Descartes himself writes in his Reply to one of the Objections: 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. 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. 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. That this straightforward test of physicality has survived all the philosophical changes of opinion since Descartes, almost unscathed, is remarkable. 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. Unlike the brain, which has physical characteristics and occupies space, it does not seem to make sense to attach spatial descriptions to it. What is characteristic of a mind, Descartes claims, is that it is conscious, not that it has shape or consists of physical matter. It is that they are different in such a way that their interaction is impossible. The difficulty is not merely that mind and body are different. This is not because it has some other shape in space, but because it is not characterized by space at all. 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. Mental entities, on the other hand, do not have these characteristics. Things in space have a position, at least, and a height, a depth, and a length, or one or more of these. According to Descartes, matter is essentially spatial, and it has the characteristic properties of linear dimensionality. There is a very common view which states that the French philosopher René Descartes discovered, or invented, this problem in the 17th century. Their characteristic properties are very different, like oil and water, which simply won’t mix, given what they are. It is also a difficulty about how they can be related and how they can affect one another. Our mind-body problem is not just a difficulty about how the mind and body are related and how they affect one another. How did my neurons contact me or my mind or consciousness, and stamp there the image of the cup of coffee for me? The next, there it is I see the cup of coffee, a foot away. One minute there are just neurons firing away, and no image of the cup of coffee. 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. The visual cortex becomes active, and I see the coffee cup. These signals are conveyed to the so-called visual cortex at the back of the brain. Then, as we have learned from physiological science, the two retinas send electrical signals past the optic chiasm down the optic nerve. 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. This article is adapted from Jonathan Westphal’s book “ The Mind-Body Problem,” part of the MIT Press Essential Knowledge series.
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