Full Idea
The identity theorist, it appears, can admit that the identity is necessary if true without substantially altering his position, but Kripke argues that the identity between pain and some brain states is not necessary.
Gist of Idea
Identity must be necessary, but pain isn't necessarily a brain state, so they aren't identical
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3) by Stephen P. Schwartz - Intro to Naming,Necessity and Natural Kinds §IV
Book Reference
'Naming, Necessity, and Natural Kinds', ed/tr. Schwartz,Stephen P. [Cornell 1979], p.35
A Reaction
This appears to depend on being able to imagine the pain occurring with a different brain state, or no brain state. Bad argument. See Idea 5819.
Related Idea
Idea 5819 Conceivability is no proof of possibility [Putnam]