If you are an economist in this day and age, and especially if you are an economist at MIT, you often get asked “Is economics really a science?”

I used to give a very complicated answer to that question, but recently I discovered a much simpler one: When the Discovery went up, the Space Administration announced that the success of the Discovery was entirely due to the scientific achievement of economists because in shifting from the Challenger to the Discovery the Space Aclrninistration had to do a lot of experiments, and they had to replace rats with economists in all these experiments. They announced three reasons for replacing rats with economists: First, there were now more economists and they were cheaper; second, you sometimes emotionally get attached to the rats; and third, there are some things that rats just won't do.

The serious answer to that question, however, is that economics is much like geology. Geologists understand earthquakes and volcanoes perfectly—it's plate tectonics. But if you ask a geologist, “Tell me the precise timing and the precise magnitude of the next earthquake in Los Angeles,” you are in trouble.

The problem is that people like to ask economists that question: “Give me the precise timing and the precise magnitude of some event.” And economists have a character weakness. They tend to answer those kinds of questions. And those kinds of questions simply can't be answered.