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Full Idea
If Hesperus and Phosphorus are one and the same, then in no other possible world can they be different.
Gist of Idea
If Hesperus and Phosophorus are the same, they can't possibly be different
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 2)
Book Ref
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.104
A Reaction
If we ask whether one object could possibly be two objects, and deny that possibility, then Kripke's novel thought seems just right and obvious.