Blackwell Reference Online

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Typically used in philosophy to mean ‘numerical identity’. A thing x is numerically identical with a thing y if x and y are one and the same . Numerical identity is standardly contrasted with ‘qualitative identity’, the relationship that holds between two things that are qualitatively similar to a high degree. Identical twins are not numerically identical but they may be qualitatively identical (to a high degree). The present essay is a discussion of numerical identity. It is arguable, however, that even ‘numerical identity’ is ambiguous. Most philosophers hold that identity satisfies Leibniz's Law ( see identity of indiscernibles ), which says that if x is identical with y then x and y share all of their properties. But there seem to be ordinary examples in which identity does not satisfy Leibniz's Law. Suppose that a piece of rubber is used to makea rubber ball. It then seems correct to say, ‘The piece of rubber and the ball are one and the same.’ But the piece of rubber and the ball do not share all of their properties; for example, only the former has the property of having existed before the ball was created. So here we seem to have a case of (numerical) identity which violates Leibniz's Law. The simplest response to this point is to distinguish between two senses of ‘(numerical) identity’. In the sense relevant to Leibniz's Law, the piece of rubber and ... log in or subscribe to read full text