This is one of my favourite proverbs. I quite often find myself turning it over in my own mind as some particularly dense person fails yet again to see the bleedin’ obvious.
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.