I recently reread Alvin Plantinga’s paper Against Materialism and needless to say I am less than impressed. Plantinga presents two “arguments” against materialism each of which is utterly ridiculous.
The first is what he calls the replacement argument (sic). It is possible, Plantinga tells us, that one could have one’s body replaced while one continues to exist; therefore one is not one’s body. Of course the obvious problem with this argument is that it at best shows that I am not identical to a particular body but it does not show that minds are not physical for it does not show that the mind exists with out any body whatsoever. To show that Plantinga needs to appeal to disembodiment and he doesn’t.
It is also clearly possible that one could have one’s immaterial substance replaced and continue to exist; thus one is not an immaterial substance. This is because there is nothing contradictory in supposing that materialism is true and what this shows, as I have argued at length before, is that these a priori arguments are of no use to us at this point.
Now Plantinga, to his credit, realizes that these kinds of intuitions are ultimately question begging so his second argument appeals to an alleged impossibility, which turns out to be none other than the problem of intentionality. The argument turns on our ability to ‘just see’ that it is impossible that a physical thing can think. Just as the the number 7 can not weigh 5 pounds neither can a brain think. Never mind computers and naturalized theories of content, those couldn’t be belief contents. Oh, I see…wait, I don’t.
But of course the real problem here is that it is even more mysterious how an immaterial substance could think. Plantinga spends some time in the paper responding to Van Inwagen’s argument along these lines. Plantinga focuses on Van Inwagen’s claim that we can’t imagine an immaterial substance. The response should be obvious: we can’t imagine lots of stuff (like what a number looks like) but that doesn’t show that they are impossible. Van Inwagen’s second swipe at immaterial substances is that we cannot see how an underlying reality that is immaterial can give rise to thinking any more than we can see how an underlying physical reality can. Plantinga’s response to this is to claim that the soul is a simple and has thinking as an essential attribute in much the same way as an electron is said to be simple and have its charge essentially.
But all of this seems to me to miss the fundamental point that Van Inwagen wants to make. The very concept of an immaterial substance is unintelligible. Attempts to make them intelligible render them into ordinary physical substances at the next level up, so to speak. And it is of course out of the question to simply say that an immaterial substance is perfectly intelligible since they are just minds (as Plantinga seems to do). It is obvious that there is thinking but it is not at all obvious that an immaterial substance could think. What would that even mean? The upshot then is that substance dualism is not a viable theory.