 Supervenience offers one modal formulation of physicalism, but it is worth taking note of a second modal formulation too. Suppose we say that a property G is necessitated by a property F just in case, in all possible worlds, if something is F then it is G; in this sense, for example, being red necessitates being colored, and being square necessitates having some extension in space. This suggests a formulation of physicalism along the following lines: