A redated post.
The most formidable argument against dualism has always been what I would call the argument from the onward march of science. Science, we are told, always pushed in a materialist direction, and it invariably resolves problems for materialist understandings of things that may have seemed insurmountable to a previous generation. So prior to the 19th century, many otherwise naturalistic thinkers were reluctant to accept full-blown atheism, because they of what they took to be the undeniable evidence of design in nature, yet Darwin came along and showed us all how to be an intellectually fulfilled atheist. Arguments of any kind against materialism can do no better than point out some explanatory gap in the present materialist understanding of the world, but just as past gaps have been close by subsequent science, so difficulties that naturalistic science faces in coming to terms with things like consciousness, intentionality, and reason, are simply bumps in the road to be got over in good materialist fashion by the future course of science.
Edward Feser thinks this argument is not as strong is it might appear to be at first. He writes:
First, the advance of science, far from settling the mind-body problem in favor of materialism seems to have made it more acute. Modern science has, as noted in chapter 2, revealed that physical objects are composed of intrinsically colorless, tasteless, and odorless particles. Colors, tastes and odors thus, in some sense, exist only in the mind of the observer. But then it is mysterious how they are related to the brain, which, like other material objects, is composed on nothing more than colorless, tasteless, and odorless particles. Science also tells us that the appearance of purpose in nature is an illusion: strictly speaking, fins, for example, don’t have the purpose of propelling fish through the water, for they have in fact no purpose at all, being the products of the same meaningless and impersonal causal processes that are supposed to have brought about all complex phenomena, including organic phenomena. Rather, fins merely operate as if they had such a purpose, because the creatures that first developed them, as a result of random genetic mutation, just happened thereby to have a competitive advantage over those that did not. The result mimicked the products of purposeful design in reality, it is said, there was not design at all. But if purposes were “mind-dependent”—not truly present in the physical world but only projected on to it by us—then this makes that act of projection, and the intentionality of which it is an instance (as are human purposes, for that matter,) at least difficult to explain in terms of processes occurring in the brain, which seem intrinsically as brutely meaningless as and purposeless as are all other purely physical processes. In short, science has “explained” the sensible qualities and meaning that seem to common sense to exist in reality only by sweeping them under the rug of the mind, that is, it hasn’t really explained them at all, but merely put off any explanation by relocating them out of the physical realm and into the mental realm. There they remain, however, forming a considerable bump under the rug, one that seemingly cannot be removed by further scientific sweeping.