It's the day after Labor Day, which means it is the first day of orientation for incoming MPAID students. Which also means that I have to give them a talk about the program and how it all fits together. In explaining our extensive and demanding curriculum, I emphasize that development is too important to be left to mushy thinking.

But having gone through an intense math camp for the last couple of weeks, taught by the inimitable Deb Hughes-Hallett, the students are a bit groggy at this point. Many wonder why they need to know about quasi-concavity and all that in order to be good at what they came here for--which is to improve the lives of the poor.

So I tell them a story about Sir W. Arthur Lewis. When I was a master's student myself at Princeton, I once attended a lecture that he gave on real wages, the commodity terms of trade, and North-South income differentials. The talk had no math in it. One of the younger faculty members of the economics department was sitting in the front row, and I could see him scratching his head in confusion throughout the talk. A few minutes after Sir Arthur was done, this young professor jumped up in excitement and went up to the board. "Now I get it!" he exclaimed and began to scribble some equations on the board. "This is the equation which relates to what you said in the first part of your talk, and this one expresses the other, and here is a third... and now finally we have three independent equations that determines your three endogenous variables..." Sir Arthur kept on his bemused smile as his lecture was explained to him in mathematical terms.

The moral of the story is that if you are smart enough to be a Nobel-prize winning economist maybe you can do without the math, but the rest of us mere mortals cannot. We need the math to make sure that we think straight--to ensure that our conclusions follow from our premises and that we haven't left loose ends hanging in our argument.

In other words, we use math not because we are smart, but because we are not smart enough.

We are just smart enough to recognize that we are not smart enough. And this recognition, I tell our students, will set them apart from a lot of people out there with very strong opinions about what to do about poverty and underdevelopment.