substitutivity

substitutive +‎ -ity

substitutivity (uncountable)

1. (philosophy) 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.
   • 1943, Willard V. Quine, "Notes on Existence and Necessity," The Journal of Philosophy, vol. 40, no. 5, p. 113:

     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.

• principle of substitutivity

• Oxford English Dictionary, 2nd ed., 1989.