The Art of Poetry on a New Plan (1761), vol. ii. p. 147.
The saying "he who fights and runs away may live to fight another day" dates at least as far back as Menander (ca. 341–290 B.C.), Gnomai Monostichoi, aphorism #45: ἀνήρ ὁ ϕɛύγων καὶ ράλίν μαχήɛṯαί (a man who flees will fight again). The Attic Nights (book 17, ch. 21) of Aulus Gellius (ca. 125–180 A.D.) indicates it was already widespread in the second century: "...the orator Demosthenes sought safety in flight from the battlefield, and when he was bitterly taunted with his flight, he jestingly replied in the well-known verse: The man who runs away will fight again".