Daniel Dennett is a brilliant and flashy writer, but his brilliance borders on sophistry. (In this regard, he is like Richard Rorty, another writer who knows how to sell books.) As John Searle rightly complains, he is not above "bully[ing] the reader with abusive language and rhetorical questions. . . ." (The Mystery of Consciousness, p. 115) An excellent example of this is the way Dennett dismisses substance dualism in the philosophy of mind:
Dualism (the view that minds are composed of some nonphysical and utterly mysterious stuff) . . . [has]been relegated to the trash heap of history, along with alchemy and astrology. Unless you are also prepared to declare that the world is flat and the sun is a fiery chariot pulled by winged horses — unless, in other words, your defiance of modern science is quite complete — you won't find any place to stand and fight for these obsolete ideas. (Kinds of Mind, Basic Books, 1996, p. 24)
There is something intellectually dishonest about this passage since Dennett must know that it makes a travesty of the dualist's position. Yes, I know he studied under Gilbert Ryle and had phrases like "ghost in the machine" drummed into him at an impressionable age; but he is smart and well-connected and has had plenty of opportunity to be set straight. A substance dualist such as Descartes does not hold that minds are composed of some extraordinarily thin intangible stuff. The dualism is not a dualism of stuff-kinds, real stuff and spooky stuff. 'Substance' in 'substance dualism' does not refer to a special sort of ethereal stuff but to substances in the sense of individuals capable of independent existence whose whole essence consists in acts of thought, perception, imagination, feeling, and the like. Dennett is exploiting the equivocity of 'substance.'
Of course, it is very difficult for the materialistically minded to understand this because they cannot understand how anything could be real that is not material. This incapacity on their part leads them to construe the Cartesian dualist as talking about some sort of rarefied matter, some sort of spook stuff, a kind of immaterial matter if you will.
The incapacity in question also leads Dennett to the preposterous notion that a defender of dualism must stand in complete "defiance of modern science." (Note en passant that Descartes was among the founders of modern science.) But where in modern science is it established that everything that exists is material or physical in nature? Which branch of physics is competent to establish this meta-physical result? Observe the difference between the following two propositions:
1. Nothing in the physical world is in principle insusceptible of natural-scientific study.
2. Nothing is in principle insusceptible of natural-scientific study.
(1) is unobjectionable, but (2) presupposes that everything is physical. But that everything is physical is a metaphysical proposition that is neither entailed by any scientific result, nor presupposed by scientific investigation.
Since Dennett is the exact opposite of stupid, one has to conclude that he is either intellectually dishonest or simply in the grip of an ideology, the ideology of scientific naturalism. He seems to think that the substance dualist, with his supposed postulation of spook stuff, is in some sort of unscientific competition with a scientific approach to the mind. But he can make this blunder only by presupposing something obviously false, namely, that the progress of natural science has shown, or is showing, that everything is at bottom physical in nature.