# S5 (modal logic)

In logic and philosophy, S5 is one of five systems of modal logic proposed by Clarence 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.[citation needed]

## Axiomatics

The following makes use of the modal operators $\Box$ ("necessarily") and $\Diamond$ ("possibly").

S5 is characterized by the axioms:

• K: $\Box(A\to B)\to(\Box A\to\Box B)$;
• T: $\Box A \to A$,

and either:

• 5: $\Diamond A\to \Box\Diamond A$;
• or both of the following:
• 4: $\Box A\to\Box\Box A$, and
• B: $A\to\Box\Diamond A$.

## Kripke semantics

In terms of Kripke semantics, S5 is characterized by models where the accessibility relation is an equivalence relation: it is reflexive, transitive, and symmetric. Alternatively, S5 can be characterized by models where the accessibility relation is "universal", that is, every world is accessible from any other. Although these characterizations produce different sets of models (since the former, but not the latter, allows for "closed" systems of worlds such that no world in one is accessible from any world in the other), they both model the theorems of S5.

Determining the satisfiability of an S5 formula is an NP-complete problem. The hardness proof is trivial, as S5 includes the propositional logic. Membership is proved by showing that any satisfiable formula has a Kripke model where the number of worlds is at most linear in the size of the formula.

## Applications

S5 is useful because it avoids superfluous iteration of qualifiers of different kinds. For example, under S5, if X is necessarily, possibly, necessarily, possibly true, then X is possibly true. Unbolded qualifiers before the final "possibly" are pruned in S5. While this is useful for keeping propositions reasonably short, it also might appear counter-intuitive in that, under S5, if something is possibly necessary, then it is necessary.

Alvin Plantinga has argued that this feature of S5 is not, in fact, counter-intuitive. To justify, he reasons that if X is possibly necessary, it is necessary in at least one possible world; hence it is necessary in all possible worlds and thus is true in all possible worlds. Such reasoning underpins 'modal' formulations of the ontological argument.