A good single sentence saying can’t require background evidencing or further explanation. It must be instantly recognizable as true. It also needs to be news to the listener. Most single sentences that people can immediately verify as true they already believe. What’s left? One big answer is things that people don’t believe or think about much for lack of wanting to, despite evidence. Drawing attention to these is called cynicism.
If you like rationalist aphorisms, you might like Aaron Haspel’s Twitter feed. Examples:
Katja Grace pointed out something interesting about aphorisms: