One thing I see over and over again in internet discussions about dualism (usually Cartesian dualism is assumed – very few people know/care about Aristotelian-Thomistic dualism) is that the materialists usually say things like, “Let’s grant that an immaterial ‘soul’ does exist. Where is this soul located? How can we detect it with our instruments? How does it interact with matter? What experiment could we run to detect this substance?”
The problem here is that the materialist is basically saying that if the immaterial exists, then we should expect it to have the properties of matter. But that’s exactly what the dualist denies in the first place. Both Cartesian and Aristotelian dualists claim that the immaterial does not, and cannot even in principle, have the properties that matter has, so we should not expect it to “bump into” particles, or “inject” energy into matter that can be detected. We should not expect it to be some ghostly kind of matter. That’s exactly what it isn’t.
Say I told you that the sentence “I am typing on my computer” has an immaterial meaning above and beyond the pixels that are illuminated on your screen. I could even point to the evidence that the sentence could be written in a different font, size, color, or even language and still have the same intrinsic, immaterial meaning, therefore there is nothing per se about the pixels that contains the meaning of the sentence.
And you replied to this assertion by saying that “If there is an immaterial meaning of ‘I am typing on my computer’ that is above and beyond the actual pixels that compose the sentence on my computer screen, then where is the immaterial sentence written that conveys this immaterial meaning? On what ghostly computer screen is it located? Where can I see this immaterial meaning with my eyes? How does it inject energy in a detectable way to my earthly computer screen?”
That would be a strange response, because the claim is not that there is some second, ghostly sentence hovering in the space above a sentence (maybe in another dimension) that makes up its immaterial meaning, nor that it interacts with my computer screen with energy or particles in the same way that matter interacts with other matter. That would lead to a crazy infinite regress (where is the super-ghostly sentence hovering over that ghostly sentence that gives it meaning, etc.?), and the claim of the dualist is that postulating more or different types of matter does not resolve the reason dualism was created in the first place – there is nothing intrinsic to matter even in principle that could carry the qualia, meaning, intentionality, etc. that we directly perceive in the world, no matter how complex matter got or how many types of matter there were. Thus, a second type of substance had to be postulated that has properties that matter cannot even in principle – as it is currently defined – have.
Of course, then there is Mormonism. A shallow reading of Joseph Smith (and a normal reading of Pratt) would lead one to believe that Mormonism holds that “spirit is matter” but that it is finer or purer than regular matter. In other words, the only form of dualism I know of that really does postulate a second, ghostly type of particles or matter, is Mormon Dualism.
I have seen Mormon philosophers do amazing back-bends around these scriptures and argue that actually most ontologies can be resolved with these Mormon scriptures (including hylemorphism), or that we don’t even know what matter is, so it’s pointless to scoff at identifying spirit with matter. But that leads me to wonder what the point of these scriptures is in the first place, if not to clarify one ontology over the other? What do we learn about the universe when we are told “spirit is matter,” if it doesn’t actually mean that spirit is composed of a special type of pure, fine atom or particle that is almost like the matter we can see? It seems to me to just add to the confusion rather than to clarify it.