I ran across this interesting post by a historian who was an undergraduate mathematics major. She found her old linear algebra notes, and was surprised to find how little of it she still understood:
Itâ€™s not just that I canâ€™t answer this question now, itâ€™s that I can barely comprehend even what it means. The terminology bounces through my brain, stirring vague imprecise echoes… Itâ€™s disconcerting enough to come across the ghost of your own self. It would be even more disconcerting to know for sure that part of your intellect is now forever closed off to you.