‘To assess physicalists' reasons for dismissing psychology as non-physical, and thus ontologically inconsequential, we must ask what makes them classify their favoured sciences as physical. What makes them count as physical not only the many diverse branches of physics itself (mechanics, electromagnetism, thermodynamics, gravity, and particle physics), but also sciences like chemistry and molecular biology? One common answer is that these other sciences are physical because they reduce to physics, which for present purposes we may take to mean that a physics enhanced with suitable bridge principles (to link its vocabulary to theirs) would entail credible approximations of all their established laws. … Some theories in other physical sciences have indeed been reduced to physics in this sense … but by no means all. But those for whom reduction to physics is the touchstone of the physical do not propose to do it in practice. They simply insist that it can be done 'in principle'. But what is the principle? It cannot be physicalism. These sciences cannot be reducible in principle because they are physical if reducibility in principle (RIP) is supposed to tell us where the bounds of the physical lie. So what other principle will tell us which sciences could 'in principle' be reduced to physics? To answer this, we must first ask to what physics the RIP principle is supposed to be applied: to present physics, or to some hypothetical future physics? This question poses a dilemma. For applying the principle to present physics entails that any future extensions of it would not be physical: that physics, the paradigm physical science, is already complete. But no one believes this. And if we apply the principle to an otherwise unspecified future physics, we shall not be able to say which sciences are physical until we know which of them that physics must cover-which is just what the principle was supposed to tell us. To use RIP to future physics to say what that physics must cover if it is to cover everything physical is obviously viciously circular. So the physical cannot be defined as what is reducible in principle to physics, either present or future.’