law of identity

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.)
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one of three principles required of logical thinking set down by Aristotle.  A thing is always itself.