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. Caecus is blind; luce is light. Iuvatur is some conjugation of Iuvo, to help. Thus, “light doesn’t help the blind”.]
[Even later update: Arguing on fb, I was pointed at “the blind shall not see” as a “sematic equivalent”. That appears to be uncommon, too. About the only ref I can find is Natural Selection and Survival of the Fittest (continued 2) from some people who don’t like atheists – Al Islam. However, that’s close in sense to “light doesn’t help the blind” I had above.]
* However, to avoid too much offence to non-fools, I need to note that I’ve taken to pointing people at this when all I mean is “I’ve explained it as well as I can; if you can’t get it from here I’m not going to try to explain any further”. Although it still carries a connotation of “you’re not trying hard enough” or “you prejudices are getting in the way of reading what is perfectly clear”.