The total number of green traffic lights must be the same as the total number of red traffic lights, because one person’s green light just is someone else’s red light. The same is true of economic exchange. Every time someone sells something, someone else must buy something. Why? Because the only way to sell something is to sell it to someone else. This may seem obvious, but a staggering percentage of the popular commentary on all sorts of economic issues loses track of this elementary equivalence.