In my humble opinion, materialist theories of mind are all of them quite hopeless. All of them founder on the reef of irreducible subjectivity. But is substance dualism in a better position than materialism when it comes to explaining the subjectivity of conscious experience?
Colin McGinn, drawing on Thomas Nagel, thinks that the same problem that afflicts the materialist returns to haunt the substance dualist. Now what was that problem again?
1. There is a categorial difference between the subjectivity of the mental and the objectivity of the physical. Conscious experiences are essentially subjective in that they are necessarily such as to be epistemically accessible only to those who enjoy them. Objective items, on the other hand, are in principle epistemically accessible to many.
My cat, for example, recognizes me, but not as a member of homo sapiens, but as the one who controls the food supply. She sees me not as a man, but as a big cat, indeed as something like her mother. (She crawls onto my belly and kneads my gut.) Associated with her take on me are certain kitty kat kwalia (cat qualia). But they are inaccessible to me, and would remain inaccessible even if I knew all there was to know about cat brains. I reckon there must be something it is like to be a cat, but I am not directly acquainted with what it is like.
The long and the short of it is that "physical facts are essentially objective in the sense that they can be understood from any point of view irrespective of the specific phenomenology of the understander's own experience." (Colin McGinn, Problem of Consciousness, p. 164) Not so for conscious experiences, whether human, feline, or Martian. (When philosophers speak of Martians, they mean extraterrestrials; the term 'Martian' is something like a synecdoche. Philosophers are weird, but they don't actually believe that there is intelligent life on Mars.)
I stress the categorial difference between the subjective and the objective. Here is second example of a categorial difference: that beween numbers (and abstracta generally) and concrete items like shoes and valve lifters. Categorial differences are unbridgeable, no matter how gradually you build the (impossible) bridge. Dennett take note.
There is an old-fashioned name for the fallacy of illicit category-jumping: metabasis eis allo genos. Perhaps we can pin said fallacy on our materialist friends.
2. The upshot is that the subjectivity of experience cannot be construed in objectivizing terms. In particular, qualia cannot be identified with states of the brain. But since it is the objectification of the subjective that is ruled out by the categorial difference between subjectivity and objectivity, rather than its materialization, it seems to follow that qualia can just as little be identified with states of a mental or spiritual substance. A res is a res whether it be a res extensa or a res cogitans. And a res is a thing, an object.
As McGinn puts it, "physicalism is not distinctively afflicted by the fact of subjectivity." (164, n. 18) Well said. Subjectivity appears to tell against both physicalism and substance dualism, and to tell against them equally. Qualia cannot be states of an object whether the object is made of material stuff, or 'spook stuff.' Stuff is stuff.
3. But is this really the case? One problem with the Nagel-McGinn parity argument is that a dualism of substances, properly understood, is not a dualism of stuffs. No sophisticated dualist recognizes his position in the 'spook stuff' strawman. See Dennett's Dismissal of Dualism.
A mental substance is a mental individual, indeed a mental continuant. It is a property-possessor, not a quantity of spook stuff. It is a parcel of consciousness with a wholly un-object-like nature. T.L.S. Sprigge, Theories of Existence, p. 46:
It is often difficult to get people to realize that the non-physical mind of which Cartesians speak is not, as some have thought it, a 'ghost in the machine' of the numan body, since ghosts and 'spirits' such as might appear in a seance are, in contrast to it, as physical, if made of finer stuff, as our ordinary bodies.
So although Nagel and McGinn are right that qualia cannot be states of any object, whether made of gross or subtle stuff, their parity argument fails to touch substance dualism. It fails because they misconstrue the claim that there are irreducible mental substances. This is not the claim that there is a special sort of stuff; it is the claim that consciousness occurs in individual 'parcels' possessing an unobjectifiable nature.
Perhaps we should speak of mental subjects rather than mental substances to avoid confusion. The correct points made by Nagel and McGinn have no tendency to show that qualia cannot be states of mental subjects.
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