 modal logic

Formal systems incorporating modalities such as necessity, possibility, impossibility, contingency, strict implication, and certain other closely related concepts.The most straightforward way of constructing a modal logic is to add to some standard nonmodal logical system a new primitive operator intended to represent one of the modalities, to define other modal operators in terms of it, and to add axioms and/or transformation rules involving those modal operators. For example, one may add the symbol L, which means "It is necessary that," to classical propositional calculus; thus, Lp is read as "It is necessary that p." The possibility operator M ("It is possible that") may be defined in terms of L as Mp = ¬L¬p (where ¬ means "not"). In addition to the axioms and rules of inference of classical propositional logic, such a system might have two axioms and one rule of inference of its own. Some characteristic axioms of modal logic are: (A1) Lp ⊃ p and (A2) L(p ⊃ q) ⊃ (Lp ⊃ Lq). The new rule of inference in this system is the Rule of Necessitation: If p is a theorem of the system, then so is Lp. Stronger systems of modal logic can be obtained by adding additional axioms. Some add the axiom Lp ⊃ LLp; others add the axiom Mp ⊃ LMp.
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branch of logic that deals with modalities (such properties of propositions as necessity, contingency, possibility, and impossibility), as opposed to truth and falsity; thus the statements “Some men may be immortal” and “Men are necessarily social animals” are modal propositions. Although modal syllogisms were considered by Aristotle, modal logic remains today an uncertain field. Modern attempts to deal with the problem are found in the manyvalued logics, which allow other truthvalues between truth and falsity, and in systems of strict implication—systems of theorems that differ somewhat depending upon the relations between the different modalities that are set forth in their axioms. Compare truthvalue.* * *
Universalium. 2010.