definition

an expression in propositional calculus meant to represent one proposition as fully equivalent to another, ¶4-2-1. In the predicate calculus a d. reduces by shifting rank to a predicate with propositional arguments, ¶4-3-2. In this book d. connects syntax and semasiology by means of reference, ¶5-5-3. Cf. also equivalence. One may note that although d. is taken as a symmetric relationship, in practice it is often used asymmetrically: in the course of a proof one side (either one) may replace the other.