Memory

Most of what a person knows never comes to mind on its own. It can be counted only by asking, and every way of asking changes the count.

Ask someone to recall a list of words they heard a few minutes ago and they will often hand back a word that was never on it.

You can know someone for ten years and find, after one bad afternoon, that the afternoon is who they are now. The decade does not average in. It is not forgotten either; it is just quieter than the most recent thing.

You can sing a song you know from the first line without trying. Ask yourself to start at the second verse and something odd happens: you go back and run up to it. That small detour is the best clue to what knowing a song is.