Swift must surely have had in his mind the passage in Horace (Ep. ii. 2. 140), where the Argive citizen upbraids his friends, when they had cured him of his madness, and had caused his 'mentis gratissimus error' to disappear. Cf. Swift's expression of this thought, not in the lightness of irony, but in the bitterness of despair, p. 60, 1. 19.