In logic

Logic

In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...

, a normal modal logic

Modal logic

Modal logic is a type of formal logic that extends classical propositional and predicate logic to include operators expressing modality. Modals — words that express modalities — qualify a statement. For example, the statement "John is happy" might be qualified by saying that John is...

is a set L of modal formulas such that L contains:

• All propositional tautologies
  Tautology (logic)
  In logic, a tautology is a formula which is true in every possible interpretation. Philosopher Ludwig Wittgenstein first applied the term to redundancies of propositional logic in 1921; it had been used earlier to refer to rhetorical tautologies, and continues to be used in that alternate sense...
  
  ;
• All instances of the Kripke schema:

and it is closed under:

• Detachment rule (Modus Ponens
  Modus ponens
  In classical logic, modus ponendo ponens or implication elimination is a valid, simple argument form. It is related to another valid form of argument, modus tollens. Both Modus Ponens and Modus Tollens can be mistakenly used when proving arguments...
  
  ): ;
• Necessitation rule: implies .

The smallest logic satisfying the above conditions is called K. Most modal logics commonly used nowadays (in terms of having philosophical motivations), e.g. C. I. Lewis's S4 and S5

S5 (modal logic)

In logic and philosophy, S5 is one of five systems of modal logic proposed byClarence Irving Lewis and Cooper Harold Langford in their 1932 book Symbolic Logic.It is a normal modal logic, and one of the oldest systems of modal logic of any kind....

, are extensions of K. However a number of deontic

Deontic logic

Deontic logic is the field of logic that is concerned with obligation, permission, and related concepts. Alternatively, a deontic logic is a formal system that attempts to capture the essential logical features of these concepts...

 and epistemic logic

Epistemic logic

Epistemic modal logic is a subfield of modal logic that is concerned with reasoning about knowledge. While epistemology has a long philosophical tradition dating back to Ancient Greece, epistemic logic is a much more recent development with applications in many fields, including philosophy,...

s, for example, are non-normal, often because they give up the Kripke schema.