The sciences of brain pathology have had a profound effect on the philosophy of mind. Clinical cases, like those detailed in Oliver Sacks's The Man Who Mistook His Wife for a Hat, reinforce the dependency of the mind on the brain. It's not merely that the cases show that the mind is dependent on the brain in some general way, but that the pathologies reveal deep and specific cognitive dependencies. There is one woman in Sacks's book, for instance, who has lost the concept of "left"; she can see and process the right side of things, but the left side of things disappears for her completely. This isn't merely a visual problem. The concept of left itself has no meaning for her. An intelligent woman, she has come up with ingenious ways of overcoming her handicap, like spinning clockwise 360 degrees to reach the left side of something.
The deep and thorough cognitive dependencies the pathologies reveal has led many to draw the philosophical conclusion that the complete dependency of the mind on the brain has been conclusively demonstrated. Although much about the mind remains mysterious, the thinking goes, one thing we know for sure right now is that there are no properties of the mind that are not ultimately traceable to the brain.
I think this conclusion is premature and, more importantly, unphilosophical. A philosophical view is necessarily a comprehensive view; in fact what distinguishes the philosophical view from others is that it is the most comprehensive view possible. As Josef Pieper has written, what the philosopher most fears is not that his conclusions lack rigor, but that his conclusions leave something out of account. So what we must hold out for in a philosophy of mind is a philosophy that leaves nothing about the mind out of account.
And this is the danger in drawing philosophical conclusions from brain pathologies. Brain pathologies reveal what the mind can't do if the brain has been injured. What we most want from a philosophy of mind, however, is an account of what the mind can do in its most profound manifestations. So it isn't the diseased mind that should concern us most in philosophy, but the healthy mind in its most profound manifestations.
Suppose, for example, that not knowing anything about cars, we wish to develop a "philosophy of the automobile." We visit a mechanic and take a tour through his garage - a tour of "car pathology." He shows us all the way cars can fail; there are cars with bald tires, broken water pumps, no batteries, worn out brakes, etc. He tells us that without the brakes a car won't be able to stop, without the battery it won't be able to start, and so on. We even get a demonstration of a car with bad steering that can only turn to the right. All it does is drive in a circle.
The tour is informative and valuable, but of limited use in developing our philosophy of the car. What we really need to know is what a car in good working order can do. How far can it go? Can it go across the county or across the country? What sort of terrain can it navigate? Are there things it just can't cross? How robust is the car to bumps in the road? What, finally, are all the things people can do with a car?
There are analogous questions that should be at the heart of the philosophy of the mind. Just what can the healthy mind do? Can it "cross the country" and know being as the classical philosophers say? Or can it get no further than the county line, i.e. at most put a construction on sense impressions, as is the typical inclination of modern philosophers? Can the mind construct and perform an empirical science that gives it true knowledge of reality? Just what does it imply about the mind that it can perform science? It is these questions that are decisive for the philosophy of mind.
In the end, we don't really need to know all the ways an automobile can fail to develop a philosophy of the automobile. Neither are the most important facts about the mind the facts of mind pathology. The important facts are at the other end, and concern what the mind can do in its most healthy and profound modes. The theory of the immaterial mind, the prime target of the modern philosophy of mind and the philosophy it is believed is decisively refuted by the clinical pathology of the mind, was always based on what the mind can most profoundly do. The Thomistic theory of the immaterial intellect, for example, is based on the argument that the intellect cannot know being unless it is substantially immaterial (Note that I used the word intellect. The Thomistic theory does not demand that all that we call "the mind" must be immaterial, but that being can only be truly known through an immaterial substance, so the mind, since it knows being, must have an immaterial component - the intellect.) Now either the mind can know being or it can't. If it can't, then the Thomistic theory of the mind is superfluous, since it offers an account of a power that doesn't exist. If it can, then the Thomistic theory of the mind is one way to account for that fact, and the question is whether it is superior to rivals. It is a non sequitur to suppose that, since an unhealthy brain causes cognitive problems, that therefore the mind in its deepest acts can be explained by the physical brain. The question is whether a healthy brain (i.e. a material organ) is capable of knowing being on its own.