From QED:
The next question is, will you understand what I'm going to tell you? Everybody who comes to a scientific lecture knows they are not going to understand it, but maybe the lecturer has a nice, colored tie to look at. Not in this case! (Feynman is not wearing a tie.)
What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school--and you think I'm going to explain it to you so you can understand it? No, you're not going to be able to understand it. Why, then, am I going to bother you with all this? Why are you going to sit here all this time, when you won't be able to understand what I am going to say? It is my task to convince you not to turn away because you don't understand it. You see, my physics students don't understand it either. That is because I don't understand it. Nobody does.
I like how honest Richard Feynman is. And when he’s talking about he himself not understanding quantum electrodynamics, he means that he doesn’t understand *why* it is the way it is. He understands the how.