What I cannot create, I do not understand

Near the end of his life, Richard Feymann left a terse statement on the blackboard: "What I cannot create, I do not understand."

What he meant, apparently, is that unless you can reconstruct every step, you don't really understand it. Understanding is a form of reverse engineering. Working back from the end-product to how everything functions. 

Unbelievers often make glib remarks about "design flaws" in nature. To which I always say, "Let's see you construct a better alternative."