Feser says that this is analogous to the modern problem of the mind. Hume and Locke swept all the "secondary qualities" (e.g. color, taste, sound) out of the world and into the mind, classifying them as subjective responses to the "real" world, which is composed solely of colorless, tasteless, soundless particles in motion. Contemporary materialist philosophers now want to dispose of the mind (i.e. color, taste, sound as we subjectively experience them) in a similar manner, which is just like trying to get rid of the dirt under the rug.
My purpose here is not to go further into Feser's metaphor, but to show that the metaphor can also be interpreted in terms of Immanuel Kant's views on the mind, views that also expose problems with the contermporary understanding of the mind but from a different perspective. My starting point will be a quote from page 51 of Nagel's book:
The existence of consciousness is both one of the most familiar and one of the most astounding things about the world. No conception of the natural order that does not reveal it as something to be expected can aspire even to the outline of completeness. And if physical science, whatever it may have to say about the origin of life, leaves us necessarily in the dark about consciousness, that shows that it cannot provide the basic form of intelligibility for this world.This is true as far as it goes, but it carries the danger of shortchanging consciousness in ways that Immanuel Kant would not approve. Writing of the existence of consciousess as a thing "about the world" misses the fact that consciousness is not a feature of the world, but is rather the constitutor of the world. Consciousness is the ground on which there is (for me) a world at all; it is the thing with reference to which anything at all can be said to be "about the world." There is nothing "about the world" for a book, a stone, the number five, or 5:00 pm, for none of these things is conscious and therefore they have no world. And as Nagel made clear in his famous 1974 article "What Is It Like To Be a Bat?", the world of a bat is likely so very different from our own that it is nigh on inconceivable to us. Now physical science is one of those things that is "about the world" for human beings; not being conscious, books and stones have no physical science, and not being men, bats don't have it either. Physical science is a creation of man and is therfore a product of his consciousness. This makes it problematic, then, to suppose that physical science can "say" anything about consciousness at all, let alone say it the same way it says things about everything else. To adapt the rug metaphor to the Kantian point I am making, it's not just secondary qualities like taste and sound that have been swept under the rug of the mind in modern philosophy; everything has been swept under the rug, including the physical sciences. The reason the physical sciences cannot ultimately grasp consciousness is because they are under the rug rather than outside it, and what is under the rug is in no position to grasp the rug itself.
Let me flesh out this point by applying it to physics. Physics deals with things like force, mass and acceleration. But force, mass and acceleration as we understand them in modern science are not things we just stumble across walking down the street. They are theoretical constructs with which the scientific mind analyzes the world. Any physics student understands how this works. In physics 101, the student is presented with problems involving blocks on a ramp or weights and pulleys. The first step in the solution, and often the most difficult step, is to analyze the blocks, ramps and weights into forces and masses. The block on the ramp is the relevant mass, the relevant forces are directed down the ramp and also straight down (the force of gravity on the block), etc. Once this analysis is complete, the student then plugs the quantities into the relevant Newtonian equations and cranks out the answer, which is then mapped back from the forces and masses back onto the blocks and pulleys (e.g. the block will take 10 sec to travel down the ramp).
Everything about this problem has an ineluctable reference to consciousness. The "block" is a block because I perceive it as such; a bat would not perceive it in this manner nor would a stone. The relationship of the block to the ramp ("the block is on the ramp") is only such because my consciousness perceives both the objects and the relationships between them. The mapping of the objects to the theoretical constructs of Newtonian physics is most obviously dependent on consciousness, as is the mapping back once the problem is complete. My point is not the solipsistic one that the world is a creation of consciousness; it is that the ground of our experience of the world is consciousness, and therefore the attempt to draw conclusions about the ultimate nature of consciousness from experience via modern scientific methods is problematic; in fact, it is doomed to failure. It is in the end an attempt to sweep the rug under itself.
Consciousness cannot be just another "thing about the world", or a variable in a physical description of something, as other things are. This is to misunderstand the relationship of consciousness to science. Take the basic elements of Newtonian physics:
What happens when we attempt to investigage consciousness as though it were as amenable to scientific investigation as anything else? Consider that Newton's Second Law (F=ma) can be written in terms of a function:
Again, X is the consciousness that serves as the ground of the scientific understanding of the world. Now a scientific account of consciousness, whatever it's specific content, can be cast in terms of a generalized function:
Can't consciousness Y just be the same as consciousness X? Even if this could be so it doesn't change the fact that consciousness has changed its essential nature in going from the constitutor of the world to a thing in the world, so what ends up being understood about consciousness in science is something much less than is necessary to ground the scientific consciousness in the first place. This is why Kant says that ultimately our rational natures are opaque to us and must remain eternal mysteries. And this accounts for the "shell game" impression given off by much of the scientific writing about consciousness these days; books have grandiose titles like Consciousness Explained or The Synaptic Self, but when these titles are cashed out in terms of what is actually explained in the works, the reader can't help but feel he has been given the bait and switch. The "consciousness" explained seems but a pale shadow of the consciousness we were hoping to be explained.
There is also the problem that an explanation should not include in its explanation that which is to be explained. F=ma can't include force on both sides of the equation if it is to explain force. But this is what happens if we take consciousness Y to be the same as consciousness X. Consciousness Y is implicit in all the terms on the right side of the equation that form the basis of the explanation of consciousness. Worse than this, it is implicit in the relations between the terms themselves. In terms of the rug metaphor, what we have here is the attempt to sweep the rug under itself.
Of course, if the rug (and the dirt under it) is going to be swept somewhere, it can only be swept under a yet larger rug that is big enough to hold the first rug and its dirt under it. Consciousness Y is that larger consciousness under which consciousness X is swept. But consciousness X is not really swept under the rug of consciousness Y, for consciousness X is in itself a constructor of the world, and it is only swept under the rug of consciousness Y as a thing in the world of consciousness Y.
One of the peculiar features of the philosophy of mind is the manner in which the participants seem to talk past each and cannot agree on even the apparently simplest things. John Searle tells us that "all of the most famous and influential theories are false." The exasperation of Daniel Dennett is palpable in much of his writing. Imputations of bad faith abound. The reason for this is that each participant takes for himself the role of consciousness Y, the transcendent consciousness or constitutor of the world, and from that vantage point tells all other consciousnesses (which he treats as things in the world) how things are. This is done in hilariously explicit manner in Dennett's Consciousness Explained:
If you want us to believe everything you say about your phenomenology, you are asking not just to be taken seriously but to be granted papal infallibility, and that is asking too much. You are not authoritative about what is happening in you, but only about what seems to be happening in you, and we are giving you total, dictatorial authority over the account of how it it seems to you, about what it is like to be you. (p. 96, emphasis in original)
Now while you may be entirely mistaken about the meaning of your consciousness (nothing in your consciousness has any metaphysical gravity until it has been subjected to heterophenomenological analysis), the author of Consciousness Explained is in no such predicament. His consciousness is reliably in contact with reality without it first needing to go through the heterophenomenological boot camp. This is how a transcendent consciousness (consciousness Y) talks to a consciousness immanent with respect to Y's scientific constructs (consciousness X). And a good thing, too, at least if you want books like Consciousness Explained to be written, for it could not be written otherwise. But then we must remember that Consciousness Explained at most explains consciousnesses of type X and never of type Y.
All this was thoroughly understood by Kant back in the 18th century and explicated in his Critique of Pure Reason. It is captured in his fancy terminology as the synthetic unity of apperception.
... it is only because I am able to comprehend the manifold of representations in one consciousness, that I call them altogether my representations, for otherwise, I should have as manifold and various a self as I have representations of which I am conscious... It is true, no doubt, that this principle of the necessary unity of apperception is itself identical, and therefore an analytical proposition; but it shows, nevertheless, the necessity of a synthesis of the manifold which is given in intuition, and without which it would be impossible to think the unbroken identity of self-consciousness... I am conscious, therefore, of the identical self with respect to the manifold of representations, which are given to me in an intuition, because I call them, altogether, my representations, as constituting one. This means that I am conscious of a necessary synthesis of them a priori, which is called the original synthetical unity of apperception...Kant is responding here to Hume's argument that, since we find no self in empirical experience, we are unjustified in thinking that there is some substantial core to ourselves that goes beyond the chain of impressions that constitutes our experience. Kant's answer is not a metaphysical argument for the self, but a consideration of "pure reason" (i.e. logic independent of empirical experience), to the effect that some unifying principle must be assumed in experience merely to allow me to claim my experiences as mine. More significantly for my purposes, Kant argues that our experience is not purely passive, but that we actively construct our experience in some measure (this is the "synthesis of the manifold") and therefore there must be some principle unifying that construction. This is all the more significant when it comes to science, for things like Newton's Second Law (F = ma) or theories of consciousness are not things we stumble over in nature; they are very definitely constructions of the human mind. Kant takes the truth that Hume saw - that there is no self encountered in empirical experience - and drew the correct conclusion that Hume missed, which is that the self is not in empirical experience because it is the ground of empirical experience.
(from Basic Writings of Kant, Modern Library, p. 73).
Kant's aim was to save science and modern philosophy from the extreme skeptical empiricism of Hume (while retaining the authentic insights of Hume), while also ruling out the metaphysical speculations of the classical philosophers. In other words, he wished to show that the nature of human thought is just what modern thinkers want it to be: Metaphysics is a waste of time, and the only true way to know the world is through empirical science supported by logical thought. Kant saw that the price to be paid for this resolution is that the conscious self must ultimately be a mystery to itself. The only legitimate way to know the world is through empirical science, but the relationship of the scientific consciousness to science is not as one of the things in the world of science, but as that which grounds and underwrites the constitution of the scientific world itself. Thus the scientific consciousness will always slip behind any attempt to understand consciousness through science.