Dualists typically criticize materialism by claiming that it doesn’t address the intelligibility of philosophical zombies, or that it can’t explain our conscious experience. It is true that these are real problems with materialism, but dualism is not without its own issues. Notably dualism is unable to solve the problem of other minds. The problem of other minds can be summarized by the question “how do I know that other people are conscious in the same way I am?” In my post Are Rocks Conscious? I showed how some versions of materialism could solve this problem, when applied to inanimate objects, and how dualism was incapable of doing so.
Not only does dualism fail to give us a reason that rocks aren’t conscious, it also prevents us from knowing with certainty that other people are conscious, as a matter of principle. The proof of this is fairly simple. First observe that all our perceptions about the world must have purely physical content. This is because the physical description of the world has its basis in our observations; if we observed an effect that wasn’t explained by our current physical description of the world it would be expanded to include an explanation, we would not conclude that the effect was “non-physical” (this is why particles too small to directly detect are considered physical; we see physical effects, and thus we have reason to postulate a physical cause). For example if there were physical “thought particles” that we could perceive we would be able to formulate laws about them, and they would become just another part of a materialistic physics. Thus for dualism not to degenerate into materialism we must assume that the mental properties or mental substances are never observable (except though the process of self-consciousness). However if the mental substance or properties aren’t observable then you can’t know when it is or isn’t present, and thus the problem of other minds will remain an open question forever.
You might argue that while we can’t observe the mental properties/substances themselves we can observe their effects, such that a system without the mental properties/substance would behave differently than a system with the mental properties/substance. However this claim degenerates into materialism as well, because in this case a physicist could examine the operation of these two systems and determine exactly were and under what circumstances the mental had an effect on the physical world. With this knowledge then the physicist could build a mental detector, and thus the mental would be observable and we are working under materialism again. This same process is how scientists learned to observe most things that were previously undetectable; they noticed that systems which were the same, as far as they could tell, were acting differently and they used this observation to build detectors for whatever it was that was causing the difference. (e.g. radiation)
You might think that the inability to address the problem of other minds might not be that big of a deal. After all can’t we simply rely on intuition or common sense to tell us who has a mind and who doesn’t? The real problem arises because who has a mind is important in most ethical theories. Those with minds generally receive special treatment and considerations that those without do not. For example it unethical to chop off someone’s hands (under most ethical theories), because they are a conscious being. However dissecting a corpse doesn’t seem like an unethical act, because it doesn’t have a mind. Unfortunately if you are a dualist you have no way of rationally dividing those with minds from those without, and thus it is possible that mistakes may be made. For example a dualist’s intuitions or common sense might lead them to believe that a certain group of people don’t have minds, and thus they end up condoning slavery or the murder of that group. Although you might think such problems are long behind us they have the potential to occur again as we create computers that are closer and closer to being human, or if we ever discover alien life forms that may be intelligent.
So while materialism may not seem to fit our intuitions, dualism has a much worse problem, it undermines the basis for an objective morality (a morality in which we don’t have to rely on our fallible intuitions to tell right from wrong).