Law of identity

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In logic, the law of identity states that an object is the same as itself: A ≡ A

Any reflexive relation upholds the law of identity. When discussing equality, the fact that "A is A" is a tautology.

In philosophy, the law is often mistakenly attributed to Aristotle, who actually wrote:

"Now 'why a thing is itself' is a meaningless inquiry (for—to give meaning to the question 'why'—the fact or the existence of the thing must already be evident—e.g., that the moon is eclipsed—but the fact that a thing is itself is the single reason and the single cause to be given in answer to all such questions as why the man is man, or the musician musical, unless one were to answer, 'because each thing is inseparable from itself, and its being one just meant this.' This, however, is common to all things and is a short and easy way with the question.)"

—Metaphysics/Book VII#Part 17

Wilhelm Wundt credits Gottfried Leibniz with the symbolic formulation, "A is A.

In 2002 Jonathon Keats held a petition drive to pass 'A = A' as statutory law in Berkeley, California. Specifically, the proposed law stated that, "every entity shall be identical to itself". Any entity caught being unidentical to itself was to be subject to a fine of up to one tenth of a cent. The law did not pass. However it did spark a copycat petition drive in Santa Cruz, California.

See also

• Aristotle's Organon
• Equality (mathematics)
• Laws of thought
• Ayn Rand and Objectivism
• John Searle
• Hilary Putnam
• David Kaplan
• Keith Donnellan
• W.V. Quine
• Saul Kripke

References

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Last updated on Tuesday February 05, 2008 at 15:47:15 PST (GMT -0800)
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