Identity and Reason

Law of Identity

Within the Western intellectual tradition, logic is usually understood as the formal expression of reason or the science of argument. That is to say, logic is a rigorous method through which principles of reason are explored and further explicated through the evaluation of arguments. Although there are a number of logical principles or laws that are seen as integral to a logical argument—the law of non-contradiction is another especially important one—the most basic is usually understood to be the law of identity.

The law of identity states that any term in a logical argument must be identical to itself. This is normally expressed as A = A. It is the most basic principle of any form of logic because it holds that our terms must be stable or always identical with themselves. So, if I use the term cat at the beginning of an argument, I can rest assured that it will mean the same thing at the end of the argument because the law of identity holds. If the law of identity did not hold, I could not be sure the terms I had previously used would have the same meaning at a future date. Again, although there are many laws important to logical argument, it seems as if all of them depend upon the terms of an argument remaining stable throughout the course of the argument. Thus, the law of identity is an indispensible concept for any system of logic and therefore a basic principle of reason.

The law of identity is usually thought to have emerged in the course of ancient Greek philosophy, especially in the writing of Parmenides (5th century BCE), and, somewhat later on, in Book VII of Aristotle's (4th century BCE) Metaphysics. Although Aristotle takes the law of noncontradiction as the most basic logical principle, he still gives us the first description of the law of identity.

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