Full Idea
Butler thought that Locke's denial of the requirement of identity of substance doomed his analysis of personal identity.
Gist of Idea
Butler thought Locke's theory was doomed once he rejected mental substance
Source
comment on John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.27.25) by John Perry - Introduction to 'Personal Identity' Intro
Book Reference
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.49
A Reaction
Butler seems to have thought that psychological criteria were a slippery slope, whereas substance gives the necessary fixed identity (such as a bishop would require). Personally I say that personal identity is the activity of a physical substance.