Substance dualism is the idea that our conscious self is a combination of both a physical and non-physical reality. That is, our consciousness is not just neurons firing in the brain, but also some kind of phenomenal self, which is separate from the physical realm.
One reason for holding to substance dualism is that it avoids the problems of monistic physicalism. Physicalism argues that our conscious self is literally the brain. There is nothing but neurons firing in the brain (okay, it’s a lot more complicated than that, but the general idea is that our brain is our “self”).
Physicalism, therefore, leads to a kind of monism–everything is matter. Depending on which physicalist philosopher one prefers, this can lead to all sorts of problems. We can see this when we examine what exactly composes a “thought.” On dualism, a “thought” is a non-physical, phenomenal experience of the “self”–which is generally referred to either as “mind” or “soul.” On physicalism, a “thought” is identical with a brain state.
In other words, on Physicalism:
Brain state A => Mental state A’
Brain state B => Mental state B’
And so on.
When I experience thought A’, it is because of a prior brain state, A. My mental states are either identical to, or supervenient upon, the physical state of my brain. The problem with this is that it relegates mental states to epiphenomenalism. That is, if a mental state is wholly dependent on a brain state, the mental state is superfluous. This is because the mental state is entirely dependent upon (or identical to) the brain state. On physicalism, a mental state does not occur without a brain state occurring prior to, or in conjunction with, it.
This, in turn, leads to epiphenomenalism because the mental state is, as I said, superfluous. If it is always the case that Brain state A=> Mental state A’, then Brain state A causes whatever actions we take, for the brain state entails the mental state, which itself is identical to or supervenient upon the brain state to exist. But then, if we cut mental state A’ out of the equation, we would still have Brain state A and the action. Thus, consciousness is entirely superfluous.
Another problem with this is that it also means consciousness doesn’t have to have any connection with the actual world. Our brain states could be causing all kinds of wild mental states which are completely unconnected to what is happening outside of our “self,” but we would never know it or act differently. I could be having the mental states of pigs flying and eating buffaloes as I write this, but it wouldn’t matter because the brain state is what is causative. The mental state is simply a byproduct of the brain state. Or, we could all be zombies, without any kind of phenomenal consciousness, and yet still be performing the same actions.
Yet another problem, on the physicalist perspective, is that there seems to be no reason for our mental states to line up with reality. Why is it that despite the fact that our brain state is causing all of our actions, or mental state seems to line up with those actions? There doesn’t seem to be any reason our mental states should line up with reality. One response could be that we have no reason to suppose they do line up with reality, but then we have no reason to trust anything we “think” and should give up whatever positions we do hold.
Of course, monistic physicalism actually argues that there is no mental state A’ generated by brain state A, but I don’t see any reason for believing this is true, for they are of two completely different kinds. One is gray mush, the other is a phenomenal image of a cat. One is composed of neurons shooting impulses to and fro, the other is the idea that “I wish I had eaten breakfast.” The law of identity states that A = A. But, according to monistic physicalism, my gray mush/neurons firing = image of cat. This is simply false.
So, I have no reason to accept physicalism on any of these formulations, and every reason to reject it. Physicalism is epiphenomenal, gives us reasons to doubt our basic intuitions, and makes any thoughts we have completely arbitrary.
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