### Single Idea 9535

#### [catalogued under 4. Formal Logic / B. Propositional Logic PL / 1. Propositional Logic]

Full Idea

Two propositions are 'contradictory' if they are never both true and never both false either, which means that ¬(A↔B) is a tautology.

Gist of Idea

'Contradictory' propositions always differ in truth-value

Source

E.J. Lemmon (Beginning Logic [1965], 2.3)

Book Reference

Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.70