If you had the power to ensure all EAs understood 1 concept back to front. What would it be?
Only one concept per answer.
Please do not read this and feel obliged to understand all these. I don't want obligation.
I am asking because I might create a test to see how well EAs do know these concepts then see if I can get content for concepts people don't know. This is meant to make things easier not harder.
The margin/marginal value.
Anyone trying to think about how to do the most good will be very quickly and deeply confused if they aren't thinking at the margin. E.g. "if everyone buys bednets, what happens to the economy?"