Concept · Foundations
Necessity
A proposition is necessary at a world when it is true in every possible world accessible from that world; written as the box operator in front of the proposition. With unrestricted accessibility this is simply truth in every possible world.
Why it matters
Necessity is the central modal concept: it is what distinguishes must-be-true from happens-to-be-true.
Where this shows up
Units and lessons using this concept
Following the links will take you to the page where this term is introduced in context.
Lessons (4)
Capstone: Modal Reasoning in Philosophy and ScienceModal Logic: Necessity, Possibility, and CounterfactualsFormalizing Modal ArgumentsModal Logic: Necessity, Possibility, and CounterfactualsThe Modal Operators and Their RulesModal Logic: Necessity, Possibility, and CounterfactualsWhat Is Modal Logic?Modal Logic: Necessity, Possibility, and Counterfactuals
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