Full Idea
In his attempted disproof of materialism about the mind, Kripke assumes that the physical description is a rigid designator, but this seems to be begging the question against the causal theory, which says the description is non-rigid.
Clarification
Rigid designators refer to an item in all possible worlds
Gist of Idea
Kripke assumes that mind-brain identity designates rigidly, which it doesn't
Source
comment on Saul A. Kripke (Naming and Necessity lectures [1970]) by David M. Armstrong - Pref to new 'Materialist Theory' p.xiv
Book Reference
Armstrong,D.M.: 'A Materialist Theory of Mind' [Routledge 1993], p.-10
A Reaction
A crucial part of this is that Armstrong believes that the laws of nature are contingent, and hence mind-brain identity has to be. Personally I incline to say that the identity is rigid, but that Kripke is still wrong.