Terminology Tuesday: Necessary / Contingent
“Squares have four sides.” is necessary.
“Stop signs are hexagonal.” is contingent.
“Pentagons are round.” is contradictory.
This distinction was traditionally associated (before Kant and Kripke) with the distinctions between a priori and a posteriori knowledge and the distinction between analytic and synthetic judgment. Necessity may also be defined de dicto in terms of the formal logical property of tautology.1
1. Philosophy Pages.