 Polger and Shapiro’s book confirms that multiple realization has of late come to be fought in the metaphysics of science literature. In keeping, they begin by offering an account of the ontological realization relation. And following the lead of their (Shapiro &amp; Polger 2012) paper (discussed in  §3.2  above) they use this account to scout the types of evidence and concerns relevant to deciding between realization physicalisms and identity theories. They restrict the scope of their arguments to actual proposed cases of realization, eschewing at least initially concern with philosophers’ imagined cases; they are solely concerned at the outset with multiple realization as an empirical hypothesis. They are also careful to point out that multiple realization is logically narrower than mere variation. Working directly with Putnam’s (1967) discussion, Polger and Shapiro articulate the “Basic Recipe” for finding occurrences of multiple realization: “Multiple realization occurs if and only if two (or more) systems perform the same function in different ways” (2016: 45). While this characterization requires further refinement, even it is enough to rule out some kinds of “sameness” that are not evidence for multiple realization, i.e., sameness of behavioral output. Two systems exhibiting the same behavior in the same circumstances, Polger and Shapiro insist, does not establish sameness of underlying psychological function generating that behavior. 