Modal logic

Essence () has various meanings and uses for different thinkers and in different contexts. It is used in philosophy and theology as a designation for the property or set of properties or attributes that make an entity the entity it is or, expressed negatively, without which it would lose its identity. Essence is contrasted with accident, which is a property or attribute the entity has accidentally or contingently, but upon which its identity does not depend.

formal logic able to express concepts such as necessity, possibility, provability, obligation, knowledge etc.

in philosophy, a complete and consistent way the world is or could have been

The field of philosphical logic and a class of formal systems that are concerned with obligation, permission, and related concepts

subfield of modal logic that is concerned with reasoning about knowledge

Gödel's formalization of the ontological argument for the existence of God using modal logic

something that is logically possible to occur

semantics for modal logics

set of modal formulas containing all propositional tautologies and all instances of the Kripke schema ◻(A→B)→(◻A→◻B) and closed under modus ponens and necessitation

theorem

possibility considered in a counterfactual

formal way of expressing the meaning of a conditional sentence

modal logic

modal logic containing the duality axiom schema ◊A↔¬◻¬A as well as the deduction rule A↔B ⊢ ◻A↔◻B

extension of modal logic

modality in linguistics

normal modal logic with the additional axioms ◻A→A and ◊A→◻◊A

Assertoric is an adjectival expression in Aristotelian logic that refers to propositions which merely assert that something is (or is not) the case. Assertoricity is the corresponding abstract noun.

type of fallacy in modal logic