Trump’s folly is a boon to economics professors looking for ways to illustrate basic insights. One is the law of diminishing returns. If you own no shoes, acquiring a pair would make your life much better. If you own 20 pairs of shoes, getting a 21st would not. If you have no fencing on the border, putting up 700 miles of it prevents some illegal crossing. But adding another 900 wouldn’t make as much of a difference, and the 1,600th mile might have zero additional effect.