Identity

Anything whatsoever has the relation of identity to itself, and to nothing else. Things are identical if they are one thing, not two. We can refute the claim that they are identical if we can find a property of one that is not simultaneously a property of the other. The concept of identity is fundamental to logic. Without it, counting would be impossible, for we could not distinguish in principle between counting one thing twice and counting two different things. When we have acquired the concept, it can still be difficult to make this distinction in practice. Misjudgments of identity are possible because one thing can be presented in many guises.

Identity judgments often involve assumptions about the nature of things. The identity of the present mature tree with the past sapling implies persistence through change. The non-identity of the actual child of one couple with the hypothetical child of a different couple is implied by the claim that ancestry is an essential property. Knowledge of what directions are involves knowledge that parallel lines have identical directions. Many controversies over identity concern the nature of the things in question. Others concern challenges to the orthodox conception just sketched of identity itself.