I must be stupid," I said. "I don't doubt that his talk is full of profound hidden truths, but I find it terribly hard to understand."
Dikaiarchos laughed. "I'll tell you a secret, Chares. I cannot understand it, either."
"You, sir?"
"Yes. It may be that we are both stupid, or it may be that there is nothing to understand. It is a good practical rule that, if a man cannot explain himself in terms that any reasonably intelligent listener can comprehend, he doesn't know what he is talking about himself.

“The trouble with you youngsters,” Joe said, “is that if you can’t understand a thing right off, you think it can’t be true. The trouble with your elders is, anything they didn’t understand they reinterpreted to mean something else and then thought they understood it. None of you has tried believing clear words the way they were written and then tried to understand them on that basis. Oh, no, you’re all too bloody smart for that—if you can’t see it right off, it ain’t so—it must mean something different.”

In my immature brain, I didn’t understand the difference between intelligence and knowledge. So I assumed I was an idiot.

Will you understand what I'm going to tell you? ... No, you're not going to be able to understand it. ... That is because I don't understand it. Nobody does.

Progress is a possibility for the animal: it can be broken in, tamed and trained; but it is not a possibility for the fool, because the fool thinks he has nothing to learn. It is his place to dictate to others and put them right, and so it is impossible to reason with him. He will laugh you to scorn in saying that what he does not understand is not a meaningful proposition. 'Why don't I understand it, then?', he asks you, with marvellous impudence. To tell him it is because he is a fool would only be taken as an insult, so there is nothing you can say in reply. Everybody else sees it quite clearly, but he will never realize it.<p>Here then, at the outset, is a potent secret which is inaccessible to the majority of people; a secret which they will never guess and which it would be useless to tell them: the secret of their own stupidity.

The root difficulty in all cases was the state of being blind and deaf to words — not seeing the words for the prose. Being adults, they had forgotten what every child understands, which is giving and taking a meaning is not automatic and inevitable

You are clever man, friend John; you reason well, and your wit is bold; but you are too prejudiced. You do not let your eyes see nor your ears hear, and that which is outside your daily life is not of account to you. Do you not think that there are things which you cannot understand, and yet which are; that some people see things that others cannot? But there are things old and new which must not be contemplate by men’s eyes, because they know – or think they know – some things which other men have told them. Ah, it is the fault of our science that it wants to explain all; and if it explain not, then it says there is nothing to explain. But yet we see around us every day the growth of new beliefs, which think themselves new; and which are yet but the old, which pretend to be young – like the fine ladies at the opera.

Someone who is using a lot of fancy words and big concepts probably doesn’t know what they’re talking about. The smartest people can explain things to a child; if you can’t do that, you don’t understand the concept.

When a wise man does not understand, he says: "I do not understand." The fool and the uncultured are ashamed of their ignorance. They remain silent when a question could bring them wisdom.

They were presented in enigmas, clad in riddles, and taught by an wise men in the most mysterious way that could be devised, not because they contain some secret evil, or are contrary to the fundamental principles of the Law (as fools think who are only philosophers in their own eyes), but because of the incapacity of man to comprehend them at the beginning of his studies:

In short, it is not merely that Johnny can't read, or even that Johnny can't think. Johnny doesn't know what thinking is, because thinking is so often confused with feeling in many public schools. [emphasis in the original]

They're talking about things of which they don't have the slightest understanding, anyway. It's only because of their stupidity that they're able to be so sure of themselves.

If you cannot understand something, then you have understood it incorrectly.

“I talked with it, boss. It’s stupid.”
“I can’t see that you have established that. Assuming that an e.-t. is stupid because he can’t speak our language well is like assuming that an Italian is illiterate because he speaks broken English. A non-sequitur.”