The primary direct argument for physicalism is the correlation between brain states and mental states. To take a few examples:
It has become possible in recent years to use magnetic and positron scanning devices to observe what is happening in different parts of the brain while people are doing various mental tasks. For example, brain scans have identified the regions of the brain involved in mental imagery and word interpretation. Additional evidence about brain functioning is gathered by observing the performance of people whose brains have been damaged in identifiable ways. A stroke, for example, in a part of the brain dedicated to language can produce deficits such as the inability to utter sentences.
Let’s extend this inference to a couple of analogous cases:
1. Using a polygraph, we can tell when a person is probably lying based on certain *physiological* (rather than neurological) correlations.
By parity of argument, wouldn't this imply the identity of mental states with physiological states like blood pressure, pulse, respiration, and skin conductivity?
But if this inference is obviously absurd, then doesn't that undercut the parallel argument from brain scans, and so on?
2. Likewise, a good poker player can predict, in large part, what a bad poker player will do, or what hand he has, by being able to *read* the player's body language.
By parity of argument, wouldn't this imply the identity of mental states with body language? But if this inference is obviously absurd...
So it seems to me that the argument for physicalism from the correlation between brain states and mental states either proves too much or too little.