'Hesperus is Phosphorus' is necessarily true — and we only found out by telescope

Kripke argued that names like 'Hesperus' and 'Phosphorus' are rigid designators picking out the same object in every possible world, so once we discover they both name Venus, that identity is necessarily true. Yet it's also a posteriori — we only learned it through astronomy, not pure logic. This broke the old assumption that every necessary truth must be knowable a priori.

— Saul Kripke, Naming and Necessity — 1970 Princeton lectures, published as a book in 1980
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