Memory

Memory is a crucial part of human identity. Maybe you think of it as just "a record of stuff that happened," but if you don't know where you've been, you can't know who you are or where you're going.

There are several types of memory: memory for events, for facts, for how to do things, and working memory, which holds ideas in our head just long enough to turn them over. They're all malleable, and they're all mysterious, which you don't really realize until they fail you.