Notes from Wikiquote
Cf. Shakespeare, Romeo and Juliet, act II, sc. iv, l. l. 186. Titus Andronicus, act 4, sc. 2, l. 144: "Two may keep counsel when the third's away." Herbert, Jacula Prudentum. Heywood, Proverbs, pt. 2, ch. 5. Franklin, Poor Richard (1735): "Three may keep a secret if two of them are dead."