no matter how much we know about an animal's brain and behavior, we can never really put ourselves into the mind of the animal and experience its world in the way it does itself. Other thinkers, such as Douglas Hofstadter
Douglas Hofstadter
Douglas Richard Hofstadter is an American academic whose research focuses on consciousness, analogy-making, artistic creation, literary translation, and discovery in mathematics and physics...

, dismiss this argument as incoherent. Several psychologists and ethologists have argued for the existence of animal consciousness by describing a range of behaviors that appear to show animals holding beliefs about things they cannot directly perceive — Donald Griffin's 2001 book Animal Minds reviews a substantial portion of the evidence.

Could a machine ever be conscious?

The idea of an artifact made conscious is an ancient theme of mythology, appearing for example in the Greek myth of Pygmalion
Pygmalion (mythology)
Pygmalion is a legendary figure of Cyprus. Though Pygmalion is the Greek version of the Phoenician royal name Pumayyaton, he is most familiar from Ovid's Metamorphoses, X, in which Pygmalion was a sculptor who fell in love with a statue he had carved.-In Ovid:In Ovid's narrative, Pygmalion was a...

, who carved a statue that was magically brought to life, and in medieval Jewish stories of the Golem
Golem
In Jewish folklore, a golem is an animated anthropomorphic being, created entirely from inanimate matter. The word was used to mean an amorphous, unformed material in Psalms and medieval writing....

, a magically animated homunculus built of clay. However, the possibility of actually constructing a conscious machine was probably first discussed by Ada Lovelace
Ada Lovelace
Augusta Ada King, Countess of Lovelace , born Augusta Ada Byron, was an English writer chiefly known for her work on Charles Babbage's early mechanical general-purpose computer, the analytical engine...

, in a set of notes written in 1842 about the Analytical Engine
Analytical engine
The Analytical Engine was a proposed mechanical general-purpose computer designed by English mathematician Charles Babbage. It was first described in 1837 as the successor to Babbage's difference engine, a design for a mechanical calculator...

 invented by Charles Babbage
Charles Babbage
Charles Babbage, FRS was an English mathematician, philosopher, inventor and mechanical engineer who originated the concept of a programmable computer...

, a precursor (never built) to modern electronic computers. Lovelace was essentially dismissive of the idea that a machine such as the Analytical Engine could think in a humanlike way. She wrote:
One of the most influential contributions to this question was an essay written in 1950 by pioneering computer scientist Alan Turing
Alan Turing
Alan Mathison Turing, OBE, FRS , was an English mathematician, logician, cryptanalyst, and computer scientist. He was highly influential in the development of computer science, providing a formalisation of the concepts of "algorithm" and "computation" with the Turing machine, which played a...

, titled Computing Machinery and Intelligence
Computing machinery and intelligence
Computing Machinery and Intelligence, written by Alan Turing and published in 1950 in Mind, is a seminal paper on the topic of artificial intelligence in which the concept of what is now known as the Turing test was introduced to a wide audience....

. Turing disavowed any interest in terminology, saying that even "Can machines think?" is too loaded with spurious connotations to be meaningful; but he proposed to replace all such questions with a specific operational test, which has become known as the Turing test
Turing test
The Turing test is a test of a machine's ability to exhibit intelligent behaviour. In Turing's original illustrative example, a human judge engages in a natural language conversation with a human and a machine designed to generate performance indistinguishable from that of a human being. All...

. To pass the test a computer must be able to imitate a human well enough to fool interrogators. In his essay Turing discussed a variety of possible objections, and presented a counterargument to each of them. The Turing test is commonly cited in discussions of artificial intelligence
Artificial intelligence
Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...

 as a proposed criterion for machine consciousness; it has provoked a great deal of philosophical debate. For example, Daniel Dennett
Daniel Dennett
Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

 and Douglas Hofstadter
Douglas Hofstadter
Douglas Richard Hofstadter is an American academic whose research focuses on consciousness, analogy-making, artistic creation, literary translation, and discovery in mathematics and physics...

 argue that anything capable of passing the Turing test is necessarily conscious, while David Chalmers
David Chalmers
David John Chalmers is an Australian philosopher specializing in the area of philosophy of mind and philosophy of language, whose recent work concerns verbal disputes. He is Professor of Philosophy and Director of the Centre for Consciousness at the Australian National University...

 argues that a philosophical zombie
Philosophical zombie
A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

 could pass the test, yet fail to be conscious.

Some philosophers have argued that it might be possible, at least in principle, for a machine to be conscious, but that this would not just be a matter of executing the right computer program. This viewpoint has most notably been advocated by John Searle
John Searle
John Rogers Searle is an American philosopher and currently the Slusser Professor of Philosophy at the University of California, Berkeley.-Biography:...

, who defended it using a thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 that has become known as the Chinese room
Chinese room
The Chinese room is a thought experiment by John Searle, which first appeared in his paper "Minds, Brains, and Programs", published in Behavioral and Brain Sciences in 1980...

 argument: Suppose the Turing test is conducted in Chinese rather than English, and suppose a computer program successfully passes it. Does the system that is executing the program understand Chinese? Searle's argument is that he could pass the Turing test in Chinese too, by executing exactly the same program as the computer, yet without understanding a word of Chinese. How does Searle know whether he understands Chinese? Understanding a language is not just something you do: it feels like something to understand Chinese (or any language). Understanding is conscious, and therefore there is more to it than merely executing a program, even a Turing-test-passing program.

In the literature concerning artificial consciousness, Searle's essay has been second only to Turing's in the volume of debate it has generated. Searle himself was vague about what extra ingredients it would take to make a machine conscious: all he proposed was that what was needed was "causal powers" of the sort that the brain has and that computers lack. But other thinkers sympathetic to his basic argument have suggested that the necessary (though perhaps still not sufficient) extra conditions may include the ability to pass not just the verbal version of the Turing test, but the robotic
Robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...

 version, which requires grounding
Symbol grounding
The Symbol Grounding Problem is related to the problem of how words get their meanings, and hence to the problem of what meaning itself really is. The problem of meaning is in turn related to the problem of consciousness, or how it is that mental states are meaningful...

 the robot's words in the robot's sensorimotor capacity to categorize
Categorization
Categorization is the process in which ideas and objects are recognized, differentiated and understood. Categorization implies that objects are grouped into categories, usually for some specific purpose. Ideally, a category illuminates a relationship between the subjects and objects of knowledge...

 and interact with the things in the world that its words are about, Turing-indistinguishably from a real person. Turing-scale robotics is an empirical branch of research on embodied cognition
Embodied cognition
Philosophers, psychologists, cognitive scientists and artificial intelligence researchers who study embodied cognition and the embodied mind believe that the nature of the human mind is largely determined by the form of the human body. They argue that all aspects of cognition, such as ideas,...

 and situated cognition
Situated cognition
Situated cognition poses that knowing is inseparable from doing by arguing that all knowledge is situated in activity bound to social, cultural and physical contexts....


Spiritual approaches

To most philosophers, the word "consciousness" connotes the relationship between the mind and the world. To writers on spiritual or religious topics, it frequently connotes the relationship between the mind and God, or the relationship between the mind and deeper truths that are thought to be more fundamental than the physical world. Krishna consciousness
International Society for Krishna Consciousness
The International Society for Krishna Consciousness , known colloquially as the Hare Krishna movement, is a Gaudiya Vaishnava religious organization. It was founded in 1966 in New York City by A.C. Bhaktivedanta Swami Prabhupada...

, for example, is a term used to mean an intimate linkage between the mind of a worshipper and the god Krishna. The mystical psychiatrist Richard Maurice Bucke distinguished between three types of consciousness: Simple Consciousness, awareness of the body, possessed by many animals; Self Consciousness, awareness of being aware, possessed only by humans; and Cosmic Consciousness, awareness of the life and order of the universe, possessed only by humans who are enlightened. Many more examples could be given. The most thorough account of the spiritual approach may be Ken Wilber
Ken Wilber
Kenneth Earl Wilber II is an American author who has written about mysticism, philosophy, ecology, and developmental psychology. His work formulates what he calls Integral Theory. In 1998, he founded the Integral Institute, for teaching and applications of Integral theory.-Biography:Ken Wilber was...

's book The Spectrum of Consciousness, a comparison of western and eastern ways of thinking about the mind. Wilber described consciousness as a spectrum with ordinary awareness at one end, and more profound types of awareness at higher levels.

Scientific approaches

For many decades, consciousness as a research topic was avoided by the majority of mainstream scientists, because of a general feeling that a phenomenon defined in subjective terms could not properly be studied using objective experimental methods. Starting in the 1980s, though, an expanding community of neuroscientists and psychologists have associated themselves with a field called Consciousness Studies, giving rise to a stream of experimental work published in journals such as Consciousness and Cognition
Consciousness and Cognition
The journal Consciousness and Cognition provides a forum for scientific approaches to the issues of consciousness, voluntary control, and self. It features empirical research and theoretical articles. Book reviews, integrative theoretical and critical literature reviews, and tutorial reviews are...

, and methodological work published in journals such as the Journal of Consciousness Studies
Journal of Consciousness Studies
The Journal of Consciousness Studies is an interdisciplinary peer-reviewed academic journal dedicated entirely to the field of consciousness studies. It was previously edited by Joseph Goguen. It has been co-edited by the philosopher of mysticism, Robert K.C...

, along with regular conferences organized by groups such as the Association for the Scientific Study of Consciousness
Association for the Scientific Study of Consciousness
The Association for the Scientific Study of Consciousness is a professional membership organization that aims to encourage research on consciousness in cognitive science, neuroscience, philosophy, and other relevant disciplines in the sciences and humanities, directed toward understanding the...

.

Modern scientific investigations into consciousness are based on psychological experiments (including, for example, the investigation of priming
Priming
Priming may refer to:* Priming , a process in which the processing of a target stimulus is aided or altered by the presentation of a previously presented stimulus....

 effects using subliminal stimuli
Subliminal stimuli
Subliminal stimuli , contrary to supraliminal stimuli or "above threshold", are any sensory stimuli below an individual's threshold for conscious perception. The large majority of research has found that subliminal messages do not produce strong or lasting changes in behavior...

), and on case studies of alterations in consciousness produced by trauma, illness, or drugs. Broadly viewed, scientific approaches are based on two core concepts. The first identifies the content of consciousness with the experiences that are reported by human subjects; the second makes use of the concept of consciousness that has been developed by neurologists and other medical professionals who deal with patients whose behavior is impaired. In either case, the ultimate goals are to develop techniques for assessing consciousness objectively in humans as well as other animals, and to understand the neural and psychological mechanisms that underlie it.

Measurement

Experimental research on consciousness presents special difficulties, due to the lack of a universally accepted operational definition
Operational definition
An operational definition defines something in terms of the specific process or set of validation tests used to determine its presence and quantity. That is, one defines something in terms of the operations that count as measuring it. The term was coined by Percy Williams Bridgman and is a part of...

. In the majority of experiments that are specifically about consciousness, the subjects are human, and the criterion that is used is verbal report: in other words, subjects are asked to describe their experiences, and their descriptions are treated as observations of the contents of consciousness. For example, subjects who stare continuously at a Necker Cube
Necker cube
The Necker Cube is an optical illusion first published as a rhomboid in 1832 by Swiss crystallographer Louis Albert Necker.-Ambiguity:The Necker Cube is an ambiguous line drawing....

 usually report that they experience it "flipping" between two 3D configurations, even though the stimulus itself remains the same. The objective is to understand the relationship between the conscious awareness of stimuli (as indicated by verbal report) and the effects the stimuli have on brain activity and behavior. In several paradigms, such as the technique of response priming
Response priming
In the psychology of perception and motor control, the term response priming denotes a special form of Priming. Generally, priming effects take place whenever a response to a target stimulus is influenced by a prime stimulus presented at an earlier time...

, the behavior of subjects is clearly influenced by stimuli for which they report no awareness.

Verbal report is widely considered to be the most reliable indicator of consciousness, but it raises a number of issues. For one thing, if verbal reports are treated as observations, akin to observations in other branches of science, then the possibility arises that they may contain errors—but it is difficult to make sense of the idea that subjects could be wrong about their own experiences, and even more difficult to see how such an error could be detected. Daniel Dennett
Daniel Dennett
Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

 has argued for an approach he calls heterophenomenology
Heterophenomenology
Heterophenomenology is a term coined by Daniel Dennett to describe an explicitly third-person, scientific approach to the study of consciousness and other mental phenomena...

, which means treating verbal reports as stories that may or may not be true, but his ideas about how to do this have not been widely adopted. Another issue with verbal report as a criterion is that it restricts the field of study to humans who have language: this approach cannot be used to study consciousness in other species, pre-linguistic children, or people with types of brain damage that impair language. As a third issue, philosophers who dispute the validity of the Turing test
Turing test
The Turing test is a test of a machine's ability to exhibit intelligent behaviour. In Turing's original illustrative example, a human judge engages in a natural language conversation with a human and a machine designed to generate performance indistinguishable from that of a human being. All...

 may feel that it is possible, at least in principle, for verbal report to be dissociated from consciousness entirely: a philosophical zombie
Philosophical zombie
A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

 may give detailed verbal reports of awareness in the absence of any genuine awareness.

Although verbal report is in practice the "gold standard" for ascribing consciousness, it is not the only possible criterion. In medicine, consciousness is assessed as a combination of verbal behavior, arousal, brain activity and purposeful movement. The last three of these can be used as indicators of consciousness when verbal behavior is absent. The scientific literature regarding the neural bases of arousal and purposeful movement is very extensive. Their reliability as indicators of consciousness is disputed, however, due to numerous studies showing that alert human subjects can be induced to behave purposefully in a variety of ways in spite of reporting a complete lack of awareness. Studies of the neuroscience of free will
Neuroscience of free will
Neuroscience of free will refers to recent neuroscientific investigations shedding light on the question of free will, which is a philosophical and scientific question as to whether, and in what sense, rational agents exercise control over their actions or decisions. As it has become possible to...

 have also shown that the experiences that people report when they behave purposefully sometimes do not correspond to their actual behaviors or to the patterns of electrical activity recorded from their brains.

Another approach applies specifically to the study of self-awareness
Self-awareness
Self-awareness is the capacity for introspection and the ability to reconcile oneself as an individual separate from the environment and other individuals...

, that is, the ability to distinguish oneself from others. In the 1970s Gordon Gallup
Gordon G. Gallup
Gordon G. Gallup, Jr. is a psychologist of the University at Albany's Psychology department, researching biopsychology. He received his Ph.D. from Washington State University in 1968, after which he joined the faculty of the Psychology Department at Tulane University...

 developed an operational test for self-awareness, known as the mirror test
Mirror test
The mirror test is a measure of self-awareness, as animals either possess or lack the ability to recognize themselves in a mirror.The test was developed by Gordon Gallup Jr. in 1970, based in part on observations made by Charles Darwin. While visiting a zoo, Darwin held a mirror up to an orangutan...

. The test examines whether animals are able to differentiate between seeing themselves in a mirror versus seeing other animals. The classic example involves placing a spot of coloring on the skin or fur near the individual's forehead and seeing if they attempt to remove it or at least touch the spot, thus indicating that they recognize that the individual they are seeing in the mirror is themselves. Humans (older than 18 months) and other great apes
Hominidae
The Hominidae or include them .), as the term is used here, form a taxonomic family, including four extant genera: chimpanzees , gorillas , humans , and orangutans ....

, bottlenose dolphin
Bottlenose Dolphin
Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphins. Recent molecular studies show the genus contains two species, the common bottlenose dolphin and the Indo-Pacific bottlenose dolphin , instead of one...

s, pigeons, and elephants have all been observed to pass this test.

Neural correlates

A major part of the scientific literature on consciousness consists of studies that examine the relationship between the experiences reported by subjects and the activity that simultaneously takes place in their brains—that is, studies of the neural correlates of consciousness
Neural correlates of consciousness
The neural correlates of consciousness constitute the minimal set of neuronal events and mechanisms sufficient for a specific conscious percept. Neuroscientists use empirical approaches to discover neural correlates of subjective phenomena...

. The hope is to find that activity in a particular part of the brain, or a particular pattern of global brain activity, will be strongly predictive of conscious awareness. Several brain imaging techniques, such as EEG
EEG
EEG commonly refers to electroencephalography, a measurement of the electrical activity of the brain.EEG may also refer to:* Emperor Entertainment Group, a Hong Kong-based entertainment company...

 and fMRI, have been used for physical measures of brain activity in these studies.

One idea that has drawn attention for several decades is that consciousness is associated with high-frequency (gamma band) EEG oscillations. This idea arose from proposals in the 1980s, by Christof von der Malsburg and Wolf Singer, that gamma oscillations could solve the so-called binding problem
Binding problem
The binding problem is one of a number of terms at the interface between neuroscience and philosophy which suffer from being used in several different ways, often in a context that does not explicitly indicate which way the term is being used. Of the many possible usages, two common versions may be...

, by linking information represented in different parts of the brain into a unified experience. Rodolfo Llinás
Rodolfo Llinás
Rodolfo R. Llinás PhD is a neuroscientist. He is presently the Thomas and Suzanne Murphy Professor of Neuroscience and Chairman of the department of Physiology & Neuroscience at the NYU School of Medicine...

, for example, proposed that consciousness results from recurrent thalamo-cortical resonance
Recurrent thalamo-cortical resonance
Recurrent thalamo-cortical resonance is an observed phenomenon of oscillatory electrical activity between the thalamus and various cortical regions of the brain. It is proposed by Rodolfo Llinas and others as a theory for the integration of sensory perception into current brain state...

 where the specific thalamocortical systems (content) and the non-specific (centromedial thalamus) thalamocortical systems (context) interact in the gamma
Gamma wave
A gamma wave is a pattern of neural oscillation in humans with a frequency between 25 to 100 Hz, though 40 Hz is prototypical.According to a popular theory, gamma waves may be implicated in creating the unity of conscious perception...

 band frequency via temporal coincidence.

A number of studies have shown that activity in primary sensory areas of the brain is not sufficient to produce consciousness: it is possible for subjects to report a lack of awareness even when areas such as the primary visual cortex show clear electrical responses to a stimulus. Higher brain areas are seen as more promising, especially the prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...

, which is involved in a range of higher cognitive functions collectively known as executive functions
Executive functions
The executive system is a theorized cognitive system in psychology that controls and manages other cognitive processes. It is responsible for processes that are sometimes referred to as the executive function, executive functions, supervisory attentional system, or cognitive control...

. There is substantial evidence that a "top-down" flow of neural activity (i.e., activity propagating from the frontal cortex to sensory areas) is more predictive of conscious awareness than a "bottom-up" flow of activity. The prefrontal cortex is not the only candidate area, however: studies by Nikos Logothetis and his colleagues have shown, for example, that visually responsive neurons in parts of the temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....

 reflect the visual perception in the situation when conflicting visual images are presented to different eyes (i.e., bistable percepts during binocular rivalry).

Biological function and evolution

Regarding the primary function of conscious processing, a recurring idea in recent theories is that phenomenal states somehow integrate neural activities and information-processing that would otherwise be independent. This has been called the integration consensus. However, it remains unspecified which kinds of information are integrated in a conscious manner and which kinds can be integrated without consciousness. Nor is it explained what specific causal role conscious integration plays, nor why the same functionality cannot be achieved without consciousness. Obviously not all kinds of information are capable of being disseminated consciously (e.g., neural activity related to vegetative functions, reflexes, unconscious motor programs, low-level perceptual analyses, etc.) and many kinds of information can be disseminated and combined with other kinds without consciousness, as in intersensory interactions such as the ventriloquism effect. Hence it remains unclear why any of it is conscious.

As noted earlier, even among writers who consider consciousness to be a well-defined thing, there is widespread dispute about which animals other than humans can be said to possess it. Thus, any examination of the evolution of consciousness is faced with great difficulties. Nevertheless, some writers have argued that consciousness can be viewed from the standpoint of evolutionary biology as an adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

 in the sense of a trait that increases fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

. In his paper "Evolution of consciousness," John Eccles argued that special anatomical and physical properties of the mammalian cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 gave rise to consciousness. Bernard Baars proposed that once in place, this "recursive" circuitry may have provided a basis for the subsequent development of many of the functions that consciousness facilitates in higher organisms.

Other philosophers, however, have suggested that consciousness would not be necessary for any functional advantage in evolutionary processes. No one has given a causal explanation, they argue, of why it would not be possible for a functionally equivalent non-conscious organism (i.e., a philosophical zombie
Philosophical zombie
A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

) to achieve the very same survival advantages as a conscious organism. If evolutionary processes are blind to the difference between function F being performed by conscious organism O and non-conscious organism O*, it is unclear what adaptive advantage consciousness could provide.

States of consciousness

There are some states in which consciousness seems to be abolished, including sleep, coma, and death. There are also a variety of circumstances that can change the relationship between the mind and the world in less drastic ways, producing what are known as altered states of consciousness
Altered state of consciousness
An altered state of consciousness , also named altered state of mind, is any condition which is significantly different from a normal waking beta wave state. The expression was used as early as 1966 by Arnold M. Ludwig and brought into common usage from 1969 by Charles Tart: it describes induced...

. Some altered states occur naturally; others can be produced by drugs or brain damage.

The two most widely accepted altered states are sleep
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

 and dream
Dream
Dreams are successions of images, ideas, emotions, and sensations that occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not definitively understood, though they have been a topic of scientific speculation, philosophical intrigue and religious...

ing. Although dream sleep and non-dream sleep appear very similar to an outside observer, each is associated with a distinct pattern of brain activity, metabolic activity, and eye movement; each is also associated with a distinct pattern of experience and cognition. During ordinary non-dream sleep, people who are awakened report only vague and sketchy thoughts, and their experiences do not cohere into a continuous narrative. During dream sleep, in contrast, people who are awakened report rich and detailed experiences in which events form a continuous progression, which may however be interrupted by bizarre or fantastic intrusions. Thought processes during the dream state frequently show a high level of irrationality. Both dream and non-dream states are associated with severe disruption of memory: it usually disappears in seconds during the non-dream state, and in minutes after awakening from a dream unless actively refreshed.

A variety of psychoactive drug
Psychoactive drug
A psychoactive drug, psychopharmaceutical, or psychotropic is a chemical substance that crosses the blood–brain barrier and acts primarily upon the central nervous system where it affects brain function, resulting in changes in perception, mood, consciousness, cognition, and behavior...

s have notable effects on consciousness. These range from a simple dulling of awareness produced by sedative
Sedative
A sedative or tranquilizer is a substance that induces sedation by reducing irritability or excitement....

s, to increases in the intensity of sensory qualities produced by stimulant
Stimulant
Stimulants are psychoactive drugs which induce temporary improvements in either mental or physical function or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others...

s, cannabis
Cannabis
Cannabis is a genus of flowering plants that includes three putative species, Cannabis sativa, Cannabis indica, and Cannabis ruderalis. These three taxa are indigenous to Central Asia, and South Asia. Cannabis has long been used for fibre , for seed and seed oils, for medicinal purposes, and as a...

, or most notably by the class of drugs known as psychedelics
Psychedelic drug
A psychedelic substance is a psychoactive drug whose primary action is to alter cognition and perception. Psychedelics are part of a wider class of psychoactive drugs known as hallucinogens, a class that also includes related substances such as dissociatives and deliriants...

. LSD, mescaline
Mescaline
Mescaline or 3,4,5-trimethoxyphenethylamine is a naturally occurring psychedelic alkaloid of the phenethylamine class used mainly as an entheogen....

, psilocybin
Psilocybin
Psilocybin is a naturally occurring psychedelic prodrug, with mind-altering effects similar to those of LSD and mescaline, after it is converted to psilocin. The effects can include altered thinking processes, perceptual distortions, an altered sense of time, and spiritual experiences, as well as...

, and others in this group can produce major distortions of perception, including hallucinations; some users even describe their drug-induced experiences as mystical or spiritual in quality. The brain mechanisms underlying these effects are not well understood, but there is substantial evidence that alterations in the brain system that uses the chemical neurotransmitter serotonin
Serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

 play an essential role.

There has been some research into physiological changes in yogis and people who practise various techniques of meditation. Some research with brain waves during meditation has differences between those corresponding to ordinary relaxation and those corresponding to meditation. It has been disputed, however, whether there is enough evidence to count these as physiologically distinct states of consciousness.

The most extensive study of the characteristics of altered states of consciousness was made by psychologist Charles Tart
Charles Tart
Dr. Charles T. Tart is an American psychologist and parapsychologist known for his psychological work on the nature of consciousness , as one of the founders of the field of transpersonal psychology, and for his research in scientific parapsychology. He earned his Ph. D...

 in the 1960s and 1970s. Tart analyzed a state of consciousness as made up of a number of component processes, including exteroception (sensing the external world); interoception (sensing the body); input-processing (seeing meaning); emotions; memory; time sense; sense of identity; evaluation and cognitive processing; motor output; and interaction with the environment. Each of these, in his view, could be altered in multiple ways by drugs or other manipulations. The components that Tart identified have not, however, been validated by empirical studies. Research in this area has not yet reached firm conclusions, but a recent questionnaire-based study identified eleven significant factors contributing to drug-induced states of consciousness: experience of unity; spiritual experience; blissful state; insightfulness; disembodiment; impaired control and cognition; anxiety; complex imagery; elementary imagery; audio-visual synesthesia
Synesthesia
Synesthesia , from the ancient Greek , "together," and , "sensation," is a neurologically based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway...

; and changed meaning of percepts.

Phenomenology

Phenomenology is a method of inquiry that attempts to examine the structure of consciousness in its own right, putting aside problems regarding the relationship of consciousness to the physical world. This approach was first proposed by the philosopher Edmund Husserl, and later elaborated by other philosophers and scientists. Husserl's original concept gave rise to two distinct lines of inquiry, in philosophy and psychology. In philosophy, phenomenology has largely been devoted to fundamental metaphysical questions, such as the nature of intentionality
Intentionality
The term intentionality was introduced by Jeremy Bentham as a principle of utility in his doctrine of consciousness for the purpose of distinguishing acts that are intentional and acts that are not...

 ("aboutness"). In psychology
Phenomenology (psychology)
Phenomenology is an approach to psychological subject matter that has its roots in the philosophical work of Edmund Husserl. Early phenomenologists such as Husserl, Jean-Paul Sartre, and Maurice Merleau-Ponty conducted their own psychological investigations in the early 20th century...

, phenomenology largely has meant attempting to investigate consciousness using the method of introspection
Introspection
Introspection is the self-observation and reporting of conscious inner thoughts, desires and sensations. It is a conscious and purposive process relying on thinking, reasoning, and examining one's own thoughts, feelings, and, in more spiritual cases, one's soul...

, which means looking into one's own mind and reporting what one observes. This method fell into disrepute in the early twentieth century because of grave doubts about its reliability, but has been rehabilitated to some degree, especially when used in combination with techniques for examining brain activity.
Introspectively, the world of conscious experience seems to have considerable structure. Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

 asserted that the world as we perceive it is organized according to a set of fundamental "intuitions", which include object (we perceive the world as a set of distinct things); shape; quality (color, warmth, etc.); space (distance, direction, and location); and time. Some of these constructs, such as space and time, correspond to the way the world is structured by the laws of physics; for others the correspondence is not as clear. Understanding the physical basis of qualities, such as redness or pain, has been particularly challenging. David Chalmers has called this the hard problem of consciousness
Hard problem of consciousness
The hard problem of consciousness is the problem of explaining how and why we have qualitative phenomenal experiences. David Chalmers contrasts this with the "easy problems" of explaining the ability to discriminate, integrate information, report mental states, focus attention, etc...

. Some philosophers have argued that it is intrinsically unsolvable, because qualities ("qualia
Qualia
Qualia , singular "quale" , from a Latin word meaning for "what sort" or "what kind," is a term used in philosophy to refer to subjective conscious experiences as 'raw feels'. Examples of qualia are the pain of a headache, the taste of wine, the experience of taking a recreational drug, or the...

"
) are ineffable
Ineffability
Ineffability is concerned with ideas that cannot or should not be expressed in spoken words , often being in the form of a taboo or incomprehensible term. This property is commonly associated with philosophy, aspects of existence, and similar concepts that are inherently "too great", complex, or...

; that is, they are "raw feels", incapable of being analyzed into component processes. Most psychologists and neuroscientists have not accepted these arguments — nevertheless it is clear that the relationship between a physical entity such as light and a perceptual quality such as color is extraordinarily complex and indirect, as demonstrated by a variety of optical illusion
Optical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...

s such as neon color spreading
Neon color spreading
Neon color spreading is an optical illusion in which a bright color appears to spread out over a white area from bright visual contours....

.

In neuroscience, a great deal of effort has gone into investigating how the perceived world of conscious awareness is constructed inside the brain. The process is generally thought to involve two primary mechanisms: (1) hierarchical processing of sensory inputs, (2) memory. Signals arising from sensory organs are transmitted to the brain and then processed in a series of stages, which extract multiple types of information from the raw input. In the visual system, for example, sensory signals from the eyes are transmitted to the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 and then to the primary visual cortex; inside the cerebral cortex they are sent to areas that extract features such as three-dimensional structure, shape, color, and motion. Memory comes into play in at least two ways. First, it allows sensory information to be evaluated in the context of previous experience. Second, and even more importantly, working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

 allows information to be integrated over time so that it can generate a stable representation of the world—Gerald Edelman
Gerald Edelman
Gerald Maurice Edelman is an American biologist who shared the 1972 Nobel Prize in Physiology or Medicine for work with Rodney Robert Porter on the immune system. Edelman's Nobel Prize-winning research concerned discovery of the structure of antibody molecules...

 expressed this point vividly by titling one of his books about consciousness The Remembered Present.

Despite the large amount of information available, the most important aspects of perception remain mysterious. A great deal is known about low-level signal processing in sensory systems, but the ways by which sensory systems interact with each other, with "executive" systems in the frontal cortex, and with the language system are very incompletely understood. At a deeper level, there are still basic conceptual issues that remain unresolved. Many scientists have found it difficult to reconcile the fact that information is distributed across multiple brain areas with the apparent unity of consciousness: this is one aspect of the so-called binding problem
Binding problem
The binding problem is one of a number of terms at the interface between neuroscience and philosophy which suffer from being used in several different ways, often in a context that does not explicitly indicate which way the term is being used. Of the many possible usages, two common versions may be...

. There are also some scientists who have expressed grave reservations about the idea that the brain forms representations of the outside world at all: influential members of this group include psychologist J. J. Gibson and roboticist Rodney Brooks
Rodney Brooks
Rodney Allen Brooks is the former Panasonic professor of robotics at the Massachusetts Institute of Technology. Since 1986 he has authored a series of highly influential papers which have inaugurated a fundamental shift in artificial intelligence research...

, who both argued in favor of "intelligence without representation".

Medical aspects

The medical approach to consciousness is practically oriented. It derives from a need to treat people whose brain function has been impaired as a result of disease, brain damage, toxins, or drugs. In medicine, conceptual distinctions are considered useful to the degree that they can help to guide treatments. Whereas the philosophical approach to consciousness focuses on its fundamental nature and its contents, the medical approach focuses on the amount of consciousness a person has: in medicine, consciousness is assessed as a "level" ranging from coma and brain death at the low end, to full alertness and purposeful responsiveness at the high end.

Consciousness is of concern to patients and physicians, especially neurologists
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

 and anesthesiologists
Anesthesia
Anesthesia, or anaesthesia , traditionally meant the condition of having sensation blocked or temporarily taken away...

. Patients may suffer from disorders of consciousness, or may need to be anesthetized for a surgical procedure. Physicians may perform consciousness-related interventions such as instructing the patient to sleep, administering general anesthesia, or inducing medical coma
Induced coma
A barbiturate-induced coma, or barb coma, is a temporary coma brought on by a controlled dose of a barbiturate drug, usually pentobarbital or thiopental...

. Also, bioethicists
Bioethics
Bioethics is the study of controversial ethics brought about by advances in biology and medicine. Bioethicists are concerned with the ethical questions that arise in the relationships among life sciences, biotechnology, medicine, politics, law, and philosophy....

 may be concerned with the ethical implications of consciousness in medical cases of patients such as Karen Ann Quinlan
Karen Ann Quinlan
Karen Ann Quinlan was an important figure in the history of the right to die controversy in the United States....

, while neuroscientists may study patients with impaired consciousness in hopes of gaining information about how the brain works.

Assessment

In medicine, consciousness is examined using a set of procedures known as neuropsychological assessment
Neuropsychological assessment
Neuropsychological assessment was traditionally carried out to assess the extent of impairment to a particular skill and to attempt to locate an area of the brain which may have been damaged after brain injury or neurological illness...

. There are two commonly used methods for assessing the level of consciousness of a patient: a simple procedure that requires minimal training, and a more complex procedure that requires substantial expertise. The simple procedure begins by asking whether the patient is able to move and react to physical stimuli. If so, the next question is whether the patient can respond in a meaningful way to questions and commands. If so, the patient is asked for name, current location, and current day and time. A patient who can answer all of these questions is said to be "oriented times three" (sometimes denoted "Ox3" on a medical chart), and is usually considered fully conscious.

The more complex procedure is known as a neurological examination
Neurological examination
A neurological examination is the assessment of sensory neuron and motor responses, especially reflexes, to determine whether the nervous system is impaired...

, and is usually carried out by a neurologist
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

 in a hospital setting. A formal neurological examination runs through a precisely delineated series of tests, beginning with tests for basic sensorimotor reflexes, and culminating with tests for sophisticated use of language. The outcome may be summarized using the Glasgow Coma Scale
Glasgow Coma Scale
Glasgow Coma Scale or GCS is a neurological scale that aims to give a reliable, objective way of recording the conscious state of a person for initial as well as subsequent assessment...

, which yields a number in the range 3—15, with a score of 3 indicating brain death
Brain death
Brain death is the irreversible end of all brain activity due to total necrosis of the cerebral neurons following loss of brain oxygenation. It should not be confused with a persistent vegetative state...

 (the lowest defined level of consciousness), and 15 indicating full consciousness. The Glasgow Coma Scale has three subscales, measuring the best motor response (ranging from "no motor response" to "obeys commands"), the best eye response (ranging from "no eye opening" to "eyes opening spontaneously") and the best verbal response (ranging from "no verbal response" to "fully oriented"). There is also a simpler pediatric version of the scale, for children too young to be able to use language.

Disorders of consciousness

Medical conditions that inhibit consciousness are considered disorders of consciousness
Disorders of consciousness
Disorders of consciousness are medical conditions that inhibit consciousness. This category generally includes minimally conscious state and persistent vegetative state, but sometimes also includes the less severe locked-in syndrome and more severe chronic coma. Differential diagnosis of these...

. This category generally includes minimally conscious state
Minimally conscious state
Minimally Conscious State is a disorder of consciousness distinct from Persistent vegetative state and Locked-in syndrome. Unlike persistent vegetative state, patients with MCS have partial preservation of conscious awareness. MCS is a relatively new category of disorders of consciousness. The...

 and persistent vegetative state
Persistent vegetative state
A persistent vegetative state is a disorder of consciousness in which patients with severe brain damage are in a state of partial arousal rather than true awareness. It is a diagnosis of some uncertainty in that it deals with a syndrome. After four weeks in a vegetative state , the patient is...

, but sometimes also includes the less severe locked-in syndrome
Locked-In syndrome
Locked-in syndrome is a condition in which a patient is aware and awake but cannot move or communicate verbally due to complete paralysis of nearly all voluntary muscles in the body except for the eyes. Total locked-in syndrome is a version of locked-in syndrome where the eyes are paralyzed as...

 and more severe chronic coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...

. Differential diagnosis
Differential diagnosis
A differential diagnosis is a systematic diagnostic method used to identify the presence of an entity where multiple alternatives are possible , and may also refer to any of the included candidate alternatives A differential diagnosis (sometimes abbreviated DDx, ddx, DD, D/Dx, or ΔΔ) is a...

 of these disorders is an active area of biomedical research
Biomedical research
Biomedical research , in general simply known as medical research, is the basic research, applied research, or translational research conducted to aid and support the body of knowledge in the field of medicine...

. Finally, brain death
Brain death
Brain death is the irreversible end of all brain activity due to total necrosis of the cerebral neurons following loss of brain oxygenation. It should not be confused with a persistent vegetative state...

 results in an irreversible disruption of consciousness. While other conditions may cause a moderate deterioration (e.g., dementia
Dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...

 and delirium
Delirium
Delirium or acute confusional state is a common and severe neuropsychiatric syndrome with core features of acute onset and fluctuating course, attentional deficits and generalized severe disorganization of behavior...

) or transient interruption (e.g., grand mal and petit mal seizures
Absence seizure
Absence seizures are one of several kinds of seizures. These seizures are sometimes referred to as petit mal seizures ....

) of consciousness, they are not included in this category.
Disorder Description
Locked-in syndrome The patient has awareness, sleep-wake cycles, and meaningful behavior (viz., eye-movement), but is isolated due to quadriplegia
Quadriplegia
Tetraplegia, also known as quadriplegia, is paralysis caused by illness or injury to a human that results in the partial or total loss of use of all their limbs and torso; paraplegia is similar but does not affect the arms...

 and pseudobulbar palsy
Pseudobulbar palsy
Pseudobulbar palsy results from an upper motor neuron lesion to the corticobulbar pathways in the pyramidal tract. Patients have difficulty chewing, swallowing and demonstrate slurred speech...

.
Minimally conscious state The patient has intermittent periods of awareness and wakefulness and displays some meaningful behavior.
Persistent vegetative state The patient has sleep-wake cycles, but lacks awareness and only displays reflexive and non-purposeful behavior.
Chronic coma The patient lacks awareness and sleep-wake cycles and only displays reflexive behavior.
Brain death The patient lacks awareness, sleep-wake cycles, and behavior.

Anosognosia

One of the most striking disorders of consciousness goes by the name anosognosia
Anosognosia
Anosognosia /æˌnɒsɒgˈnəʊsɪə/ is a condition in which a person who suffers disability seems unaware of the existence of his or her disability. Unlike denial, which is a defense mechanism, anosognosia is rooted in physiology...

, a Greek-derived term meaning unawareness of disease. This is a condition in which patients are disabled in some way, most commonly as a result of a stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, but either misunderstand the nature of the problem or deny that there is anything wrong with them. The most frequently occurring form is seen in people who have experienced a stroke damaging the parietal lobe
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...

 in the right hemisphere of the brain, giving rise to a syndrome known as hemispatial neglect
Hemispatial neglect
Hemispatial neglect, also called hemiagnosia, hemineglect, unilateral neglect, spatial neglect, unilateral visual inattention, hemi-inattention or neglect syndrome is a neuropsychological condition in which, after damage to one hemisphere of the brain, a deficit in attention to and awareness of...

, characterized by an inability to direct action or attention toward objects located to the right with respect to their bodies. Patients with hemispatial neglect are often paralyzed on the right side of the body, but sometimes deny being unable to move. When questioned about the obvious problem, the patient may avoid giving a direct answer, or may give an explanation that doesn't make sense. Patients with hemispatial neglect may also fail to recognize paralyzed parts of their bodies: one frequently mentioned case is of a man who repeatedly tried to throw his own paralyzed right leg out of the bed he was lying in, and when asked what he was doing, complained that somebody had put a dead leg into the bed with him. An even more striking type of anosognosia is Anton–Babinski syndrome, a rarely occurring condition in which patients become blind but claim to be able to see normally, and persist in this claim in spite of all evidence to the contrary.

The stream of consciousness

William James
William James
William James was a pioneering American psychologist and philosopher who was trained as a physician. He wrote influential books on the young science of psychology, educational psychology, psychology of religious experience and mysticism, and on the philosophy of pragmatism...

 is usually credited with popularizing the idea that human consciousness flows like a stream, in his Principles of Psychology of 1890. According to James, the "stream of thought" is governed by five characteristics: "(1) Every thought tends to be part of a personal consciousness. (2) Within each personal consciousness thought is always changing. (3) Within each personal consciousness thought is sensibly continuous. (4) It always appears to deal with objects independent of itself. (5) It is interested in some parts of these objects to the exclusion of others". A similar concept appears in Buddhist philosophy, expressed by the Sanskrit term Citta-saṃtāna, which is usually translated as mindstream
Mindstream
Mindstream in Buddhist philosophy is the moment-to-moment "continuum" of awareness. There are a number of terms in the Buddhist literature that may well be rendered "mindstream"...

 or "mental continuum". In the Buddhist view, though, the "mindstream" is viewed primarily as a source of noise that distracts attention from a changeless underlying reality.

In the west, the primary impact of the idea has been on literature rather than science: stream of consciousness as a narrative mode means writing in a way that attempts to portray the moment-to-moment thoughts and experiences of a character. This technique perhaps had its beginnings in the monologues of Shakespeare's plays, and reached its fullest development in the novels of James Joyce
James Joyce
James Augustine Aloysius Joyce was an Irish novelist and poet, considered to be one of the most influential writers in the modernist avant-garde of the early 20th century...

 and Virginia Woolf
Virginia Woolf
Adeline Virginia Woolf was an English author, essayist, publisher, and writer of short stories, regarded as one of the foremost modernist literary figures of the twentieth century....

, although it has also been used by many other noted writers.

Here for example is a passage from Joyce's Ulysses
Ulysses (novel)
Ulysses is a novel by the Irish author James Joyce. It was first serialised in parts in the American journal The Little Review from March 1918 to December 1920, and then published in its entirety by Sylvia Beach on 2 February 1922, in Paris. One of the most important works of Modernist literature,...

 about the thoughts of Molly Bloom:

See also

  • Blindsight
    Blindsight
    Blindsight is a phenomenon in which people who are perceptually blind in a certain area of their visual field demonstrate some response to visual stimuli...

  • Causality
    Causality
    Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

  • Centipede's dilemma
    Centipede's dilemma
    "The Centipede's Dilemma" is a short poem which has given its name to an effect in psychology: the centipede effect . This is when a normally automatic or unconscious activity is disrupted by consciousness of it or reflection on it...

  • Cognitive neuroscience
    Cognitive neuroscience
    Cognitive neuroscience is an academic field concerned with the scientific study of biological substrates underlying cognition, with a specific focus on the neural substrates of mental processes. It addresses the questions of how psychological/cognitive functions are produced by the brain...

  • Cognitive psychology
    Cognitive psychology
    Cognitive psychology is a subdiscipline of psychology exploring internal mental processes.It is the study of how people perceive, remember, think, speak, and solve problems.Cognitive psychology differs from previous psychological approaches in two key ways....

  • Explanation
    Explanation
    An explanation is a set of statements constructed to describe a set of facts which clarifies the causes, context, and consequencesof those facts....

  • Explanatory gap
    Explanatory gap
    The explanatory gap is the claim that consciousness and human experiences such as qualia cannot be fully explained just by identifying the corresponding physical processes. Bridging this gap is known as "the hard problem"...

  • Functionalism (philosophy of mind)
    Functionalism (philosophy of mind)
    Functionalism is a theory of the mind in contemporary philosophy, developed largely as an alternative to both the identity theory of mind and behaviourism. Its core idea is that mental states are constituted solely by their functional role — that is, they are causal relations to other mental...

  • Hard problem of consciousness
    Hard problem of consciousness
    The hard problem of consciousness is the problem of explaining how and why we have qualitative phenomenal experiences. David Chalmers contrasts this with the "easy problems" of explaining the ability to discriminate, integrate information, report mental states, focus attention, etc...

  • Merkwelt
    Merkwelt
    The merkwelt is a concept in robotics, ethology and biology that describes a creature or android's capacity to view things, manipulate information and synthesize to make meaning out of the universe...

  • Mind
    Mind
    The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

  • Mind-body problem
  • Mirror neuron
    Mirror neuron
    A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors" the behaviour of the other, as though the observer were itself acting. Such neurons have been directly observed in primate and other...

  • Neuropsychological assessment
    Neuropsychological assessment
    Neuropsychological assessment was traditionally carried out to assess the extent of impairment to a particular skill and to attempt to locate an area of the brain which may have been damaged after brain injury or neurological illness...

  • Neuropsychology
    Neuropsychology
    Neuropsychology studies the structure and function of the brain related to specific psychological processes and behaviors. The term neuropsychology has been applied to lesion studies in humans and animals. It has also been applied to efforts to record electrical activity from individual cells in...

  • Phenomenology
  • Philosophical zombie
    Philosophical zombie
    A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

  • Philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

  • Problem of other minds
    Problem of other minds
    The problem of other minds has traditionally been regarded as an epistemological challenge raised by the skeptic. The challenge may be expressed as follows: given that I can only observe the behavior of others, how can I know that others have minds? The thought behind the question is that no matter...

  • Reverse engineering
    Reverse engineering
    Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

  • Sentience
    Sentience
    Sentience is the ability to feel, perceive or be conscious, or to have subjective experiences. Eighteenth century philosophers used the concept to distinguish the ability to think from the ability to feel . In modern western philosophy, sentience is the ability to have sensations or experiences...

  • Solipsism
    Solipsism
    Solipsism is the philosophical idea that only one's own mind is sure to exist. The term comes from Latin solus and ipse . Solipsism as an epistemological position holds that knowledge of anything outside one's own mind is unsure. The external world and other minds cannot be known, and might not...

  • Turing test
    Turing test
    The Turing test is a test of a machine's ability to exhibit intelligent behaviour. In Turing's original illustrative example, a human judge engages in a natural language conversation with a human and a machine designed to generate performance indistinguishable from that of a human being. All...



External links

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