Law of identity
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In logic, the law of identity states that 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 attributed to Aristotle, who 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
The law of identity has deep impact on Aristotle's ethics as well. In order for a person to be morally praiseworthy or blameworthy for an action, he or she must be the same person before the act as during the act and after the act. Without the law of identity, Aristotle notes, there can be no responsibility for vice (see Nicomachean ethics).
