What I'm going to tell you now is what we tell physics students in their third or fourth year of graduate school. I will tell them. You think you'll understand too, do you? No, you will not understand anything. So why am I bothering you with all this stuff? Why are you sitting across from me for so long when you can't understand what I'm going to say? My job is to convince you not to turn your back just because you don't understand. Look, my physics students don't understand this. Because I don't understand, nobody understands.
Quantum Electrodynamics - The Cat: The Strange Theory of Light and Matter
I remember the effort it took to get a real degree from Princeton, and getting honorary degrees without any effort on the same platform as me, and I felt that the so-called "honorary diploma" devalued the idea of "a diploma that certifies the accomplishment of a particular job". It's kind of like an "honorary electrician certificate" I think. If one day I was offered such a diploma, I swore that I would not accept it.
Dr. Letter to George W. Beadle (Feynman with Letters)
I truly believe that quantum mechanics is a fundamental truth and that all this is actually a psychological problem. It is extremely difficult to get used to it. Because it is a requirement of our common sense and general knowledge that something should behave one way or the other when you are not looking. You say: “What do you mean, can't we even say 'it's like this or that' when we're not looking? It must be one of the two.” You can't say no because you're in trouble! This might not be such a bad thing. So prayer is not what we think it is.
Lecture given at the Esalen Institute, “Quantum Mechanics' View of Reality (Part 1)” October 1984
Now it's time for theoretical physics' greatest embarrassment. Field theories have been around for the last twenty-five years—in fact, even more so, almost forty years—but no one can still calculate their consequences with certainty. For example, even the predictions of Yukawa's field theory cannot be calculated exactly. The only reason we can make approximate calculations in electrodynamics is because the coupling is small. We can apply the series expansion to the coupling constant. For example, if we do not apply series expansion, it seems that we are too stupid to calculate the results. That's a sin. And it's one of the reasons we haven't made much progress.
Lecture on particles at Caltech. 1973