What is the connection between the mental and the physical? How are the mind and the body related? Is the mental realm reducible to, or identical with the material realm? Or is the mental perhaps subsisting in its own realm of reality, distinct from the physical world? Or maybe the mind is wholly superfluous to the body, a sort of illusion, completely void of any causal influence? Questions like these have been haunting philosophers for centuries.

 

In the current philosophy of mind, for the past forty or fifty years, one view on these issues has been ruling over the others: nonreductive physicalism. The appeal of this view is easy to understand, for it seems to offer us what we’ve always craved for: thoroughly physicalistic ontology seasoned with distinct, genuine and nonreducible mental features. How could that be? Because of multiple realization, the nonreductive physicalists reply: each mental state, or higher-level feature in general, is physically realized (this is the physicalism part), but not uniquely so. Rather, each mental state is multiply realizable, and hence not reducible to, or identical with, the physical basis that realizes it (this is the nonreductive part). So there are no separate minds, or souls, or mental substances hovering in their own ghostly realm of reality. What there is, are mental features, phenomena and processes, occurring in physical, bodily states. But the former are still distinct from the latter, the nonreductive physicalists claim, because the very same mental feature, phenomenon or process can be realized by different physical states.

 

In The Multiple Realization Book, Thomas Polger and Lawrence Shapiro offer the first book-length analysis of the multiple realization thesis. Given the relatively long history of the topic, and the central place it occupies in the fervent debates on reductionism, it is quite surprising that we have had to wait this long for a book focusing specifically on this issue. Multiple realization has of course featured in literally hundreds of books, and in many of them the thesis has occupied a pivotal role in the argumentation, but no book has been written with the sole, direct focus on the thesis (although both authors have themselves come close to doing that in their earlier books (Polger 2004; Shapiro 2004). So this is a warmly welcome publication indeed.

 

This is not a fan book, however. On the contrary: the authors are very critical of the multiple realization thesis, and their aim is to expose the loose thinking surrounding the topic. According to them, the philosophical community has been wrong in thinking that “multiple realization is obvious, omnipresent, and inevitable” (p. 168). What it is, at best, is a rare and highly special phenomenon, not robust and wide-spread enough to support thoroughly nonreductionist metaphysics. The currently received view in the philosophy of mind is wrong, the authors maintain.

 

Although the discussion in the book is much more nuanced, the basic problem with multiple realization can be summarized the following way. It is a simple truism that everything in the world is in a constant change, and variation is ubiquitous, and hence not all variation should count as multiple realization. What multiple realization actually requires is that something is constant — namely the thing that is being multiply realized. In other words, the proponents of the thesis are forced to walk a very fine line between variation and constancy; they would need to point to things that are both different and the same at the same time — things that are “differently the same” — to support the thesis. And these sorts of things, the authors claim, are much more difficult to find than people have thought. In a nutshell, and as suggested by this conceptual background, purported cases of multiple realization tend to dissolve either into cases where the different realizers get unified (when it is understood that they are not that different after all), and there is thus no multiplicity to ground the multiple realization anymore, or, conversely, into cases where the realizers are actually different, but so different that they tear the purportedly multiply realized kind apart, and there is no single, unified thing left to become multiply realized anymore.

 

Consider convergent evolution, for example. Appealing to evolutionary considerations has been one popular strategy in arguing for the multiple realization thesis. Isn’t biology full of perfectly natural examples of how similar traits can evolve and hence become differently realized? Birds got wings, but so do bats and insects — aren’t these all clear examples of different structures realizing the same function? Isn’t it rather obvious how different evolutionary pathways, independent of each other, can converge on the same result? Yes, but it is far from clear that all this amounts to multiple realization. For genuine multiple realization to take place, these traits would need to be, not only relevantly the same, but also relevantly different. And one can doubt that that’s really the case. They are all wings after all, and the evolution has converged on the same result, so why should we suppose that they are different then? In other words, why not just conclude that they are all the same thing, in the relevant sense, and there is no multiplicity that would ground genuine multiple realization. Sure, there are differences in them, but should we let that count? Houses made from timber and bricks are houses all the same, and simple differences in materials and manufacturing history do not make such houses multiply realized. Or, if we want to make an issue of the evolutionary and structural differences of wings, why not then divide this seemingly unified trait into separate traits (bird-wings, bat-wings, insect-wings)? They are different after all, so what makes us think they are the same?

 

The book contains a lot of data and convincing argumentation pointing to the conclusion that many apparent cases of multiple realization ultimately become dissolved in this way. What remains to be settled is the ramifications of this deconstruction to the received metaphysical view in the philosophy of science and the philosophy of mind. And this, in the last two chapters of the book, is where the story takes a strange turn. The main message of the book is that there is no such thing as multiple realization, at least not in any really significant sense, and that we should renounce the distinctness of the mental and the physical, and embrace the identity of the two. Whether you buy the arguments against multiple realization or not, there is little doubt that rejecting multiple realization and embracing identity theory creates a coherent whole. And what’s typically thought to result from this is the rejection of the idea of psychology as an autonomous discipline. Yet Polger & Shapiro hasten to resist such a conclusion.

 

The background to all this is in metaphysical considerations related to causal explanation. The reality of the mental and the autonomy of psychology have for some time been put into doubt by the exclusion argument (e.g. Kim 1998, 2005): if the mental is distinct from the physical, and the physical realm is causally complete (in the sense that every physical effect has a complete physical cause), then the mental seems to be left without a causal role to play. Now, it is clear that if one wants to save the causal efficacy of the mental, one can simply accept the idea that the mental and the physical are identical. However, that option is not available for a nonreductive physicalist, who is committed to keeping the two apart (by relying on the multiple realization thesis) (cf. Pernu 2014).

 

So we are now faced with the following two choices. You can either hold that the mental and the physical are distinct (via multiple realization), in which case you would also have grounds for holding psychology as an autonomous discipline, but you would be risking causal exclusion of the mental (and true significance of psychology with it). Or you can reject the distinctness assumption, and identify the mental with the physical, in which case you would have no trouble with explaining the causal efficacy of the former, but you would be losing the autonomy of psychology. “It’s a package deal” (p. 195), Polger & Shapiro admit — but still they want to resist it.

 

The position that Polger & Shapiro are putting themselves into is difficult to understand. It feels like they are trying to smuggle back in from the backdoor something that they have just escorted out from the front door: the very distinctness of the mental and the physical. How do they do that? By separating explanations from the things that are being explained: psychological causal explanations and neuroscientific causal can be distinct, they maintain, simply because “explanations can be distinct even if the phenomena that they explain are not” (p. 209). Doesn’t this take us back to square one?

 

It is very difficult — and nothing in the book makes it less so — to see how the view advocated by Polger & Shapiro could be made to stand any serious criticism. True, explanations always have an epistemic aspect to them, and different types of explanatory schemes carry information about the world in different ways, as Polger & Shapiro are eager to stress. Of course explanations can vary depending on to whom the explanation is offered, and for what purposes. But how does acknowledging this truism suddenly lift mental phenomena back up to their own autonomous realm of reality? We already knew that we utilize both psychological and neuroscientific explanations, and that we have good reasons to do so. But we also know that often the two are in a tension with each other, and that one appealing remedy to that is to renounce the idea that the two are distinct, and, consequently, the idea that psychology is autonomous in any philosophically interesting sense. This was the whole issue to begin with! Surely the scientific legitimacy of psychology was never in doubt; no one has been claiming that studying psychology is futile. What was at issue was the metaphysical autonomy of the enterprise, the question whether psychology postulates entities and powers that are fundamentally different to physical entities and powers. A negative answer to this question does not have to erode the epistemic usefulness of psychological explanations, but it does confer the physical view on the world a more important and fundamental role. It is clear that explanation of celestial mechanics amounts to a different thing to Einstein than to the man on the Clapham omnibus, and that both explanations can be useful in their own contexts. But at the same time it is equally clear that the former explanation is somehow better and more accurate — and indeed, when push comes to shove, that the former is right and the latter is wrong. The ontic aspects of explanations trump their epistemic aspects; there are no alternative facts, and there are no alternative explanations.

 

Polger & Shapiro fail to convince with their conclusions. However, they will attract supporters, no doubt, for people still want to be told how psychology is special. If you can do that — if you could do that — without compromising the identity of the mental and the physical you would be making many people very happy. This book will thus have no trouble finding its way into the hands of enthusiastic readers. And even if you would not buy the ultimate message of the book, it still contains plenty of valuable material and enlightening critical analyses of various aspects of the multiple realization thesis. This is an important book, and neither friends nor foes of multiple realization can afford to ignore it.

 

 

 

 

 

References

 

Kim, Jaegwon (1998). Mind in a Physical World: An Essay on the Mind-Body Problem and Mental Causation. Cambridge MA: MIT Press.

 

Kim, Jaegwon (2005). Physicalism, Or Something Near Enough. Princeton NJ: Princeton University Press.

 

Pernu, Tuomas K. (2014). “Causal exclusion and multiple realizations”. Topoi 33, p. 525-530.

 

Polger, Thomas W. (2004). Natural Minds. Cambridge, MA: MIT Press.

 

Shapiro, Lawrence A. (2004). The Mind Incarnate. Cambridge, MA: MIT Press.

 

 

 

© 2017 Tuomas K. Pernu

 

 

Dr Tuomas K. Pernu, Visiting Research Fellow,Research Associate in Philosophy & Medicine at King’s  College London