Modal Logic, including: Gödel's Ontological Proof, Barcan Formula, Temporal Logic, Logical Possibility, Essence, De Dicto And De Re, Rigid Designator, ... Linear Temporal Logic, Hybrid Logic

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More info: Modal logic is a type of formal logic that extends the standards of formal logic to include the elements of modality (for example, possibility and necessity). Modals qualify the truth of a judgment. For example, if it is true that "John is happy," we might qualify this statement by saying that "John is very happy," in which case the term "very" would be a modality. Traditionally, there are three "modes" or "moods" or "modalities" represented in modal logic, namely, possibility, probability, and necessity.