[dioib]

IMHO, whatever does not challenge the man will not contribute to his growth; but simply because his brain cannot survive in total chaos with no conforming pattern, he can't push it to the very limit, it's just not defined there. This leaves it up to him, to decide, how is he going to answer the (eventually) equivalent question of "what have you done with your age?". To me, everything can be broken down to "justifiable" in some way or another, and that's what renders the criterion futile.