In philosophy, supervenience refers to a relation between sets of properties or sets of facts. X is said to supervene on Y if and only if some difference in Y is necessary for any difference in X to be possible. Equivalently, X is said to supervene on Y if and only if X cannot vary unless Y varies. Here are some examples. In philosophy, supervenience refers to a relation between sets of properties or sets of facts. X is said to supervene on Y if and only if some difference in Y is necessary for any difference in X to be possible. Equivalently, X is said to supervene on Y if and only if X cannot vary unless Y varies. Here are some examples.