Full Idea
Granted the contingency of the laws of nature, the identification of dispositions with their categorical bases can be contingent only.
Clarification
Their 'categorical bases' are (roughly) their properties
Gist of Idea
The identity of mental states with physical properties is contingent, because the laws of nature are contingent
Source
David M. Armstrong (Pref to new 'Materialist Theory' [1992], p.xvi)
Book Reference
Armstrong,D.M.: 'A Materialist Theory of Mind' [Routledge 1993], p.-8
A Reaction
Personally I am not willing to grant the contingency of the laws of nature, but I suppose Armstrong is right about identity if he is right about laws. Presumably an identity could happen to be invariant across possible worlds, without being necessary.