Multiple realization and compositional variation

It has often been thought that compositional variation across systems that are similar from the point of view of the special sciences provides a key point in favor of the multiple realization of special science kinds and in turn the broadly nonreductive consequences often thought to follow from multiple realization. Yet in a series of articles, and culminating in The Multiple Realization Book, Tom Polger and Larry Shapiro argue that an account of multiple realization demanding enough to yield such nonreductive consequences implies that compositional variation is far less significant for the multiple realization of special science kinds than many have supposed. I argue, in contrast, that even on this demanding account, lower-level compositional variation may frequently support the multiple realization of special science kinds across the systems of interest, and that there is a good explanation for where Polger and Shapiro go wrong in drawing the contrary conclusion. I consider but reject Carl Gillett’s claim that different views about the significance of compositional variation for multiple realization phenomena should be traced to implicit disagreement about the metaphysics of realization.