Law of Identity

The Law of identity states that each thing is identical with itself. It is the first of the historical three laws of thought, along with the law of noncontradiction, and the law of excluded middle.

Quotes

 * The proposition A = A is axiomatic and self-evident. It is the primitive measure of truth for all other propositions; however much we may think over it we must return to this fundamental proposition. It is the principle of the distinction between truth and error; and he who regards it as meaningless tautology, as was the case with Hegel and many of the later empiricists (this being not the only surprising point of contact between two schools apparently so different) is right in a fashion, but has misunderstood the nature of the proposition. A = A, the principle of all truth, cannot itself be a special truth. He who finds the proposition of identity or that of non-identity meaningless does so by his own fault. He must have expected to find in these propositions special ideas, a source of positive knowledge. But they are not in themselves knowledge, separate acts of thought, but the common standard for all acts of thought. And so they cannot be compared with other acts of thought. The rule of the process of thought must be outside thought. The proposition of identity does not add to our knowledge; it does not increase but rather founds a kingdom.
 * Otto Weininger, Sex and Character (1906)


 * Memory, then, is a necessary part of the logical faculty. … The proposition A = A must have a psychological relation to time, otherwise it would be At1 = At2.
 * Otto Weininger, Sex and Character (1906)