One can see that with half an eye.

Polybius, 18, 17, 4. Rejoinder of Philip V (of Macedon) to the one-eyed Ætolian commander, Phæneas, in the Second Macedonian War, 198 BC | Or, as related in Livy, 32, 34, 3, Apparet id quidem etiam cæco.—"Even a blind man can see that." | Cited in Classical and Foreign Quotations (1904), no. 141