In examining consciousness from a materialist perspective, Rosenberg concludes that there is no such thing. This is because neurons simply by firing can’t really be about anything outside of themselves, in the way we think of our thoughts as being about things.
In using the example of thinking about Paris, he writes:
The Paris neurons aren’t about Paris in the same way, for example, that a picture postcard or a diorama or pop-up book’s three-dimensional layout is about Paris. (The Atheist’s Guide to Reality, pp. 174-175)
He goes on to consider the idea that some other neurons might interpret those neurons as being about Paris (the way we interpret language). But this leads him to the problem that the neurons doing the interpreting would first have to know what Paris is. That is, they would have to have a thought about Paris. Thus, he adds:
What we need to get off the regress is some set of neurons that is about some stuff outside the brain without being interpreted— by anyone or anything else (including any other part of the brain)— as being about that stuff outside the brain. (ibid, pp. 178-179)
There is simply no way of doing this while adhering to materialism.
Rosenberg is so convinced of materialism that he concludes that humans don’t actually have thoughts about anything. Now, if you find that a long list of objections to this conclusion forming in your mind, you are not alone. Many issues have been raised here, but space only permits me to address the most basic two:
First, the existence of one’s own thoughts is undeniable. It is not simply that it is better evidence than materialism, but that it is something we cannot even doubt without assuming it to be true. What is doubt, after all, if it doesn’t involve thinking about an idea outside of one’s neurons?
Second, because we cannot doubt our thought without assuming it, Rosenberg’s argument is actually logically incoherent. Though it follows from his materialism, this argument undermines itself. If neither Rosenberg nor the reader can think about, say, the idea that our thoughts might be illusions, there is no reason at all to believe it is true.
So, if materialism contradicts the idea that we have thoughts, then, so much the worse for materialism.