We have a conception of at least two different kinds of things that exist in the world, mental and physical. Here are a few examples of members of each category:
Relatively speaking, these categories seem clear and unproblematic. At least, it's clear that all of the things I have listed as mental are mental, and all the things I have listed as physical are clearly physical; and any of us could generate a long list of additional clear cases of physical things and clear cases of psychological things. And we know that both of these types of things, mental phenomena and physical phenomena, are part of our world.
But we want to get a unified, overall picture of the world, and scientists have been trying to provide such a picture using certain physical concepts. What is the connection between the two categories? How are the mental and the physical related, if at all, and how does the former fit into our general conception of the world? These questions constitute the mind/body problem, which Schopenhauer, quite rightly I think, referred to as "the world knot." But these are vague questions, and it is my aim here to clarify what they are asking and why it is a philosophical problem.
If we take a look at some of the traditional answers proposed by philosophers, perhaps we will get a better grasp of what the question is. One answer says that there is basically no connection whatever between any mental phenomena and any physical phenomena. This view is called parallelism. It says that mental phenomena and physical phenomena exist, as it were, in two utterly separate realms, going on independently of each other. Mental events have no effect on any physical events, and physical events have no effect on any mental events.
Another answer to the problem says that there exist two distinct entities, body and soul, that interact with each other causally, though it is not known how. This is called Cartesian dualism, after Descartes.
Another view says that there simply are no mental phenomena. There is only the physical world. The existence of consciousness, therefore, must be some kind of massive delusion: contrary to popular opinion, nobody has any opinions, desires, or feelings. We are all just mindless automata. This lunatic view may be called radical materialism or eliminative materialism.
A fourth view is that there are no physical phenomena, there are only ideas in our minds. Contrary to popular opinion, therefore, there really aren't any pencils, mountains, or matter. The whole physical world is all in our minds. This lunatic view is called idealism, and it was held by Bishop Berkeley, who preferred, however, to say that pencils were ideas rather than that pencils don't exist.
A fifth view is that mental phenomena are, surprisingly, a subset of physical phenomena. All mental states, it turns out, are really states of the central nervous systems of animals. "Pain" just happens to be another word for a certain kind of brain state, just as "light" happens to be another word for electromagnetic radiation within a certain range of wavelengths. This view is called the mind/brain identity theory.
Sixth, there is the view that mental properties represent a distinct aspect of certain physical objects - that is to say, some objects, like people, have two different kinds of properties, mental properties and physical ones. This is called property dualism. It differs from Cartesian dualism in that it postulates distinct properties but not a special, distinct entity to have those properties. Consciousness is not a property of an immaterial substance, the soul, but of plain physical objects.
Another view, which could be compatible with property dualism or Cartesian dualism, is called epiphenomenalism. It says that physical events cause mental events and physical events cause behavior, but mental events don't cause anything. The mind is a kind of helpless spectator.
All of these theories can strike one as strange and most as counter-intuitive. What problem are they trying to solve, why is it so difficult, and what considerations could lead someone to a view as crazy as eliminative materialism or idealism? This I shall try to make plain presently.
Consider the following five tenable theses:
For any system, every fact about the whole is a necessary consequence of the nature and relations of the parts.
People are made of atoms.
Atoms are purely physical objects, with nothing but physical properties and physical relations to one another.
People have mental states.
No statement ascribing a mental predicate can be derived from any set of purely physical descriptions.
Now, I think that the mind/body problem can be viewed as a paradox resulting from the conflicting claims of these five statements, and the various theories of the mind/body relationship can be viewed as attempts each to deny one or more of the above theses. But first these theses must be clarified.
(1) should be read as saying that given a complete knowledge of every property of every part of some system, plus a knowledge of how these parts are arranged, every property of the whole follows logically. The nature of the whole can be predicted a priori from the nature and relations of the parts, which is to say that the whole is 100% explicable in terms of the parts. "Necessary" here means logically necessary.
(3), of course, is not talking about collections of atoms but about atoms considered as such, that is, individually.
(4) affirms that people experience pains, emotions, desires, and so on.
In (5), "derived" means "logically derived," that is, derived in the sense in which the fundamental theorem of calculus can be derived from the axioms of arithmetic and some definitions. It does not mean "caused". This means that it would not be possible to deduce the quality of someone's conscious experiences from a physical description of him or of anything else. If a physical description of the universe is given, it will always be an additional piece of information, e.g., to note that someone is in pain.
All of these theses can be defended, as I will show below, but the first thing to note is that they cannot all be true. #1-3 imply that a complete explanation of human beings as physico-chemical mechanisms exists, while #4-5 entail that such an account is intrinsically impossible. For if atoms are physical, and people are made of atoms, and the whole can be explained in terms of the parts, then people can be explained physically. But yet, if mental phenomena cannot be derived from physical facts and mental phenomena take place in people, then people can not be explained physically. At least one of these five tenable theses must be false.
Eliminative materialism is a straightforward denial of (4), Cartesian dualism a straightforward denial of (2), idealism a denial of (2) and (3) insofar as it implies there aren't any atoms, etc. It is difficult to say which thesis some of the other theories of the mind/body relation deny, but it is clear that any satisfactory resolution to the paradox must deny at least one. Property dualism might be a denial either of (1) or of (3) and the mind/brain identity theory could deny either (1) or (5), for instance.
Well, what reasons are there for thinking each of these theses is true?
Take #1: it seems to be just a conceptual truth about the relationship between wholes and things that we can think of as their parts. After all, a whole is not something over and above its parts; it is merely a matter of considering all of the parts as a whole. I could arbitrarily select any set of particles and choose to think of them as one system, but how could this give them any new properties? Since the whole does not have any independent existence, its nature must be wholly dependent on its constitution, and therefore there should be an explanation of everything that the whole can do in terms of what its parts do. But if you can not derive the properties of the whole from the properties and relations of the parts, then in some sense it remains unexplained why the whole has the properties it has. This kind of holism - saying as it does that for a complete knowledge of the universe it would be necessary to treat whole systems as wholes rather than simply describing the parts - many people feel runs contrary to the scientific world-view and is completely mysterious and incomprehensible.
What about #2? Well, if you cut open a person, you won't find anything but atoms. The denial of (2) would mean, presumably, postulating the existence of ghosts - i.e., spirits that exist apart from the physical world, and that certainly runs against the scientific world-view. Our best physical theories afford no possible explanation of the existence of such entities. As far as we know, the universe originated in the big bang, our earth was formed by swirling gases, etc., and then organisms evolved, starting with microorganisms. Now where did the ghosts come in? They don't seem to be part of the story about the matter and energy present in the big bang, nor the story about simple replicators developing into microorganisms, so when did souls get created, and how? And once they did appear, how did they manage to connect up with physical objects in such a way as to change the course of atoms that had previously gone on their courses according to natural laws - how, in other words, does the interaction between mental things and physical things work? Thought of in this way, Cartesian dualism sounds silly.
We might try questioning (3). Maybe atoms have some properties besides charge, position, mass, etc., that we haven't discovered yet. Perhaps electrons have a kind of minimal, proto-consciousness. The problem with this hypothesis, besides that it sounds pretty weird, is that it won't help any. For suppose that atoms are just a teensy bit conscious. All the same, that would not help one iota in explaining why we are conscious. An ascription of consciousness to a number of elements does not imply any ascription of consciousness to the group. For instance, every citizen of the United States has a mind, but that doesn't mean that the country as a whole has a mind of its own - some kind of collective super-consciousness. Suppose on the other hand that atoms have some other properties that we don't know about - not mental states but some non-mental properties hitherto not recognized. It is hard to see how this could help. The discovery of new physical properties would leave us in the same position of not being able to derive any subjective, conscious experiences; and even the discovery, say, of a new kind of non-mental but yet non-physical property doesn't look as if it would help. The problem is that, as Descartes noted (Sixth Meditation), the mind is simple as opposed to composite. That is, it does not have any parts. Nor, as Descartes argues, can it be said that thinking, feeling, imagining, etc., are different parts, since it is one and the same mind that thinks, feels, imagines, etc. The mind in this case should be thought of not as a bunch of conscious experiences but as the subject of experiences. When I have a pain, that is different from you having a pain in that the pain is felt from a different point of view (mine, rather than yours). If this is right, then the difficulty is not just with physical parts explaining the mind but with any kind of description of parts explaining the mind.
Some people, including a surprising number of contemporary philosophers, have chosen to deny (4), but it is hard to take this seriously. (4) is a classic exemplar of Cartesian certainty: I don't find it even possible to doubt that I am conscious. Doubt is a mental state, so even if I doubt that I have consciousness, it follows that I'm wrong since I have to have a mind in order to doubt. In order for me to be deluded about anything, I must first be conscious, so it follows that I cannot be deluded about thinking I have consciousness. Even the way I described the position of eliminative materialism shows that it is false. The materialist still thinks of himself as expressing his views and showing other people their mistakes. He bases his opinions on the information that he has gathered from scientists - for he has not done all of the research and experimentation required to verify atomic and evolutionary theory by himself - but that requires his ability to discern the beliefs that scientists have had based on the observations they have made. All of this is laden with mental concepts, so it would be inconsistent to deny the existence of the mental.
Finally, there is (5). What reason is there for thinking that is true? Well, we can compare it with a number of similar principles to get the general idea. In moral philosophy, there is a principle sometimes called Hume's law that says it is not possible to derive a normative judgement from a descriptive judgement. A normative judgement is a judgement about what is good or bad, right or wrong, and a descriptive judgement is basically anything else. Another way this is stated is that you cannot deduce an "ought" from an "is": you can't derive what ought to be the case solely on the basis of what is the case. This principle is almost universally acknowledged. And it is merely part of a more general pattern. For example, you can't derive a statement describing distances from any set of statements that don't describe distances. You can not derive a statement about colors from any set of non-color statements. You can't derive geometrical statements from non-geometrical ones. And generally, if you have an inference in which the conclusion talks about one thing and the premises talk about something else, the inference is invalid. In the same way, it is a conceptual truth that you cannot derive a mental description from a physical description. After all, just consider some physical concepts, such as spatial/geometrical properties, mass, force, and electric charge. Is it plausible that there is any way that these concepts could be used to explain what it feels like to be in pain? Say whatever you like about masses, positions, and forces of particles, you will not have ascribed any mental states to anything.
Descartes argued that he could clearly and distinctly conceive of any physical state existing without any conscious experiences accompanying it, so it follows that conscious experiences cannot be derived solely on the basis of the existence of any given physical state. And it looks as if he is right. We can forgo the Cartesian lingo about clear and distinct conceptions and say that for any physical description, that description could have been true even if there were no such thing as consciousness. Another way to put this is the following: for any physical property P, it is an open question whether a thing that has P is conscious. It makes sense to ask, "Granted that it has the feature P, but can it really think/feel/etc.?" and it's a significant question, so it follows that saying a thing has P is different from saying it is conscious, so having P can't be the same thing as thinking/feeling/etc. Of course, it might turn out that everything that has P is conscious, but that would be an additional piece of information beyond the fact that everything that has P has P.
But from this it follows that physical descriptions do not explain why there is consciousness, since they don't say anything about consciousness. They talk about a different subject matter.
Now it might be tried to challenge the sharp mental/physical dichotomy that has been drawn, to claim that mental and physical are not two mutually exclusive categories corresponding to completely different subject matters but that mental things can also simultaneously be physical. This view flies in the face of common sense (as well as the above argument), and I don't find it very comprehensible. But moreover, it won't help matters any because I could just as well have substituted "non-mental" for "physical" everywhere in the statement of the five theses and their defense. That is, we can simply restate (3) as, properties of atoms are all non-mental, and (5) as, non-mental properties cannot entail mental properties, and the problem is back in full force without having to assume that "physical" = "non-mental".
At this point, we seem to have an insoluble problem on our hands. All five theses, considered individually, are very difficult to doubt, but they can't all be true. I don't know what the answer to this is. It is in my view a major, perhaps the major, challenge to our fundamental world-view, and it would require a major world-view-revision to resolve the difficulty. As far as I can tell, no philosophical theory has come anywhere near to solving this problem in any coherent and vaguely plausible way, because no one has dealt with the arguments that I offered in behalf of the five theses. New discoveries in physics and biology are not helping - merely adding more detail to physical descriptions does not bring us any closer to being able to derive mental statements from them, and I can see no significant progress over the past 300 years.
The problem in a nutshell is that since people are composed of chemicals, they must be physically explicable, but yet since they have consciousness they can't. If this paper has a theme, it is that, unfortunately, the mind/body problem is alive and well.