Tyler Cowen asks for one sentence containing the most insights of macroeconomics. Sumner weighs in. People are very cycle-focused now (for obvious reasons), but we shouldn't forget growth theory. I would offer: "Generally speaking, the short run is demand constrained and the long run is supply constrained". It seems like a sentence that grabs a lot from the 18th century to today, and it covers growth and the cycle. And of course I think it's essentially true, which is nice too.
This of course butts up againts Tyler's point that historically most downturns are real - I'm happy to add a caveat about market economies or financial economies or whatever seems right to add, but I don't think that's really necessary.