Memory

Memory is a mental instrument and depends on the formation of the brain. Your brain is constantly growing, unless it begins to degenerate, but still its growth can continue for a very, very long time, much longer than that of the body. And in this growth, necessarily some things will take the place of others. And as the mental instrument develops, things which have served their term or the transitory moment in the development may be wiped out to give place to the result.
So the result of all that you knew is there, living in itself, but the road traversed to reach it may be completely blurred. That is, a good functioning of the memory means remembering only the results so as to be able to have the elements for moving forward and a new construction. That is more important than just retaining things rigidly in the mind.

Ref: Questions and Answers 1954