TITLE: Colloquial catchy statements encoding serious mathematics
QUESTION [62 upvotes]: As the title says, please share colloquial statements that encode (in a non-rigorous way, of course) some nontrivial mathematical fact (or heuristic).  Instead of giving examples here I added them as answers so they may be voted up and down with the rest.
This is a community-wiki question: One colloquial statement and its mathematical meaning per answer please!

REPLY [2 votes]: Adding an axiom saying that something's provable
  doesn't help you prove it.

A characterization for Lob's theorem from reddit user noop_noob.