What is it to say that something is multiply realized? More precisely, and perhaps even more importantly, what is it to say that something is multiply realizable? In The Multiple Realization Book, the authors argue that the concept of multiple realization—despite enjoying popular use in philosophy of mind, especially for those thinkers who take a functionalist approach—fails to find adequate empirical support. In short, despite the insistence by functionalist proponents of philosophy of mind, such as Hilary Putnam, that minds are multiply realizable, Thomas Polger and Lawrence Shapiro urge thinkers to reconsider this assumption. Suspending the assumption of multiple realization is warranted particularly in light of empirical evidence that demonstrates that the question of multiple realizability is still an open one.
With René Descartes’ separation of mind and body, scientists and philosophers alike have sought to situate mind within a physical world. Specifically, thinkers have sought to create a theory of mind that meets four desiderata for mental/psychological states: realism, causal efficacy, generality, and psychological explanation. Briefly, adequate theories of mind must maintain the reality of mental states, must acknowledge the causal role mental states have (they are not a mere epiphenomenon), must account for the cross-species generality of mental states, and must allow for mental states to figure in legitimate psychological explanations. Consider that although Cartesian dualism may satisfy the realist and generality requirements, it fails to ascribe any causal power to mental states nor would one be able to invoke mental states when giving a psychological explanation.
Accepting the four desiderata outlined above, Polger and Shapiro focus on two contemporary theories of mind: the realization theory and the brain state theory (hereafter, the identity theory). The authors endeavour to demonstrate over the course of this book that “the best overall model of psychological and brain sciences is one that includes substantial and important explanatory identifications of psychological and cognitive process kinds with neuroscientific process kinds” (35). They are motivated to defend the identity theory against the realization theory primarily because of the lack of a philosophically rigorous investigation of multiple realization. Further, once one conducts such an investigation, the authors argue that there is no strong empirical data to suggest that mental or psychological states are multiply realized, or even multiply realizable.
The popularity of realization theories, which insist on the multiple realizability of mental states, stems from their satisfaction of the four desiderata outlined above to a higher degree than identity theories, specifically in regards to the criterion of generality. As Polger and Shapiro note, realization theories were preferred over identity theories primarily because it was argued that “no single reduction to a particular realizer would match the potential Generality of a functionalist or realization theory—at best we [proponents of identity theory] can get local or species-specific reduction” (38). Further, attempts by identity theorists to match the purported generality of realization theories often sacrifice simplicity for generality, with the result being the same: “functionalist or realization theories are taken to have the upper hand; and either way, the advantage stems from the fact of multiple realization” (38).
Yet the question remains as to what counts as a realization relation and whether such a realization is one among others such that there is a genuine case of multiple realizability. Indeed the strength of the realization theory hinges on the existence of such relations. Polger and Shapiro therefore formulate an “Official Recipe” by which we can delineate between multiple realizability and mere realization, and apply this recipe to commonly cited examples of evidence for the existence of multiple realization (67). The insistence on this methodology is a common theme throughout the book, but especially in Part II when Polger and Shapiro examine direct and indirect evidence for multiple realization. Moreover, given that “both functionalism and the identity theory were intended as empirical hypotheses,” Polger and Shapiro make liberal and extensive use of relatively complex phenomena in neuroscience and biology to persuade readers that, contrary to what a realization theorist might expect, genuine cases of multiple realizability appear to be unsupported and uncommon (83).
Beyond investigating realization theory and its purported success as a more general theory of mind, Polger and Shapiro address criticisms levelled at identity theory that aim to undermine its satisfaction of the other desiderata listed above, namely generality and explanatory value. They conclude that realization theories have no advantage in terms of generality over identity theories in light of the fact that the “actual practices of the cognitive and psychological sciences are compatible with the mind-brain identity theories” and that, moreover, identity theories are compatible with the autonomy of psychology (176). Ultimately Polger and Shapiro believe that they have demonstrated that identity theories of mind ought to be preferred over functionalist theories of mind, given that multiple realizability—a doctrine central to functionalism—is neither as ubiquitous nor as easily empirically verified as was previously thought.
