Can you give me an example of a rule that is true ("holds across a broad range of environments") and the truth of which does not depend on a "specific (and clos

Can you give me an example of a rule that is true ("holds across a broad range of environments") and the truth of which does not depend on a "specific (and closed) set of assumptions"? How does one arrive at "rules" like this using mathematics? I think that's quite a revolution in the foundations of mathematics...