By “physicalism” I mean that doctrine which says that only the physical or material world exists and that all causes are physical causes. By “rationalism” I mean that doctrine which says that we have access to information about the physical world which does not come from our physical senses alone. By “empiricism” I mean the denial of rationalism.
Physicalism and empiricism are often thought to go together, but in fact a consistent empiricist must deny that there is any evidence for physicalism. To see this, we note that the empiricist is committed to the position that all of our knowledge comes from our physical senses; there is a sort of wall of separation between our minds and the physical world such that all information about the physical world must come through a fixed and limited sort of pipeline of information, namely the senses.
Our epistemological situation as envisioned by the empiricist can be modeled as follows: a man sits in a room which has a slot in the wall. A paper tape issues from the wall, and on that tape is a sequence of letters. As time goes on, more of the tape issues from the wall with more letters. The man has no information about where the tape comes from or how the letters are being produced.
Suppose now that by examining the tape, the man is able to discover a pattern. Say that a sequence of three A’s is always followed by a B. Clearly the man can use discovered patterns to predict future letters, so long as the pattern continues to be followed, but what can the man infer about the causality that leads to the pattern? He has no idea how the pattern is being produced; it could be by another person, by a computer (in which case he has no knowledge about the algorithm), by some sort of mechanical device, or anything else.
It would be reckless for the man to come to any conclusions about the cause behind the pattern, but it would be more than reckless–it would be downright bizarre–to say that the letters themselves cause the pattern. It would be odd to say that the A’s combine their forces in some way to produce a B.
This bizarre position is the position of an empiricist physicalist. The empiricist can see nothing but the letter, which corresponds to a physical phenomenon; the mechanism behind the wall–the noumenon– is entirely unavailable. As a physicalist, he concludes that the letters themselves–the phenomena–are the causes. This is a great leap of faith, and a bizarre one, for an empiricist to take since nothing in his observation, pared of all intuition and expectation, can possibly tell him that phenomena have causal power of any sort. All his senses tell him is that phenomena exist and follow patterns.
The only way to justify the idea that phenomena have causal power is to rely on physical intuition. When we take a stick and push a ball, we have an immediate sense that it was our hand that made the stick move and the stick that made the ball move, but relying on such intuition is a rationalist position, not an empiricist position.
So, just the position that some phenomena are caused by other phenomena, is a rationalist position, must less full-on physicalism which asserts that all phenomena are caused by other phenomena. Even more a violation of empiricism is the idea that there is nothing behind the wall at all. How could an empiricist possibly know? Consequently, anyone who is a physicalist can’t be considered an empiricist.