Q: What is instead denied is a radical privacy - the idea that no-one could experience or understand that feeling in principle, regardless of their physical brain state.
A: That's not it at all. What is being denied is the primacy of the subjective, the fact that the subjective nature of experience can never be understood in wholly third-person terms, from outside of experience, but is a simply another kind of phenomenon. Part of this is the idea that humans are perfectly reducible to objective analysis (which is a central aspect of Dennett's 'scientism'.)
Furthermore Dennett explicitly argues that the idea of the subject and subject-hood, generally, are at best epiphenomal illusions which in reality are simply the consequences of cellular transactions. Humans are in some real sense automata, they do what they are programmed to do by the 'Darwinian algorithm'. He explicitly, if humorously, says that humans really are 'moist robots', and then says 'so, what's the problem'? It's the fact that he doesn't understand why this is a problem, that's the problem!
Nagel's review of Dennett's latest book is called
Is Conscousness an Illusion? Nagel says:
Our manifest image of the world and ourselves includes as a prominent part not only the physical body and central nervous system but our own consciousness with its elaborate features—sensory, emotional, and cognitive—as well as the consciousness of other humans and many nonhuman species. In keeping with his general view of the manifest image, Dennett holds that consciousness is not part of reality in the way the brain is. Rather, it is a particularly salient and convincing user-illusion, an illusion that is indispensable in our dealings with one another and in monitoring and managing ourselves, but an illusion nonetheless.
You may well ask how consciousness can be an illusion, since every illusion is itself a conscious experience—an appearance that doesn’t correspond to reality. So it cannot appear to me that I am conscious though I am not: as Descartes famously observed, the reality of my own consciousness is the one thing I cannot be deluded about. The way Dennett avoids this apparent contradiction takes us to the heart of his position, which is to deny the authority of the first-person perspective with regard to consciousness and the mind generally.
...Dennett believes that our conception of conscious creatures with subjective inner lives—which are not describable merely in physical terms—is a useful fiction that allows us to predict how those creatures will behave and to interact with them.
The way in which this conscious life is allegedly illusory is finally explained in terms of a “user illusion”, such as the desktop on a computer operating system. We move files around on our screen desktop, but the way the computer works under the hood bears no relation to these pictorial metaphors. Similarly, Dennett writes, we think we are consistent “selves”, able to perceive the world as it is directly, and acting for rational reasons. But by far the bulk of what is going on in the brain is unconscious, low-level processing by neurons, to which we have no access. Therefore we are stuck at an “illusory” level, incapable of experiencing how our brains work.
Dennett's philosophy can be summed up as follows - molecules are the only real agents, real 'doers', in the Universe, and everything else we see is the product of their activities, which are really not so much intelligent as a kind of elaborate chemical reaction:
The true nature of things is evident only at the bottom, and so we must understand life from the bottom up.
• What we find at the bottom are scraps of molecular machinery.
• Through the power of natural selection — which operates like a mindlessly mechanistic algorithm (Dennett) or a blind, unconscious automatism (Dawkins) — these low-level molecular machines slowly evolve into the kind of apparently purposeful, complex entities we recognize as organisms, including ourselves.
• Whatever we are to make of this appearance of meaning and purpose — including my own intentions as I write this and yours as you read it — we are both urged to shed our prejudices and acknowledge that we with our intentions somehow arise from more basic, underlying processes that are essentially dumb, meaningless, and mindless.
Love it or hate it, phenomena like this [i.e. organic molecules] exhibit the heart of the power of the Darwinian idea. An impersonal, unreflective, robotic, mindless little scrap of molecular machinery is the ultimate basis of all the agency, and hence meaning, and hence consciousness, in the universe.
Darwin's Dangerous Idea, Dennett, page 202.
Dennett's opponents, in essence, claim there is a subjective aspect to every act of knowing, even the so-called 'hard sciences' such as physics (a fact which has manifested in the 'observer problem'). The reason Dennett can even make his preposterous argument, is that the subject is ubiquitous and is implicitly invoked in every conversation, every 'speech act', even without being made manifest or explicitly referred to. It's unconscious, but not in the way Dennett claims: not because it comprises purely material actions on the cellular level acting out of an algorithm of which we're unaware, but because we so deeply embody it, that it is never an object of perception, it is never a 'that' to us. That is why Dennett can apparently argue for it's unreality; because you can never really pick out what mind is, or where it is, as it precedes any speech act, thought, or gesture. You can act like it's not there, except for if it really weren't there, you'd be dead, and Dennett wouldn't be able to tell you anything.