Way back in the 1990s I heard a modern commentary on this Mishnah to the effect that it applies where a person possesses -- as most of us now do -- a reasonable set of sefarim from which he can easily and conveniently look up the things he forgets but he chooses not to do so, whether through lethargy or indifference.
Unfortunately I do not recollect who said this. Can any reader please point me to a source for this explanation?
