But even when his mistakes and failures are to be explained in ophthalmological or optical terms, this fact does not by itself prove the disheartening general proposition that nobody’s eyes are ever any good for anything, with its tacitly implied rider that no proof-readers can really find out for certain whether there are misprints on a page or not. The existence of disabilities is evidence against and not in favour of the non-existence of abilities.
Source: wiktionary