just wanted to share this quora response -->
What did Richard Feynman mean when he said, "What I cannot create, I do not understand"?
Feynman answered this question himself.
The quote is taken from his blackboard at the time of his death. Right underneath, it says, "Know how to solve every problem that has been solved."
When Feynman said "create", he did not literally mean that in order to understand particle physics, he had to go Tony Stark on us and build his own accelerator. Instead, he meant that, starting with a blank piece of paper and the knowledge already in his mind, he could take any theoretical result and re-derive it. ("Any" is probably an exaggeration, but he could likely derive whatever he was interested in.)
Feynman thought that ability was the true marker of understanding something because the only way to be able to work something out yourself is to have a firm understanding of each step of the reasoning involved. Further, if you try this, even with relatively simple concepts you think you understand well already, you'll find that you frequently come away from the process with a much deeper appreciation for the problem.
An even more extreme position from Feynman was:
Feynman meant here that understanding something is not just about working through advanced mathematics. One must also have a notion that is intuitive enough to explain to an audience that cannot follow the detailed derivation.
I've seen a few more sources that spell out Feynman's position on this in detail.
The spinning plate story describes how Feynman felt that curiosity about simple things, and working them out for himself, helped him retain an attitude of play towards his professional work that got him out of a slump:
http://books.google.com/books?id=7papZR4oVssC&lpg=PP1&dq=surely%20you're%20joking%20mr.%20feynman&pg=PA173#v=onepage&q=plate&f=false
Feynman's Tips on Physics, an extension to the Feynman lectures, has a chapter about how to learn physics, emphasizing that memorizing formulas is hopeless in the long run, and that by knowing a few key things and understanding the principles, you can work out whatever details you need. I can't find this chapter online, though.
Finally, Feynman's Lost Lecture is a fantastic example of precisely what Feynman meant. In it, he describes his own elementary proof that the inverse square law for gravity leads to elliptical orbits.