 The weak sortal relativity claim consists of saying that the identity relation for sets is different from those for numbers, which is different from those for people and artifacts like cars, and numbers. For sets it involves members of whatever kind, for numbers relations to numbers and for persons and artifacts continuity of life and of function. For abstract non-mathematical things the relation is even less clear—the question when two ideas are the same idea is often only settled in a court case about copyright or patent infringement. 