The rank absurdities of Alex Rosenberg's The Disenchanted Naturalist's Guide to Reality are being subjected to withering criticism at Ed Feser's weblog here, here and here. But a correspondent wants me to throw in my two cents, so here's a brief comment.
In the ComBox to the article linked to above, Rosenberg, responding to critics, says this among other things:
If beliefs are anything they are brain states—physical configurations of matter. But one configuration of matter cannot, in virtue just of its structure, composition, location, or causal relation, be “about” another configuration of matter in the way original intentionality requires (because it cant [sic] pass the referential opacity test). So, there are no beliefs.
This is a valid argument. To spell it out a bit more clearly: (1) If beliefs are anything, then they are brain states; (2) beliefs exhibit original intentionality; (3) no physical state, and thus no brain state, exhibits original intentionality; therefore (4) there are no beliefs.
But anyone with his head screwed on properly should be able to see that this argument does not establish (4) but is instead a reductio ad absurdum of premise (1) according to which beliefs are nothing if not brain states. For if anything is obvious, it is that there are beliefs. This is a pre-theoretical datum, a given. What they are is up for grabs, but that they are is a starting-point that cannot be denied except by lunatics and those in the grip of an ideology. Since the argument is valid in point of logical form, and the conclusion is manifestly, breath-takingly, false, what the argument shows is that beliefs cannot be brain states.
Now why can't a smart guy like Rosenberg see this? Because he is in the grip of an ideology. It is called scientism, which is not to be confused with science. (Rosenberg talks nonsense at the beginning of his piece where he implies that one does not take science seriously unless one embraces scientism.) Rosenberg thinks that natural-scientific knowledge is the only knowledge worthy of the name and, to cop a line from Wilfrid Sellars, that "science is the measure of all things, of what is that it is, and of what is not, that it is not." (Science, Perception and Reality, p. 173). That is equivalent to the view that reality is exhausted by what natural science (physics, chemistry, evolutionary biology) says exists. This is why Rosenberg thinks that, if beliefs are anything, then they are brain states. Given scientism, plus the assumption (questioned by A. W. Collins in The Nature of Mental Things, U of ND Press, 1987) that beliefs need to be identified with something either literally or figuratively 'inner,' what else could they be? Certainly not states of a Cartesian res cogitans.
The trouble with scientism, of course, is that it cannot be scientifically supported. 'All genuine knowledge is natural-scientific knowledge' is not a proposition of any natural science. It is a bit of philosophy, with all the rights, privileges, and debilities pertaining thereunto. One of the debilities is that it is self-vitiating. For if all genuine knowledge is scientific knowledge, then that very proposition, since it is not an item of scientific knowledge, cannot count as a piece of genuine knowledge. Nor can it ever come to be known.
That won't stop people like Rosenberg from believing it as they are entitled to do. But then scientism it is just one more philosophical belief alongside others, including others that imply its negation.
I think it is clear what a reasonable person must say. The (1)-(4) argument above does not establish (4), it reduces to absurdity (1). The only support for (1) is scientism which we have no good reason to accept. It is nothing more than a bit of ideology.
