Mind: A Brief Introduction (John R. Searle) / quote 54 of 138
Leibniz's law against the identity theory
There were a number of objections to the identity theory. I find it useful to distinguish between the technical objections and the common-sense objections. The first technical objection was that the theory seemed to violate a principle of logic called “Leibnitz’s Law.”11 The law says that if any two things are identical, then they must have all their properties in common. So if you could show that mental states had properties that could not be attributed to brain states, and brain states had properties that could not be attributed to mental states, it looks like you would refute the identity theory.