Postscript on qualia
Why do I think that the sensory side of psychology, as it is constituted in our world, is deducible in principle from enough about the world' s physical nature? Our knowledge of the sensory side of psychology has a causal source. Seeing red and feeling pain impact on us, leaving a memory trace which sustains our knowledge of what it is like to see red and feel pain on the many occasions where we are neither seeing red nor feeling pain. This is why it was always a mistake to say that someone could not know what seeing red and feeling pain is like unless they had actually experienced them: false 'memory' traces are enough. This places a constraint on our best opinion about the nature of our sensory states: we had better not have opinions about their nature which cannot be justified by what we know about the causal origin of those opinions. Now the precise connection between causal origin and rational opinion is complex, but for present purposes the following rough maxim will serve: do not have opinions that outrun what is required by the best theory of these opinions' causal origins. 1 Often it will be uncertain what the best theory is, or the question of what it is will be too close to the question under discussion for the maxim to be of much use. But in the case of sensory states, the maxim has obvious bite. We know that our knowledge of what it is like to see red and feel pain has purely physical causes. We know, for example, that Mary' s transition from not knowing what it is like to see red to knowing what it is like to see red will have a causal explanation in purely physical terms. (Dualist interactionism is false.) It follows, by the maxim, that what she learns had better not outrun how things are physically.