 This issue has received less attention in the literature than the questions we have been studying so far. But it is just as important. Without any understanding of what the physical is, we can have no serious understanding of what physicalism is. After all, what does it mean to say that everything is physical, as opposed to chemical or financial. (The point here is quite general: if Thales says that everything is water, we don’t understand what he says unless he says something about what water is. The physicalist is in the same boat.)