Consider the human body, with everything in it, including internal and external organs and parts — the stomach, nerves and brain, arms, legs, eyes, and all the rest. Even with all this equipment, especially the sensory organs, it is surprising that we can consciously perceive things in the world that are far away from us. For example, I can open my eyes in the morning and see a cup of coffee waiting for me on the bedside table. There it is, a foot away, and I am not touching it, yet somehow it is making itself manifest to me. How does it happen that I see it? How does the visual system convey to my awareness or mind the image of the cup of coffee?
The answer is not particularly simple. Very roughly, the physical story is that light enters my eyes from the cup of coffee, and this light impinges on the two retinas at the backs of the eyes. Then, as we have learned from physiological science, the two retinas send electrical signals past the optic chiasm down the optic nerve. These signals are conveyed to the so-called visual cortex at the back of the brain. And then there is a sort of a miracle. The visual cortex becomes active, and I see the coffee cup. I am conscious of the cup, we might even say, though it is not clear what this means and how it differs from saying that I see the cup.
One minute there are just neurons firing away, and no image of the cup of coffee. The next, there it is; I see the cup of coffee, a foot away. How did my neurons contact me or my mind or consciousness, and stamp there the image of the cup of coffee for me?
It’s a mystery. That mystery is the mind-body problem.
Our mind-body problem is not just a difficulty about how the mind and body are related and how they affect one another. It is also a difficulty about how they can be related and how they can affect one another. Their characteristic properties are very different, like oil and water, which simply won’t mix, given what they are.
There is a very common view which states that the French philosopher René Descartes discovered, or invented, this problem in the 17th century. According to Descartes, matter is essentially spatial, and it has the characteristic properties of linear dimensionality. Things in space have a position, at least, and a height, a depth, and a length, or one or more of these. Mental entities, on the other hand, do not have these characteristics. We cannot say that a mind is a two-by-two-by-two-inch cube or a sphere with a two-inch radius, for example, located in a position in space inside the skull. This is not because it has some other shape in space, but because it is not characterized by space at all.
What is characteristic of a mind, Descartes claims, is that it is conscious, not that it has shape or consists of physical matter. Unlike the brain, which has physical characteristics and occupies space, it does not seem to make sense to attach spatial descriptions to it. In short, our bodies are certainly in space, and our minds are not, in the very straightforward sense that the assignation of linear dimensions and locations to them or to their contents and activities is unintelligible. That this straightforward test of physicality has survived all the philosophical changes of opinion since Descartes, almost unscathed, is remarkable.
This issue aroused considerable interest following the publication of Descartes’s 1641 treatise “Meditations on First Philosophy,” the first edition of which included both Objections to Descartes, written by a group of distinguished contemporaries, and the philosopher’s own Replies. Though we do find in the “Meditations” itself the distinction between mind and body, drawn very sharply by Descartes, in fact he makes no mention of our mind-body problem. Descartes is untroubled by the fact that, as he has described them, mind and matter are very different: One is spatial and the other not, and therefore one cannot act upon the other. Descartes himself writes in his Reply to one of the Objections:
The whole problem contained in such questions arises simply from a supposition that is false and cannot in any way be proved, namely that, if the soul and the body are two substances whose nature is different, this prevents them from being able to act on each other.
Descartes is surely right about this. The “nature” of a baked Alaska pudding, for instance, is very different from that of a human being, since one is a pudding and the other is a human being — but the two can “act on each other” without difficulty, for example when the human being consumes the baked Alaska pudding and the baked Alaska in return gives the human being a stomachache.
The difficulty, however, is not merely that mind and body are different. It is that they are different in such a way that their interaction is impossible because it involves a contradiction. It is the nature of bodies to be in space, and the nature of minds not to be in space, Descartes claims. For the two to interact, what is not in space must act on what is in space. Action on a body takes place at a position in space, however, where the body is. Apparently Descartes did not see this problem. It was, however, clearly stated by two of his critics, the philosophers Princess Elisabeth of Bohemia and Pierre Gassendi. They pointed out that if the soul is to affect the body, it must make contact with the body, and to do that it must be in space and have extension. In that case, the soul is physical, by Descartes’s own criterion.
In a letter dated May 1643, Princess Elisabeth wrote to Descartes,
I beg you to tell me how the human soul can determine the movement of the animal spirits in the body so as to perform voluntary acts—being as it is merely a conscious substance. For the determination of the movement seems always to come about from the moving body’s being propelled—to depend on the kind of impulse it gets from what it sets in motion, or again, on the nature and shape of this latter thing’s surface. Now the first two conditions involve contact, and the third involves that the impelling [thing] has extension; but you utterly exclude extension from your notion of soul, and contact seems to me incompatible with a thing’s being immaterial.
Propulsion and “the kind of impulse” that set the body in motion require contact, and “the nature and shape” of the surface of the site at which contact is made with the body require extension. We need two further clarifications to grasp this passage.
The first is that when Princess Elisabeth and Descartes mention “animal spirits” (the phrase is from the ancient Greek physician and philosopher Galen) they are writing about something that plays roughly the role of signals in the nerve fibers of modern physiology. For Descartes, the animal spirits were not spirits in the sense of ghostly apparitions, but part of a theory that claimed that muscles were moved by inflation with air, the so-called balloonist theory. The animal spirits were fine streams of air that inflated the muscles. (“Animal” does not mean the beasts here, but is an adjective derived from “anima,” the soul.)
The second clarification is that when Princess Elisabeth writes that “you utterly exclude extension from your notion of soul,” she is referring to the fact that Descartes defines mind and matter in such a way that the two are mutually exclusive. Mind is consciousness, which has no extension or spatial dimension, and matter is not conscious, since it is completely defined by its spatial dimensions and location. Since mind lacks a location and spatial dimensions, Elisabeth is arguing, it cannot make contact with matter. Here we have the mind-body problem going at full throttle.
Descartes himself did not yet have the mind-body problem; he had something that amounted to a solution to the problem. It was his critics who discovered the problem, right in Descartes’s solution to the problem, although it is also true that it was almost forced on them by Descartes’s sharp distinction between mind and body. The distinction involved the defining characteristics or “principal attributes,” as he called them, of mind and body, which are consciousness and extension.
Though Descartes was no doubt right that very different kinds of things can interact with one another, he was not right in his account of how such different things as mind and body do in fact interact. His proposal, in “The Passions of the Soul,” his final philosophical treatise, was that they interact through the pineal gland, which is, he writes, “the principal seat of the soul” and is moved this way and that by the soul so as to move the animal spirits or streams of air from the sacs next to it. He had his reasons for choosing this organ, as the pineal gland is small, light, not bilaterally doubled, and centrally located. Still, the whole idea is a nonstarter, because the pineal gland is as physical as any other part of the body. If there is a problem about how the mind can act on the body, the same problem will exist about how the mind can act on the pineal gland, even if there is a good story to tell about the hydraulics of the “pneumatic” (or nervous) system.
We have inherited the sharp distinction between mind and body, though not exactly in Descartes’s form, but we have not inherited Descartes’s solution to the mind-body problem. So we are left with the problem, minus a solution. We see that the experiences we have, such as experiences of color, are indeed very different from the electromagnetic radiation that ultimately produces them, or from the activity of the neurons in the brain. We are bound to wonder how the uncolored radiation can produce the color, even if its effects can be followed as far as the neurons in the visual cortex. In other words, we make a sharp distinction between physics and physiology on the one hand, and psychology on the other, without a principled way to connect them. Physics consists of a set of concepts that includes mass, velocity, electron, wave, and so on, but does not include the concepts red, yellow, black, and the like. Physiology includes the concepts neuron, glial cell, visual cortex, and so on, but does not include the concept of color. In the framework of current scientific theory, “red” is a psychological term, not a physical one. Then our problem can be very generally described as the difficulty of describing the relationship between the physical and the psychological, since, as Princess Elisabeth and Gassendi realized, they possess no common relating terms.
Was there really no mind-body problem before Descartes and his debate with his critics in 1641? Of course, long before Descartes, philosophers and religious thinkers had spoken about the body and the mind or soul, and their relationship. Plato, for example, wrote a fascinating dialogue, the Phaedo, which contains arguments for the survival of the soul after death, and for its immortality. Yet the exact sense in which the soul or mind is able to be “in” the body, and also to leave it, is apparently not something that presented itself to Plato as a problem in its own right. His interest is in the fact that the soul survives death, not how, or in what sense it can be in the body. The same is true of religious thinkers. Their concern is for the human being, and perhaps for the welfare of the body, but mainly for the welfare and future of the human soul. They do not formulate a problem with the technical precision that was forced on Princess Elisabeth and Gassendi by Descartes’s neatly formulated dualism.
Something important clearly had changed in our intellectual orientation during the mid-17th century. Mechanical explanations had become the order of the day, such as Descartes’s balloonist explanation of the nervous system, and these explanations left unanswered the question of what should be said about the human mind and human consciousness from the physical and mechanical point of view.
What happens, if anything, for example, when we decide to do even such a simple thing as to lift up a cup and take a sip of coffee? The arm moves, but it is difficult to see how the thought or desire could make that happen. It is as though a ghost were to try to lift up a coffee cup. Its ghostly arm would, one supposes, simply pass through the cup without affecting it and without being able to cause it or the physical arm to go up in the air.
It would be no less remarkable if merely by thinking about it from a few feet away we could cause an ATM to dispense cash. It is no use insisting that our minds are after all not physically connected to the ATM, and that is why it is impossible to affect the ATM’s output — for there is no sense in which they are physically connected to our bodies. Our minds are not physically connected to our bodies! How could they be, if they are nonphysical? That is the point whose importance Princess Elisabeth and Gassendi saw more clearly than anyone had before them, including Descartes himself.
Jonathan Westphal is a Permanent Member of the Senior Common Room at University College, Oxford, and the author of “The Mind-Body Problem,” from which this article is adapted.