For an excellent introductory discussion of physicalism, Jackson, and Nagel, see Graham Oddie's outlines for his lectures on these subjects at:

(1) Notes on Physicalism

(2) The Qualia Objection

Quotes from Oddie here are from these sources. Oddie teaches philosophy at the University of Colorado at Boulder.

[NOTE: Sorry, links to Graham Oddie's papers above do not work anymore, and I have not been able to find them online anywhere else.]

The term "physicalism" has several related meanings:

1. The view that everything real is, in some sense, physical. In this sense "physicalism" means the same thing as "materialism".
2. The view, in philosophy of science, that any piece of factual knowledge can be formulated as a statement about physical objects and activities. "Physical" means "able be described in a language based on intersubjective observation." Thus, if physicalism is correct, the language of science is entirely third-person.
3. Graham Oddie (1) puts it this way: "Physicalism is the thesis that everything is physical, that a complete description of the physical universe is a complete description of the universe, period."

Physicalists do not deny the existence of apparently "mental" phenomena: things like beliefs, desires, hopes, fears, etc. (We are following Peter Smith's convention of "mental" meaning "mental without Cartesian baggage.") But physicalists say that all mental states are really just physical states. For example, Identity Theorists say that mental states are identical to physical states of the central nervous system.

In "Epiphenomenal Qualia", Frank Jackson argues against physicalism in the second and third senses. So first we need to get a clear idea of what physicalism amounts to.

Philosophers don't usually have a problem with the idea that things like tables and chairs and cars and bodies are physical. Physicalism is controversial only because it claims that so-called "mental" events are also really physical.

P1: Mental states cause behavior.

P2: Behavior involves bodily motions in the physical world.

P3: Only physical things can cause bodily motions in the physical world.

C: Therefore, mental states must ultimately be physical things.

I'll discuss each step in the argument.

This seems intuitively obvious. As Peter Smith says, I raise the glass (physical movement) because I want to drink the beer (mental state — wanting). I run (physical movement) because I think the lion is chasing me (mental state — thought).

From Oddie (1): "The causal theory of mental concepts claims that mental concepts (like the concept of poison) are all causal. What is pain? Pain is a mechanism for warning an organism that its body is damaged and to take appropriate action, so that it is an inner state of a person which is (typically) brought about by bodily damage and which (typically) brings about pain-behavior. When we see somebody whose body is damaged, and who is writhing and groaning, we say 'he is in pain'. But we do not simply mean that he is exhibiting pain behavior. Rather, we are conjecturing the presence of an inner mental state of the person (pain) which is brought about by the bodily damage, and which brings about the pain-behavior. Using arrows to indicate causal links, and X to indicate the position of pain in the causal sequence, we can depict the causal role like this:

bodily damage ---> X ---> pain behavior.

Whatever actually fills the gap marked by X is pain.

Other mental concepts, so the theory goes, can be analyzed in a similar way."

This way of accounting for mental concepts is basically functionalist, since it says that pain is whatever has the function of causally linking bodily damage and pain behavior.

P1 leaves open the nature of the inner states that play the causal roles. P1 alone doesn't commit you to physicalism. As far as P1 goes, "the mental state of pain may be a non-physical state of a Cartesian mind, or it may be a physical state of the central nervous system. Which one it is will depend on the way the world is constructed, and will not itself be open to purely philosophical investigation. In order to find out what pain is, for example, we have to do some hard empirical research."(Oddie(1))

The world, however, seems to be constructed according to P3.

This seems obvious.

This seems well-supported. As a matter of fact, we find that the mental supervenes on the physical, i.e., mental events are correlated with brain events, and different types of mental events are correlated with different types of brain events.

Once you acknowledge this premise, you're forced to conclude that the mental states are most reasonably thought of as physical states of the nervous system.

For example, suppose stimulation of C-fibers in the brain fills the gap between bodily damage and pain behavior. In other words, "bodily damage typically brings about such stimulation, and such stimulation, via the motor center in the brain, brings about typical pain behavior. That is, we can fill in the X position in the diagram above:

bodily damage ---> C-fiber stimulation ---> pain behavior

Since whatever fills the gap previously marked by X is pain, we would have an argument that pain just is the stimulation of C-fibers." (Oddie(1)) The physicalist would say that's all there is to pain. In any true statement about "pain" you could substitute the expression "stimulation of C-fibers" for "pain" and truth value would be unaffected.

"John is in pain" = "John's C-fibers are being stimulated."

Using the physicalist approach, you can in effect get rid of all references to the mental and replace them with references to the physical. At least, that's the idea. According to the physicalist, there's nothing else to say about pain once you discover its physical nature. All true statements about pain can be recast as statements about C-fiber stimulation; there are no true statements about pain that cannot be so recast.