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Full Idea
There is such a thing as synthetic and a posteriori identity that is nonetheless genuine identity, as in lightning being electrical discharge, and the Morning Star being Venus.
Clarification
'synthetic a posteriori' means it is about the world, and is discovered
Gist of Idea
'Lightning is electric discharge' and 'Phosphorus is Venus' are synthetic a posteriori identities
Source
William Lycan (Introduction - Ontology [1999], p.5)
Book Ref
'Mind and Cognition (2nd Edn)', ed/tr. Lycan,William [Blackwell 1999], p.5
A Reaction
It is important to note that although these identities are synthetic a posteriori, that doesn't make them contingent. The early identity theorists like Smart seemed to think that it did. Kripke must be right that they are necessary identities.