There is supervenience when and only when there cannot be a difference of some sort A (for example, mental) without a difference of some sort B (for example, physical). When there cannot be an A-difference without a B-difference, then but only then A-respects supervene on B-respects. Supervenience claims are thus modal claims. They are claims to the effect that necessarily, there is exact similarity in A-respects whenever there is exact similarity in B-respects. So if, for example, mental properties supervene on physical properties, then, necessarily, individuals that are physically indiscernible (exactly alike with respect to every physical property) are mentally indiscernible (exactly alike with respect to every mental property). Thus, A-properties supervene on B-properties just in case how something is with respect to A-properties is a function of how it is with respect to B-properties.

Supervenience has been invoked in nearly every area of analytical philosophy. In addition...