"The new basic principle is that in order to learn to avoid making mistakes we must learn from our mistakes." – Karl Popper, In Search of a Better World.
"It is an attitude which does not lightly give up hope that by such means as argument and careful observation, people may reach some kind of agreement on many problems of importance; and that, even where their demands and their interests clash, it is often possible to argue about the various demands and proposals, and to reach – perhaps by arbitration – a compromise which, because of its equity, is acceptable to most, if not to all." - Karl Popper, The Open Society and Its Enemies Volume 2.
I'd let you pick one or the other of these sentences, and be fine with either choice.
So apparently Richard Feynman once said:
I could be missing something, but this strikes me as a terrible answer.
When was the atomic hypothesis confirmed? If I recall correctly, it was only when chemists started noticing that the outputs of chemical reactions tended to factorize a certain way, which is to say that it took millennia after Democritus to get the point where the atomic hypothesis started making clearly relevant experimental predictions.
How about, "Stop trying to sound wise and come up with theories that make precise predictions about things you can measure in numbers."
I noticed this on Marginal Revolution, so I shall also state my candidate for the one most important sentence about macroeconomics: "You can't eat gold, so figure out how the heck money is relevant to making countries actually produce more or less food." This is a pretty large advance on how kings used to think before economics. I mean, Scott Sumner is usually pretty savvy (so is Richard Feynman btw) but his instruction to try to understand money is likely to fall on deaf ears, if it's just that one sentence. Think about money? Everyone wants more money! Yay, money! Let's build more gold mines! And "In the short run, governments are not households"? Really, Prof. Cowen, that's what you'd pass on to the next generation as they climb up from the radioactive soil?
*Cough.* Okay, I'm done. Does anyone want to take their own shot at doing better than Feynman did for their own discipline?