I know that most men — not only those considered clever, but even those who are very clever and capable of understanding most difficult scientific, mathematical, or philosophic, problems — can seldom discern even the simplest and most obvious truth if it be such as obliges them to admit the falsity of conclusions they have formed, perhaps with much difficulty — conclusions of which they are proud, which they have taught to others, and on which they have built their lives.

What makes a subject difficult to understand — if it is significant, important — is not that some special instruction about abstruse things is necessary to understand it. Rather it is the contrast between the understanding of the subject and what most people want to see. Because of this the very things that are most obvious can become the most difficult to understand. What has to be overcome is not difficulty of the intellect but of the will. [Nicht eine Schwierigkeit des Verstandes, sondern des Willens ist zu überwinden.]

Be patient with the belligerence of the simple-minded. It is not easy to understand that one doesn’t understand

As for acquiescing in what a man understands not, or cannot reconcile to his reason, they know best the fruits of it that practice it. For my part, I'm a stranger to it, and cannot reconcile myself to such a principle. On the contrary, I am pretty sure he pretends in vain to convince the judgment, who explains not the nature of the thing. A man may give his verbal assent to he knows not what, out of fear, superstition, indifference, interest, and the like feeble and unfair motives: but as long as he conceives not what he believes, he cannot sincerely acquiesce in it, and remains deprived of all solid satisfaction. ... But he that comprehends a thing, is as sure of it as if he were himself the author.

The most ignorant are the most conceited. Unless a man knows that there is something more to be known, his inference is, of course, that he knows every thing. Such a man always usurps the throne of universal knowledge, and assumes the right of deciding all possible questions. We all know that a conceited dunce will decide questions extemporaneous which would puzzle a college of philosophers, or a bench of judges. Ignorant and shallow-minded men do not see far enough to see the difficulty. But let a man know that there are things to be known, of which he is ignorant, and it is so much carved out of his domain of universal knowledge. And for all purposes of individual character, as well as of social usefulness, it is quite as important for a man to know the extent of his own ignorance as it is to know any thing else. To know how much there is that we do not know, is one of the most valuable parts of our attainments; for such knowledge becomes both a lesson of humility and a stimulus to exertion.

Albert Einstein said, “If you can’t explain something simply, you don’t understand it well enough.

What a wise man knows seems so plain and simple to himself that he easily makes the mistake of thinking it to be so for others.

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 ability of nonintelligent people to understand the most complicated mechanisms and to use them has always been to me a cause of astonishment: their inability to understand simple questions is even more astonishing. The general acceptance of simple ideas is difficult and rare, and yet it is only when simple, fundamental, ideas have been accepted that further progress becomes possible on a higher level.

A man doesn't know what he knows until he knows what he doesn't know.

Two things are to be remembered: that a man whose opinions and theories are worth studying may be presumed to have had some intelligence, but that no man is likely to have arrived at complete and final truth on any subject whatever. When an intelligent man expresses a view which seems to us obviously absurd, we should not attempt to prove that it is somehow true, but we should try to understand how it ever came toseemtrue. Thisexercise of historical and psychological imagination at once enlarges the scope of our thinking, and helps us to realize how foolish many of our own cherished prejudices will seem to an age which has a different temper of mind.

I know that most men, including those at ease with problems of the greatest complexity, can seldom accept even the simplest and most obvious truth if it be such as would oblige them to admit the falsity of conclusions which they have delighted in explaining to colleagues, which they have proudly taught to others, and which they have woven, thread by thread, into the fabric of their lives.

The simple minded know many things that the wise ignore

I can’t understand anything in general unless I’m carrying along in my mind a specific example and watching it go. Some people think in the beginning that I’m kind of slow and I don’t understand the problem, because I ask a lot of these “dumb” questions: