“Every sentence I utter must be understood not as an affirmation but as a question.[A caution he gives his students, to be wary of dogmatism.]”

“There are trivial truths and there are great truths. The opposite of a trivial truth is plainly false. The opposite of a great truth is also true.”

“Prediction is very difficult, especially about the future.”

“The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.”

“You can recognize a small truth because its opposite is a falsehood. The opposite of a great truth is another truth.”