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."
I fully agree with Dr.Vallicella. It's simply implausible that an smart and seasoned professional philosopher like Dennett would actually believe in so silly positions and arguments against dualism. Even if dualism is false, you need to argue against its better defenders and in its most charitable formulation, not against straw men or easy targets.
So, the most reasonable position is to think that Dennett is driven by an ideology (the ideology of ontological materialism and metaphysical naturalism) and, fully knowing that most academic philosophers share his ideology, they won't critically examine his simplistic anti-dualistic arguments or dismissals.
And just for the record, there are exist some contemporary nonreligious philosophers who defend dualism. An example is philosopher Robert Almeder (philosophy professor in Georgia State University), who defends dualism and afterlife on empirical grounds:
Hear also this long interview with professor Almeder here.
Another contemporary philosopher who defends dualism is David Lund (see for example his book in defense of an immaterial self and his lastest book about the empirical evidence for an afterlife.).
Also, you'll find sophisticated defenses of dualism by leading philosophers of religion, like Alvin Plantinga:
So it's simply and demostrably false Dennett's assertion that "you won't find any place to stand and fight for these obsolete ideas" (this is another example of Dennett's misdirection and misrepresentation of the facts)
Links of interest:
-My post on Thomas Nagel and the cosmic authority problem
-Paper by physicist Eugene Wigner on the scientific case for dualism.
-Philosopher Chris Carter's paper on consciousness and dualism.
-My post on one of Daniel Dennett's arguments against dualism.