When I was young I could remember everything just by studying it repeatedly. Now I basically cannot remember anything (except what I learned well when young) unless I "discover it myself". The quotes mean that I need not really discover it independently, it is enough to sit down with a pen and try to work it out alone, even if drawing subconsciously on latent memories of previous knowledge of it. It also helps to try to simplify it so much that I can teach it to a bright but unsophisticated person, like a young child.

Nonetheless, I will still forget it again after some time away from it, so it is very helpful, as stated above, to write it out very clearly, in a recoverable medium, once I have mastered it.

I also find that things that are really clearly explained, simply and succinctly, by a great master, are much more easily recalled, since the master's version contains exactly the heart of the matter. E.g. I still cannot forget Lagrange's explanation of the quadratic formula (due to Diophantus?), Euler's explanation of "Cardano's" cubic formula, Auslander's explanation of Nakayama's lemma, Riemann and Roch's exposition of their theorem, Euclid's discussion of the concept of gcd of two integers, Kempf's exposition of Mumford's proof of the Riemann singularities theorem, etc,....