 It is desirable to have a more explicit statement of physicalism, and here too Lewis’s example gives us direction. He says that, in the case of the picture, supervenience means that “no two pictures can be identical in the arrangement of dots but different in their global properties”. Similarly, one might say that, in the case of physicalism, no two possible worlds can be identical in their physical properties but differ, somewhere, in their mental, social or biological properties. To weaken this slightly, we might say that if physicalism is the case at our world – that is, is true of our universe and everything in it – then no other world can be physically identical to our world without being identical to it in all respects. This suggests the following general account of what physicalism is (in the following formulation and in subsequent ones, we use “iff” to abbreviate “if and only if”):