I recently read that the Polish mathematician Stanislas Ulam once challenged one of the great names in twentieth-century economics, Paul Samuelson, to "name me one proposition in all of the social sciences that is both true and non-trivial".$^1$ Samuelson's vote went to the idea of comparative advantage.
I am interested to know what other professional economists would volunteer as answers. I absolutely can't ask this question on Econ SE since it's opinion-based, neither on the subreddit r/AskEconomics for the same reason.
What are some other communities where this kind of question will be entertained?
$^1$ The Collected Scientific Papers of Paul A. Samuelson, vol. 3 (Cambridge, MA: MIT Press, 1966), p. 683