In our last post of the week we will discuss some of the broader implications of our view of multiple realization. But first we want to put the arguments in The Multiple Realization Book into the context of our earlier individual and joint work, in part to make clear the scope or ambition of our conclusions.
In Larry’s first book, The Mind Incarnate (2004), he explicitly restricts his attention to actual multiple realization, rather than multiple realizability. He contrasts the thesis that psychological states are multiply realized—the multiple realization thesis—(MRT) with the thesis that there are strong constraints on realization—the mental constraint thesis (MCT). He goes on to argue that the mental constraint thesis has better support, with careful attention to scientific evidence.
Tom’s first book, Natural Minds (2004), he defends the identity theory against various arguments including arguments from multiple realization and multiple realizability. The approach in that book relies on carefully distinguishing various theses concerning realization and functionalism and then arguing that there is no single notion of realization that vindicates functionalism in a way that makes it superior to the identity theory. The upshot is that the identity theory is a live option as a view of the metaphysics of minds. Consequently the view is candid about its modal commitments, and the argument applied to both possible multiple realization (multiple realizability) as well actual multiple realization. But in contrast to Larry’s work, Tom’s was a more traditional philosophical approach that relied very little on actual scientific evidence.
Our collaboration began after these books were published, and the initial steps involved making explicit how the distinctions made by each of us individually can be combined into the basis of a consistent approach to thinking about multiple realization. Really, we were just combining our toolboxes. For a variety of reasons, Tom’s approach to thinking about multiple realization was pushed and pulled more toward that of philosophy of science, and he came to fully endorse Larry’s emphasis on evidence. But at the same time, Tom pushed Larry to drop his empiricist anxieties about modal claims and embrace ontological commitments—not least of which, in a published review of Larry’s book. During the time period the subdiscipline of the “metaphysics of science” started to get some recognition, carve out space at conferences, and ultimately get its own society, the Society for the Metaphysics of Science. The Multiple Realization Book consolidates our approaches.
Title aside, The Multiple Realization Book is about both multiple realization and multiple realizability. An important part of our approach is to insist that modal claims about multiple realization are not sui generis or immune from scientific evidence. In particular, the premise that psychological states are multiply realizable is not an easy and evidence-impenetrable back-up for the premise that mental states are actually multiply realized. If there isn’t evidence of actual multiple realization, then it should be more demanding to show that multiple realization is possible—not less demanding.
We recount this developmental story in order to emphasize the scope of our ambitions. Janet Levin comments, her very kind review of The Multiple Realization Book, that “Polger and Shapiro state that their goal is to rebut the empirical arguments for multiple realization…. They are not interested in addressing the logical or conceptual arguments against the type-identity theory. As they put it (p. 57), ‘stories about Commander Data are not data’.” This is mostly right, but could be misleading. It’s true that we focus on empirical arguments, those for which data are relevant; and it’s true that we are somewhat dismissive of conceivability arguments. But we are not just dividing and conquering—empirical arguments now, conceptual arguments some other day. Nor do we think that empirical arguments are relevant to multiple realization and conceptual arguments relevant to multiple realizability.
Our view, rather, is that all arguments for multiple realizability, whether they use the premise of actual multiple realization or go directly to the modal possibility—to multiple realizability—are subject to evaluation in terms of evidence. Evidence is relevant because the choice between accounts of the nature of minds—the metaphysical nature of minds—is a question about the best overall theory (i.e., model, explanation, etc.) of mental phenomena in our world, and evidence is always relevant to theory choice.
Viewing the choice of theories of mind as broadly empirical in this way is not just a matter of taste. As naturalists—something like “natural born naturalists” a la Penelope Maddy’s “Second Philosopher”—we don’t have any other criteria we could use. Multiple realization, to do the job we assign it—following Putnam, Fodor, Kitcher, and many others—has to be a feature of the world, a fact about the kinds of variation that we can find in nature. There is no shortcut to facts about the world that is immune to evidence, i.e., immune to facts about the world. So even if our concepts are “functional” so that they take multiple semantic values (per David Lewis’s original use of the expression, “multiply realized”), this will not answer the question of whether the kinds of variation in the world (however denoted) actually or possibly fit the pattern of multiple realization. That’s a question that requires empirical evidence.
So while it’s true that we “are not interested in addressing the logical or conceptual arguments against the type-identity theory,” that should not be mistaken for restricting our ambitions to the actual or yielding the modal claims to thought experiments. The Multiple Realization Book argues that there is insufficient reason to think that psychological states are multiply realized—and that there is insufficient reason to think that they are multiply realizable, either. We have a principled reason for thinking that arguments that claim to be in principle immune from evidential evaluation are irrelevant, because we do not recognize any special epistemic route to modal facts about the world could be that is invulnerable to evidence about how the world is. (This is not to say that every interesting question is settled by scientific evidence, either. That is a question for another day, but we recognize that sometimes empirical evidence makes little difference, as the second factor does in Gillian Russell’s analogy of multiplying by zero.)