As an undergraduate math major, I found economics confusing. I understood self-interest: I have a TV, you have a $100 bill, both of us would prefer what the other has and so we’d like to trade. Good enough, but why, I kept asking my professor, when — as was likely — one of us has both items in hand, why didn’t the trading stop? Why wouldn’t one of us keep both the money and the TV? But, in that era of institution-free Samuelsonian economics, such questions were irrelevant and my grades suffered accordingly.