I’ve come across these two words in Plato’s texts (together with the word ἀμαθία which is mentioned to bring out his conception of ignorance as a malady of the soul), and become confused by their meanings which seem similar to each other although I feel that the word ἄνοια consists in a stronger emphasis on “lack of understanding” or “mindlessness” (because it implies the lack of νοῦς). What is the real difference between these words? Are there any philological/scholarly writings on these words? (I’ve found a piece of writing of this kind by R. Hackforth, titled “Moral Evil and Ignorance in Plato’s Ethics”, but it seems a bit laconic.)
In general, I would venture to say that ἄνοια means something closer to “folly” or even “madness” (as you note, “lack of νοῦς”, “mindlessness”, “being out of one’s mind”) rather than just “ignorance”, “lack of knowledge”, which would be ἄγνοια. However, though I’ve read a certain amount of Plato in Greek, I’m not an academic Plato specialist, and I’m not aware of any scholarly discussion of your question. I think an academic study would of course have to look at the way other authors use these terms, too.