What interests me here is the analogy Rosenberg uses to get across his view of how the brain might produce the illusion of intentionality without there really being any intentionality at all. I think Rosenberg's analogy is actually a good way to show why intentionality can't be completely an illusion. Here is the analogy:
A single still photograph doesn't convey movement the way a motion picture does. Watching a sequence of slightly different photos one photo per hour, or per minute, or even one every 6 seconds won't do it either. But looking at the right sequence of still pictures succeeding each other every one-twentieth of a second produces the illusion that the images in each still photo are moving. Increasing the rate enhances the illusion, though beyond a certain rate the illusion gets no better for creatures like us. But it's still an illusion. There is noting to it but the succession of still pictures. That's how movies perpetrate their illusion. The large set of still pictures is organized together in a way that produces in creatures like us the illusion that the images are moving. In creatures with different brains and eyes, ones that work faster, the trick might not work. In ones that work slower, changing the still pictures at the rate of one every hour (as in time-lapse photography) could work. But there is no movement of any of the images in any of the pictures, nor does anything move from one photo onto the next. Of course, the projector is moving, and the photons are moving, and the actors were moving. But all the movement that the movie watcher detects is in the eye of the beholder. That is why the movement is illusory.
The notion that thoughts are about stuff is illusory in roughly the same way. Think of each input/output neural circuit as a single still photo. Now, put together a huge number of input/output circuits in the right way. None of them is about anything; each is just an input/output circuit firing or not. But when they act together, they "project" the illusion that there are thoughts about stuff. They do that through the behavior and the conscious experience (if any) that they produce.
The sentence "Of course, the projector is moving, and the photons are moving..." is written in passing put it points out a consideration fatal to the analogy. Without the actual motion of the projector and the photons, there wouldn't be any illusory motion through the film. The projector stops, the illusion stops. So the illusory motion in the film, far from providing evidence that there is no actual motion, is conclusive proof that there is actual motion in the world. Yes, the viewer may be mistaken as to the actual nature of the motion, but Rosenberg's point with the analogy is that there is no intentionality at all in the world. For the analogy to support such a conclusion, it must support the analogous conclusion that all motion is an illusion. It does just the opposite.
The standard fallback here is to say that all analogies are limited. True, they are, but in a good analogy, those limitations arise only when the analogy is pushed beyond the limited point it is designed to make. The fact that in Rosenberg's analogy the illusory motion of a film proves rather than refutes the actual reality of motion is not an irrelevant point, but is the point. And it is not an accident. There is no way to design an analogy showing the illusion of motion without also establishing the actuality of motion.
The deeper point is that we can't be mistaken about fundamental categories like motion (or change).
This is Aristotle's answer to Parmenides. Change is an undeniable metaphysical reality, for if there were no such thing as motion, then we couldn't possibly know it, since thought itself is a kind of motion. (And if you say that thought is an illusion, and the underlying matter is unchanging, then you need to explain how a non-moving projector can produce the illusion of movement in a film). We may be mistaken about the content of change, but the fact of change is literally self-evident.
Neither can we be mistaken about the fact of intentionality. It is really as simple as saying that if there were no intentionality in the world, there wouldn't be any intentionality, and we wouldn't experience any. For the illusory intentionality must have a source in some real intentionality, just as illusory motion must have a source in real motion. If there weren't some real intentionality somewhere, neither would there be any illusory intentionality anywhere. "Intentionality" would be something that simply didn't exist in the world even as an idea. The discovery of counterfeit money is not proof that there is no real money, but that there must be real money, for there is no sense in counterfeiting something that doesn't exist. Are you in any danger of falling for a counterfeit Martian dollar?
From the subjective side, we are only subject to the illusion of motion in the film because it mimics actual motion in the world. If there were no actual motion in the world, just what would illusory motion in the film mean to us? We are receptive to the illusion of motion in the film because it appeals to a part of our nature that is receptive to actual motion. Animals who live for generations in the darkness of a cave or the depths of the sea gradually lose their sight as it is of no use to them. Eventually they are unable to detect light at all. It disappears for them. At this point, they are in no danger of falling for "fake light" (whatever that might be) because they can't detect any light at all. The creature loses his ability to fall for the illusion as he loses his ability to detect the reality. The fact that we experience intentionality at all is proof enough that there is something "intentional-like" in reality.
