 Naturalism depends on a number of presuppositions which are open to debate. But even if these presuppositions are granted, the naturalistic project of looking to science for methodological guidance within philosophy faces a major difficulty, namely how to ‘read off’ from actual scientific practice what the underlying methodological principles are supposed to be. Burgess, for example, argues that what the patterns of scientific behavior show is not a concern with multiplying entities per se, but a concern more specifically with multiplying ‘causal mechanisms’ (Burgess 1998). And Sober considers the debate in psychology over psychological egoism versus motivational pluralism, arguing that the former theory postulates fewer types of ultimate desire but a larger number of causal beliefs, and hence that comparing the parsimony of these two theories depends on what is counted and how (Sober 2001, pp. 14–5). Some of the concerns raised in Sections 1 and 2 also reappear in this context; for example, how the world is sliced up into kinds effects the extent to which a given theory ‘multiplies’ kinds of entity. Justifying a particular way of slicing becomes more difficult once the epistemological naturalist leaves behind the a priori, metaphysical presuppositions of the rationalist approach.