Several people in this thread offer the argument that since damage to specific brain regions causes function-specific deficits in behaviors (e.g. damaging Broca's area causes Broca's aphasia -- the loss of ability to produce grammatically correct sentences), dualism -- or at least interactionist dualism -- must be false. Just to play devil's advocate -- it seems to me that the argument depends a lot on the details of "soul-theory." For example, if your picture is roughly Cartesian interactionism, the argument isn't persuasive. (Though a related argument might be.) Recall that Descartes said something like, "The soul wiggles the pineal gland, which wiggles the brain, which wiggles the body." Descartes could have said that damaging something like Broca's area prevented the soul from producing grammatical sentences because the pineal gland couldn't wiggle the right mechanisms anymore.
The causal picture for Descartes (or any one-point-of-contact interactionist) would be something like Soul --> Pineal Gland (or whatever the point of contact is) --> Broca's Area (or whatever the relevant brain area is) --> Grammatical Language Production (or whatever the function of the relevant brain area is).
The one-point-of-contact interactionist should predict that there will be a single brain region such that lesioning it will shut down all intelligent behavior. And so it is vulnerable to empirical criticism.
Alternatively, one might be a multiple-point-of-contact interactionist. One might suppose that the soul is as complex as the brain itself, and one might suppose that each soul-component interacts directly with a specific brain region. Anyway, you can see how this might go.
As I see it, the likelihood of the evidence being considered here (local brain damage causes specific behavioral deficits) on either of these theories is as high as it is on physicalism.
(As with Richard, I am not trying to defend the existence of souls. I think dualism is false. But the argument on offer is not a very good one.)
I can't think of better arguments agains souls than the one on offer. It is among the most observation-based arguments we have and observation-based arguments are usually much stronger than a priori reasoning.
Soul --> Pineal Gland --> Broca's Area --> Grammatical Language Production
The problem with this (and related theories) is that the soul believers believe that the soul itself can live and think without the body. Much of thinking is mediated by language. I don't think a believer in soul would accept that their soul after death will be inca...
No plot spoilers here, just wanted to flag a bit of poor reasoning that shows up in Chapter 39:
This is a surprisingly common fallacy. Just because X depends on Y, it doesn't follow that X depends on nothing but Y. A phenomenon may involve more than just its most obvious failure point.
To illustrate: Suppose I'm trapped in a box, and my only way to communicate with the outside world is via radio communication. Someone on the other end argues that I don't really exist -- "There's no person beyond the radio receiver, for if there was then there wouldn't be any such thing as damaged radios!" Pretty silly, huh? But people say this kind of thing in defense of physicalism all the time.
(N.B. This is not to defend the existence of souls. It's just to point out that this particular argument against them is invalid.)