Multiple realization and the commensurability of taxonomies

The past two decades have witnessed a revival of interest in multiple realization and multiply realized kinds. Bechtel and Mundale’s (Philos Sci 66(2):175–207, 1999) illuminating discussion of the subject must no doubt be credited with having generated much of this renewed interest. Among other virtues, their paper expresses what seems to be an important insight about multiple realization: that unless we keep a consistent grain across realized and realizing kinds, claims alleging the multiple realization of psychological kinds are vulnerable to refutation. In this paper I argue that, intuitions notwithstanding, the terms of their recommendation make it impossible to follow, while also misleadingly insinuating that its application virtually guarantees mind-brain identity. Instead of a matching of grains, what multiple realization really requires is a principled method for adjudicating upon differences between tokens. Shapiro’s (J Philos 97(12):635–654, 2000) work on multiple realization can be understood as an attempt to adumbrate just such a method. While his “causal relevance” criterion can easily be mistaken for Bechtel and Mundale’s grain requirement, my analysis reveals exactly where and why these two tests diverge.