substitutivity

A logical relationship in which two terms can be mutually substituted without affecting the truth value of any propositions in which the terms occur, thereby establishing that the terms are identical.
One of the fundamental principles governing identity is that of substitutivity—or, as it might well be called, that of indiscernibility of identicals. It provides that, given a true statement of identity, one of its two terms may be substituted for the other in any true statement and the result will be true.
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