And yet I discover that it doesn’t appear to be a “standard” proverb, at least as revealed by 5 mins of not-very-exhaustive google searching. If you know better, tell me.
The meaning, of course, is that once a certain minimal level of literal or metaphorical illumination has been shed on a subject, increasing the level of illumination or quantity of explanation will not allow the foolish to understand any more. Very useful for the GW debate.
I know it from John Crowley’s masterpiece, The Deep. Perhaps Crowley invented it.
[Late exciting update: see http://www.freieskunstforum.de/hosch_2008_kauffman_brugger_bild.pdf, p 47, footnote 68: "Eigentlich Zeichen der Weisheit, des Durchblicks aber nur im Dunkel, vgl. Emblem aus Gabriel Rollenhagen, Nucleus emblematum, 1613, Nr. 95: „Caecus nil luce iuvatur“ - Dem Blinden helfen weder Fackeln noch sonstiges Licht, die törichte Eule sieht am hellen Tage nicht.; vgl. Henkel/Schöne, Emblemata, Stgt. 1967/1996, Sp. 896, vgl. Abb. 27a." Its close I tell you! I've added the pic.
Oh dear. It turns out that this is the top google hit for "Caecus nil luce iuvatur" so I'd better provide my gloss for it. Caesus is blind; luce is light. Iuvatur is some conjugation of Iuvo, to help. Thus, "light doesn't help the blind".]