So apparently Richard Feynman once said:

If, in some cataclysm, all scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or atomic fact, or whatever you wish to call it) that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence you will see an enormous amount of information about the world, if just a little imagination and thinking are applied.

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?