…is from my GMU professor Robin Hanson, as quoted in Quora:
Quora: What is the most important little known economic fact or idea?
Hanson: Honestly, supply and demand. It is the part of economics with which more people have a passing familiarity, yet where even so a deeper fuller understanding would bring big gains. Someone who deeply understands just supply and demand understands a great deal.
For example, understanding all of the many ways in which the real world deviates from a supply and demand world can give one a deep understanding of the likely places to expect both market failures and useful innovations.