Normajean: You once summarized Quine stating that physical facts do not logically entail mental facts, just as physical facts do not logically entail moral facts. Getting an "about" from an "is" is just as impossible as getting an "ought" from an is, and for much the same reason.
VR: The analogy to the problem of getting an ought from an is is mine, not Quine's. But the idea is that many people argue, and I think they are right about this, that if you add up all the physical truths, broadly conceeived, you cannot get to any conclusion like "Jones ought to stop beating his wife." The physical facts are facts of the wrong type to entail any moral truths. Such truths, if they existed would, as J. L. Mackie puts it, be queer pieces of furniture in a physicalistic world.
At the same time, if you add up all the physical facts, it doesn't seem to me that what someone's thought is about is strictly entailed. Physical facts, by their very nature, are going to underdetermine mental states like beliefs, and even just the entertaining of propositions. Whatever the state of the physical world is, it is compatible with a multiplicity of mental states, or even, with no mental states at all (in which case we'd all be zombies). Physical states don't entail the existence of determinate mental states. But, whatever exists must be determined by the physical, then it follows that there is no fact of the matter as to what our thoughts are about, and that has all sorts of disturbing implications. For example, it means that we don't literally add, subtract, multiply, divide, or take square roots of numbers.