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Full Idea
An identity statement between names (such as 'Hesperus' and 'Phosphorus'), when true at all, is necessarily true, even though one may not know it a priori.
Clarification
Hesperus (the evening star) and Phosphorus (the morning star) turned out to be the planet Venus
Gist of Idea
"'Hesperus' is 'Phosphorus'" is necessarily true, if it is true, but not known a priori
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3)
Book Ref
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.108
A Reaction
This seems correct, but one should not read too much into it. What should we say if Venus fissions into two, one for the morning, one for the evening? That identity implies x=x doesn't prove the existence of unchanging essences.
| 8856 | Quine's indispensability argument said arguments for abstracta were a posteriori [Quine, by Yablo] |
| 2796 | For Quine the only way to know a necessity is empirically [Quine, by Dancy,J] |
| 5476 | Essentialists say natural laws are in a new category: necessary a posteriori [Ellis] |
| 9174 | It is necessary that this table is not made of ice, but we don't know it a priori [Kripke] |
| 2408 | Kripke has demonstrated that some necessary truths are only knowable a posteriori [Kripke, by Chalmers] |
| 4960 | "'Hesperus' is 'Phosphorus'" is necessarily true, if it is true, but not known a priori [Kripke] |
| 4966 | Theoretical identities are between rigid designators, and so are necessary a posteriori [Kripke] |
| 14631 | How can you show the necessity of an a posteriori necessity, if it might turn out to be false? [Jackson] |
| 16421 | Critics say there are just an a priori necessary part, and an a posteriori contingent part [Stalnaker] |
| 15171 | The necessary a posteriori is statements either of identity or of essence [Sidelle] |