Let’s say that you are, for some reason, a kind of dualist. Perhaps you are convinced that the phenomenal properties, like qualia, must exist in an irreducible way. But then you must be an epiphenomenalist, as I will argue below. Which might not seem too bad initially. However, epiphenomenalism undermines the very basis for saying that the phenomenal properties exist in the first place. And so dualism is thus shown to undercut itself. Far from remaining a coherent position in which the mind and the physical world both exist distinctly, it instead implies that the phenomenal can be eliminated completely.
A modern dualist usually concedes two things. First they concede that psychological properties can be identified with the material world. What exactly you are thinking, your dispositions, those they concede are physical. However, they maintain that an extra “feel” also exists, that comes along with those psychological properties in conscious beings. These “feels” are the phenomenal properties, and they make consciousness what it is. And secondly they concede that the physical world is causally closed, that for every event there is a sufficient physical cause. To say otherwise is laughable in light of modern science.
Do these two concessions force a dualist theory to be an epiphenomenal one? I think they do. There are four major proposals put forth by dualists that attempt to explain how the physical world can be causally closed, and at the same time how the phenomenal properties can have a causal effect. The first is that some physical events are causally over-determined, meaning that some of them have both physical causes and phenomenal causes. This is simply false because by definition you can only say an event is causally over-determined when if either cause had been present, without the other, the event would still have occurred. This is not the case in the real world because the physical causes of an event are also necessary (meaning that nothing happens without a suitable physical cause). The second possibility is that causation is simply a regularity in events, and in our world the phenomenal properties occur before the physical results with the required regularity. But this fails on two grounds. First causation is more than regularity; at the very least it involves counterfactual considerations (for example, it is a regularity that twins have almost the same DNA, but this doesn’t make the DNA of one the cause of the DNA of the other). And secondly the cause and effect occur between the very smallest constituents of the world, large scale causation is simply a convenient way of talking about these connections in aggregate. But there are only a few causal laws on the smallest level, and although some of those events make up a conscious beings, many more of the same type are independent of consciousness, therefore phenomenal properties cannot be said to occur regularly with these events. The third possibility is that the causal relations themselves are somehow the phenomenal properties, such that they causally intermediate between physical causes and physical effects. But this doesn’t prevent them from being epiphenomenal, since the physical laws operate in the same way everywhere, and thus meaning that the phenomenal properties (perhaps some intrinsic part of the causal relations) don’t have an influence on what physical effect results from a physical cause. Again, this is epiphenomenalism. And the fourth and final possibility is that the phenomenal properties are extra intrinsic properties of the fundamental constituents of nature, such that one electron might have phenomenal property P, while another might have phenomenal property Q, even though the two are physically indistinguishable. But again, physics shows us that in terms of the physical properties of subsequent moments these phenomenal properties have no effect. Even if they are being carried along with the basic physical stuff they seem to have no influence on how the physical stuff changes over time. And again, this is the same as saying that they are epiphenomenal with respect to the physical world. So, no matter how you look at it dualism, even property dualism, is forced to be a epiphenomenal theory.
But maybe, you reason, we can live with epiphenomenalism. Sure it seems unintuitive, and contrary to our experience, but so does materialism to many people. But some would say that anything epiphenomenal should simply be considered to be non-existent; after all we wouldn’t believe in the existence of anything else that was claimed to have no effect on the world; what evidence could we possibly have for its existence? But the phenomenal properties are different, right? We can just feel that they exist, they are an undeniable part of our experience. Let’s analyze that thought. In terms of content, the content that asserts that I do indeed experience certain phenomenal and non-reducible properties is a psychological property (not to be confused with the “feel” of the thought, which is its phenomenal property). And psychological properties can be explained physically. But this means, since the physical world is causally closed and the phenomenal properties are epiphenomenal, that you could have had that thought even if there were no phenomenal properties. In fact there is no way to prove that you do have phenomenal properties, even to yourself. A duplicate you could exist without those phenomenal properties, and it would act in the same way, and it would even have thoughts with the same content (it must, since the content of a thought has a causal effect on the actions taken). This duplicate would feel equally strongly that it too had phenomenal properties. And thus you have no evidence for thinking that you experience the phenomenal properties in question, because you might very well be such a phenomenal property-less being only thinking it had phenomenal properties. But if you have no reason to believe they exist then why insist on their existence? (This is the same line of argument which leads the atheist to be unconvinced by the experience of god other people claim to have, if why you think you experienced god can be explained without invoking god then the experience doesn’t imply that god exists; likewise if you can explain why people think they experience these phenomenal properties then those thoughts don’t imply that the properties exist.) The world would be exactly the same without them, and you would have the same thoughts, say the same things; you couldn’t tell the difference. So serious dualism seems to lead to its own dismissal.