The following is a portion of Larrys Text that has been wikified; further wikification is encouraged
The mind-body problem is the problem of determining the relationship between the human body and its mind. Are our minds something different from our physical bodies? Suppose we think that the mind is a substance of some sort -- a mental substance. We might still ask: Is there some way to explain what the mind, a mental substance, is, in terms of physical substance? Or will we maintain that the mind is something totally different from physical bodies, and that we cannot explain what the one is in terms of the other at all?
Suppose instead that we deny that the mind is some mysterious substance, and we hold instead that there are only mental events and that "the mind" designates no more than a series of mental events? We can still inquire about the relation between mind and body in a different way, in terms of the relation between mental events and physical events. We can ask: Are mental events totally different from physical events, so that you can't explain what mental events are in terms of physical events; or are mental events somehow explainable as being the same as physical events? For example, when John feels a pain, a mental event is occurring; now is that pain even possibly the same as something that occurs in John's brain, such as the firing of some special group of neurons? Now this question we will examine.
The mind-body problem can be introduced more fully with an example. Suppose John decides to walk across the room, whereupon he does in fact walk across the room. John's decision is a mental event and his walking across the room is a physical event. On anyone's accounting, there is another physical event involved, namely, something happens in John's brain, which tells John's legs to start walking. This brain event is closely connected with John's decision; the brain event happens at about the same time, or right after, John decides to walk across the room. We might ask: How is it possible that a decision, which is something mental, resulted in something in your brain, which is something physical? If we say that the mental and the physical are totally different sorts of things, then how can one have any causal impact on the other? How can a mere mental event, a decision, actually cause neurons in my brain to start firing? The very idea might seem absurd.
On one view (see philosophical view), a better description of the situation is this: John's decision is itself a physical event. When John decides to take my trip across the room, a group of neurons fire in his brain. He is not aware of those neurons; but the firing of those neurons is itself just the same as his decision. There isn't any more to the decision than that physical event. So, on the view in question, there's no trouble thinking about how a mental event can have a physical effect; mental events are themselves physical. Ultimately, everything is physical.
To many people it sounds really strange to say that a mental process is no more than a special kind of physical process. The mind, they say, is more spiritual, ethereal, and so it simply is not the sort of thing that can be physical. And they have other reasons as well for rejecting this reduction of the mental to the physical.
So in fact what some philosophers have believed instead - but hardly anyone anymore - is that the reduction goes the other way. We should explain what bodies are in terms of mental goings-on; so the physical can be reduced to the mental. When John walked across the room, really that was happening only in John's mind and perhaps also in each of our minds individually at the same time. There is, on this view, nothing more to John's walking across the room than our having the thought, or the perception, that that happens. This view would also solve the problem of how the mental can affect the physical. Since physical events are themselves nothing more than a special kind of mental event, then of course there is no trouble about how a decision, which is obviously a mental event, can result in our bodies moving, which is also a mental event, although less obviously so.
The three above-described views about the relationship between the mental and the physical have names:
There are, then, three basic choices: mental and physical events are totally different, and cannot be reduced to each other (which is dualism); mental events are to be reduced to physical events (which is materialism); and physical events are to be reduced to mental events (which is phenomenalism). To put it in terms of what exists "ultimately", we could say that according to dualism, both mental and physical events exist ultimately; according to materialism, only physical events exist ultimately; and according to phenomenalism, only mental events exist ultimately. Materialism and phenomenalism are both varieties of monism; of monism there is one further variety, namely neutral monism.
This is already a lot of positions to consider, but it gets worse: within each category there are further refinements to be made.
See also