This short book is on a topic familiar to all students of philosophy, and a subtopic familiar at least to specialists in the philosophy of mind, but it is written from an uncommon point of view. The topic is the venerable mind-body problem, now almost four centuries old. The subtopic is the much-discussed thesis of the supervenience of the mental on the physical, the claim that minds could not have been different without bodies having been different. The point of view from which these issues are approached is that of a logician, one who has come to the question from modal logic rather than philosophical psychology. The overarching theses of the work are two, one in the foreground, the other in the background.
First, I argue that there are several reasons to suspect that the supervenience thesis deserves less attention than it has received, and that the real division of philosophical importance over mind and body lies elsewhere. It lies in the difference between those who — whether they call themselves dualists or physicalists or neither, and whether they oppose or support or are indifferent to the supervenience claim — recognize that there is something that a physicalistic account of the world leaves out, and those who are in denial about such omissions.
Second, I try to demonstrate that attention to the historical source of our problem remains of continuing relevance. Only if we remember that something was deliberately left out of the conceptual range of physical science when the subject was launched four hundred years ago can we fully appreciate the fact that it is still left out today.
To keep the work to a manageable size I have set aside many subtopics with no more than an oblique reference to external sources. Contrasting views that are amply and ably represented in the literature I generally no more than summarize, and not always impartially, in overview. I have also tried to avoid all unnecessary technical jargon. Above all, I have avoided any elaborate apparatus of possible worlds. I have used such apparatus in some of my work on other topics, but I do not find it useful here.
This book is a kind of sequel to my Saul Kripke: Puzzles and Mysteries. As such it shares some of the intellectual debts of that earlier book, but the debts to other scholars connected with this one are mainly to their writings, rather than to direct personal contact. The authors whose works have been most important to me are listed at the end of the first section of the first chapter of the text, and the whole list need not be repeated here. But I do have some more direct personal debts that it is a pleasure to acknowledge in this place, to Ned Block, David Chalmers, Daniel Garber, Stevan Harnad, Frank Jackson, Mark Johnston, Oliver Marshall, Thomas Nagel, Neil Tennant, Alexander Williams, Jack Woods, and doubtless others I am forgetting. To some I owe inspiring conversations before undertaking this project, to others opportunities to present work in progress, and to yet others comments at greater or lesser length on an earlier draft of this work. Besides these figures I should mention in gratitude my late colleague David Lewis. It was a remark of his, to the effect that all positions in contemporary philosophy of mind would probably have been considered forms of materialism by Victorians, that first set me off thinking about these topics.
I Descartes and Dualism
The problem of the relation of mind to body, or mental to physical, as it is discussed and debated among philosophers and psychologists today, goes back some four centuries to René Descartes, at the very beginning of modern philosophy and modern science. To understand the terms of the problem, we need to take a backward look at its source.
Mental versus Physical and Possible versus Necessary
The issue as we know it retains from its Cartesian origins a couple of features that distinguish our mind-body problem from what have been viewed as similar or related problems in other intellectual and cultural traditions: first, a focus on the brain as the part of the body most relevant to the mind; and second, the mathematical character of the science that informs the conception of the physical that is contrasted with the mental. The topic of this book will be a third feature that the contemporary mind-body problem shares with that of four hundred years ago, the curious entanglement of the issue mentality and physicality with the notions of necessity and possibility.
Descartes’ position as a whole has few if any defenders today, but there are present-day defenders of two of his main theses: first, that mind and body are a duo, two distinct things; and second, that there is no necessary connection between the two. What exactly this latter claim means will emerge only gradually, but at the outset it may be noted that since the first law of the logic of these notions is that not necessarily is equivalent to possibly not, denial of the necessity of a connection is equivalent to affirmation of the possibility of a disconnection.
This circumstance accounts for another curious feature of recent discussion of mind and body, namely, that much of it is concerned with ghosts and zombies, minds disconnected from bodies and bodies disconnected from minds. Perhaps a century or so ago, in the days of William James and the Society for Psychical Research, there may have been philosophers or psychologists who seriously endorsed claims of the actual existence of such beings; but not so to any noticeable degree today. The fact that such beings are nonetheless mentioned in the contemporary literature is indicative of how today, as in the days of Descartes, discussion still, or again, often concerns possibility rather than actuality.
My aim in this book will be to argue, largely from a point of view otherwise sympathetic to many Cartesian positions, for disentanglement of the mind-body issue from issues about possibility and necessity. To make a case for disentanglement is, however, only my final destination, and I attach less importance to arriving at it than to journeying towards it, enlarging our understanding of the mental and physical, and of the necessary and possible, along the way. I know no better place to begin that journey than with Descartes, and the remainder of this introductory chapter will be devoted to contrasting Descartes’ position with the kind of contemporary position that I especially wish to consider.
For reasons of space among others, in making my comparisons I will deal with a picture of Descartes simplified in places to a caricature; and what I contrast with him will not be a gallery of realistic portraits of his most distinguished present-day heirs, either. What I contrast with Descartes will be a simplified composite sketch, which to be frank will be partly a self-portrait, of a kind of contemporary philosopher of mind who has been influenced by key works of Ned Block, David Chalmers, Stevan Harnad, Frank Jackson, Saul Kripke, Joseph Levine, Colin McGinn, Thomas Nagel, John Searle, and other such thinkers, without becoming an entirely faithful disciple of any one of them, not that anyone could be an entirely faithful disciple of more than one.
In the process of carrying out my comparison, I will gradually narrow my focus in from the vast topic of the relation of mind and body, which there can be no hope of treating adequately in a book of modest size, towards a more manageable subtopic. Various other subtopics will be set aside one by one, each the subject of a large literature of its own, though I will mention no more in any case than a bit of the jargon characteristically used in that literature.
These mentions will appear inside double quotation marks (and indeed inside such “scare-quotes” inside parentheses), where otherwise, as in much of the linguistics literature, I use italics, rather than quotation marks when I wish to mention rather than use a word — and dashes rather than parentheses to set off material from the main text — while, as in some mathematics textbooks, I use boldface rather than italics, to highlight a word that is going to be used repeatedly as a term of art when it first appears so used, where I give at least an informal explanation, if not a rigorous definition, of the special sense of term. Displayed propositions are numbered if I plan to refer back to them, but the numbering begins anew from (1) in each new section of text.
The scare-quoting is not just a stylistic tic, and not just a signal that previous familiarity on the part of the reader with the expression quoted is not being assumed. It plays a more important role: if the scare-quoted expression(s) is/are typed exactly as quoted into the most common search engine, together with “stanford encyclopedia” as an additional search term, the reader interested in the issue I am leaving aside will be led at once to the latest version — while whatever version if any is listed in the references here can only be that current at the time of this writing — of one or more generally excellent and frequently-updated on line survey expository articles touching on whatever issue is in question, with bibliographies leading on to yet further material. The reader who carries out this exercise every time a bit of technical jargon is parenthetically mentioned will probably never make it to the end of this first chapter, but will certainly learn a great deal about the philosophy of mind. I hope some readers will choose to defer enough of these intellectual excursions to stay with me to the chapter’s end, for many as are the subissues I will be setting aside, there will still be many left.
Descartes’ Context and Ours
Though Descartes and the kind of contemporary philosopher I want to consider are both dualists, holding that the mental and physical are distinct, with no connection between them necessary, or equivalently, with disconnection between them possible, and though both are thus much concerned with modality, the category to which the notions of necessity and possibility belong, still there are inevitably many differences between philosophers four hundred years apart. Let me begin the process of comparison with consideration of the overall intellectual situation of Descartes on the one hand and of my hypothetical contemporary dualist, or indeed any present-day philosopher of mind, on the other.
At the outset note must be taken of some major differences as regards the two features of scientific thought about mind and body already mentioned as Descartes’ most enduring legacy in this area: the neuralor encephalic orientation, or the emphasis on the nerves or brain, and the mathematicization of physical science, its concentration on what is quantitative or measurable. The main point is that while today both are so much taken for granted that it takes some effort for us even to see them as substantive assumptions, in Descartes’ day both were contentious novelties.
Descartes’ advocacy of them was part of a comprehensive campaign against the still powerful and baleful influence of Aristotle and his Scholastic disciples in physiology, physics, and beyond. In physiology Descartes championed not only concentration on the brain as the bodily organ most relevant to mental life, but also on the heart as the key organ of a system that circulated fluids around the body. On both points he had to oppose the authority of Aristotle, who took the heart to be the primary organ for thinking, and the brain to be an organ for cooling the blood. The new approach to physics that Descartes favored involved not only mathematicization but even more importantly avoidance of Scholastic-style pseudo-explanations of events in terms of supposed purposes or goals (“Aristotelian teleology”). Such would-be explanations seem to involve attributing human-like motives — and what could be less mathematically representable than a human-like motive? — to inanimate objects, as when heavy objects falling towards the center of the earth were said to be seeking their natural place, as if they were travelers impatient to get home.
Closely related with the main point that Descartes’ ideas in physiology and physics were new and disputed is the fact that there was very little established science along the lines he advocated for him to make use of in his philosophizing. He was thus obliged to fall back on his own resources, carrying out his own investigations in various domains. It is hardly surprising that, though he scored some partial successes, among other things with phantom limb pain in amputees and with the refraction of light, many of his efforts were unsuccessful. Having correctly focused on the brain, he notoriously then went wrong by focusing on the pineal gland, the epiphysis cerebri or conarium of anatomists, as supposedly the crucial part of the brain. Having developed his gloriously successful application of algebra to geometry, he then went ingloriously wrong in trying to apply such mathematics to physics; so that today, while Cartesian coordinates are taught to middle school students the world over, Cartesian vortices are forgotten by all but historians of the arcana of seventeenth-century protoscience.
As regards the brain, the present-day philosopher of mind faces the opposite of Descartes’ situation: there is not too little scientific information but too much. There are so many new neurophysiological and neuropsychological results arriving so rapidly that it is a large task to try to pick out from the vast quantity of scientifically interesting material being generated the small fraction that may be philosophically relevant.
As regards mathematicization, the present-day philosopher of mind is also in a different situation. Today it goes without saying that physics is expected to be formulated mathematically, that chemistry is being slowly absorbed by physics, that biology is being slowly absorbed by chemistry, and so on. The fact that the original project of mathematicization was controversial because it deliberately excluded certain factors from consideration may have been lost sight of, as it never could be for Descartes.
That is an issue to which we will have to return later. For the present, the next thing that needs noticing is that in proportion as the influence of science was smaller four centuries ago, the influence of religion was greater. To be sure, a distinctive domain of philosophy, which unlike theology may make no appeal to faith, revelation, tradition, authority, or the like, but only to reasoned argument and the evidence of experience, had been recognized in principle since the middle ages. But in practice very often argumentation that passed for philosophical was directed towards offering proofs of conclusions already dictated in advance by theology; and Descartes’ work on mind and body, as he himself presents it, is a case in point. Descartes professed a faith according to which the human mind or soul survives death and exists in a disembodied state pending eventual resurrection and re embodiment; and he maintained that this doctrine of survival after death is not merely something true and known by faith, but also something that could and should be proved philosophically, as part of a defense of that faith.
By contrast, contemporary philosophical writers on the mind-body question virtually never announce overt theological agendas behind their philosophical theses and arguments. At most there may remain a suspicion in some quarters of some kind of implicit or covert connection between dualism and theism, and between antidualism and atheism. Such suspicions, even or especially when they have no basis in fact, may engender tendencies on the part of the secularly oriented to be more easily struck by weaknesses in the arguments of the dualist side than by weaknesses in the counterarguments of the antidualist side, and to engender opposite tendencies on the part of those oppositely oriented; but all this remains deep in the background, and seldom openly discussed.
Zombies, the Zombie World, and Supervenience
Now despite his claim that human survival after death can be proved philosophically, in his first and best-known philosophical treatise, the Meditations on First Philosophy, what Descartes offers is not a purported philosophical proof that the human mind actually does survive death, but only that it possibly could. This is precisely where modality comes in for him. Admittedly, the fact that his conclusion is a modal one is not conspicuously advertised.
On the contrary, the publisher’s title page to the first, Latin, edition erroneously claims that the book offers a proof of the immortality of the soul, though the text contains nothing that even purports to be such a proof. And the corrected title page to the second, French, edition claims only that the book contains a proof of the real distinction between the human soul and body, without mentioning that the possibility of immortality is supposed to follow (that being implicit in how Descartes distinguishes a “real distinction” from a “rational distinction”). But conspicuously advertised or not, the modal aspect is there.
This brings us to a crucial difference between seventeenth-century and twenty-first-century dualism in the nature of their modal claims. The possibility that concerned Descartes was, in line with his religious commitments, that of disembodied souls, while the contemporary dualist is more concerned with the possibility of the opposite combination, soulless bodies. Other differences and asymmetries are connected with this one. Descartes is interested in disembodied souls and the possibility of an afterlife for their own sake, so to speak. Contemporary dualists seem to be interested in soulless bodies mainly for the sake of what their alleged possibility tells us about the nature of physical science and the prospects for mental science.
Above all, Descartes claimed that minds could exist without bodies, but did not claim that minds could exist just as they are, with all their mental properties, in the absence of bodies. Rather, he held that some features of mental life were more independent of embodiment than others. By contrast, contemporary dualism does maintain not merely that bodies could exist without minds, but that bodies could exist just as they are, with all their physical properties, without minds.
For while any claim at all about the existence of bodiless minds is a substantive one, a mere claim of the existence of mindless bodies is a triviality, since there indisputably do exist some bodies without minds, namely, those we call corpses or cadavers. There is a broader sense of body in which we speak, for instance, of heavenly bodies and fruiting bodies, and taken in this broader sense, meteors, which never were alive, and mushrooms, which still are alive, join mummies, which once were but no longer are alive, as examples of mindless bodies. What the contemporary dualist maintains is that, though there do not actually exist any, there possibly could have existed mindless bodies that, unlike mummies, let alone meteors or mushrooms, moved around just as we do when we act in pursuit of our preferences and desires, and emitted sounds just as we do when we speak in expression of our opinions and beliefs.
Such are zombies, in the philosophical sense popularized by Chalmers, as opposed to a folkloristic or horror-movie sense. The contemporary dualist indeed maintains that there could have existed nothing but zombies, so to speak, that the world could have been physically just as it actually is, without there being any mental activity at all in it.
Philosophers nowadays often, instead of saying the world could possibly have been thus-and-so, will say there is a possible world that is thus-and-so, and instead of saying the world is actually thus-and-so, will say the actual world is thus-and-so. This possible world talk was popularized by Kripke, who later retreated from it somewhat, as having a potential to create confusions if taken too seriously. Like Kripke, I will use it here only as a picturesque manner of speaking. So speaking, the dualist hypothesis is that of a zombie world, a possible world that is just like the actual world physically, but is devoid of mental life.
The characteristic contemporary antidualist claim is one that strongly contradicts this hypothesis: the thesis of the so-called supervenience of the mental on the physical. Here X is said to superveneon Y if some difference in Y would be necessary for any difference in X to be possible. To make somewhat plainer how necessary and possible are being understood here (pending future elaboration on “metaphysical modality”), to claim supervenience of the mental on the physical amounts to claiming that the mental could not have different from how it is, without the physical having been in some way different from how it is; or equivalently, that in order for the mental to have been different, the physical would have to have been different. Or at least, this will do for the moment.
Being wholly absent is the most extreme way of being different, so the supervenience thesis is stronger than denial of the zombie world hypothesis, though generally those who deny that hypothesis do so by subscribing to supervenience. I said earlier that the topic of this book will be the entanglement of the mental with the modal; I can now say more specifically that the topic will be the supervenience thesis.
Kinds of Minds
Further differences between Cartesian and contemporary thinking on the mind-body problem pertain to what portion of that which I have heretofore been nebulously calling the mental is of central concern. First under this heading comes the question of whose mental life is at issue. A half-dozen types at least of real or imaginary minds have been considered over the past four centuries, with considerable disagreement as to which are real and which are imaginary: