|Equivalence: Relation between sentences. It exists if both sides have the same truth value, so that they are both true or both false.|
|Geach, Peter T.
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Equivalence/Biconditional/GeachVsBlack "is material equi" is not synonymous with "if and only if" - (symbol with three lines) is often read as "materially equivalent" - equivalence only between sentences, not the names of sentences - Problem: "Tom loves Mary ↔ Mary loves Tom" is only significant if "↔" (three lines) is read as "iff" rather than "materially equi".
Logic Matters Oxford 1972