Life (cf. biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...

) is a characteristic that distinguishes objects
Physical body
In physics, a physical body or physical object is a collection of masses, taken to be one...

 that have signaling and self-sustaining processes
Biological process
A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation....

 (i.e., living organisms
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

) from those that do not, either because such functions have ceased (death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

), or else because they lack such functions and are classified as inanimate. Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 is the science concerned with the study of life.

Living organisms undergo metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

, maintain homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

, possess a capacity to grow
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...

, respond to 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....

, reproduce and, through natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

, adapt to their environment in successive generations.

Brief and powerless is man's life; on him and all his race the slow, sure doom falls pitiless and dark.

Bertrand Russell, Mysticism and Logic, ‘A free Man's Worship.’

I compare human life to a large mansion of many apartments, two of which I can only describe, the doors of the rest being as yet shut upon me.

John Keats, letter to J. H. Reynolds, 3rd May 1818.

Life is a pill which none of us can bear to swallow without gilding.

Samuel Johnson, in Anecdotes of Johnson by Mrs Piozzi.

Life is a process of becoming, a combination of states we have to go through. Where people fail is that they wish to elect a state and remain in it. This is a kind of death.

Anaïs Nin, in D. H. Lawrence : An Unprofessional Study (1932)

Life's but a walking shadow, a poor player that struts and frets his hour upon the stage, and then is heard no more: it is a tale told by an idiot, full of sound and fury, signifying nothing.

William Shakespeare in Macbeth

Life is not measured by the number of breaths we take, but by the moments that take our breath away.

Claimed by Erica Frandsen in

Life is the distance between dreams and reality.

Leonid S. Sukhorukov|Leonid S. Sukhorukov, All About Everything (2005)

Life is what happens to you while you're busy making other plans.

John Lennon, in "Beautiful Boy (Darling Boy)" (1980)

Life shrinks or expands according to one's courage.

Anaïs Nin, as quoted in French Writers of the Past (2000) by Carol A. Dingle, p. 126