Rabbi Hanina used to say “I learned a lot from my teachers, and from my friends more than my teachers, and from my pupils the most.” I believe this means that there are in fact three levels of learning:
Level 1 - Learning - this is a passive learning of the material, where one inputs the material.
Level 2 - Experiencing - in this level you work with the material you learned, and try to implement what you’ve learned and integrate it. This requires more understanding, because you have to work with the material.
Level 3 - Teaching - in this level you teach the material to someone else. This requires the most understanding because you need to organise it properly and convey it to someone else.
Perhaps there’s a fourth level - Science in which the knowledge is expanded. However, this implies that to truly understand the material, one needs to experiment with it (preferably in production) and better yet teach it to someone else.
The old adage “He who can - does. He who cannot - teaches.” which was said by George Bernard Shaw is amusing, but simply not true, as I’ve demonstrated here. Being a great teacher is much more difficult than being a great doer, and is much more enlightening. [those_who_can]
[those_who_can] What is true, in my opinion. is that “Those who can - do. Those who can’t - complain.” However, often people who can and do, still complain. I recall this quote being attributed to Linus Torvalds , but it predates him.