Jos, that's a good question. I guess it's true that a proverb offers advice. Here's one that does, but it doesn't seem to at first. It's from my husband:

Gravity never gives up.

The reason it's advice is that it's usually spoken when something is missing, and the missing thing is apt to be found under a piece of furniture or somewhere else that gravity took it to.

Let's be careful. Defining proverbs could turn into one of those Mudcat wrangles that never ends, just like defining a folksong.