Burn the math

By Eric Crampton 20/02/2014

I love it when Paul Krugman channels Alfred Marshall.

Here’s Krugman:

I once talked to a theorist (not RBC, micro) who said that his criterion for serious economics was stuff that you can’t explain to your mother. I would say that if you can’t explain it to your mother, or at least to your non-economist friends, there’s a good chance that you yourself don’t really know what you’re doing.

Math is good. Sometimes jargon is good, too. But plain language and simple intuition are important to keep you grounded.

And here’s Alfred Marshall, in a letter to Bowley (or see here), explaining how his approach to mathematics evolved:

I went more and more on the rules—(1) Use mathematics as a shorthand language, rather than as an engine of inquiry. (2) Keep to them till you have done. (3) Translate into English. (4) Then illustrate by examples that are important in real life. (5) Burn the mathematics. (6) If you canʼt succeed in 4, burn 3. This last I did often.

Krugman is really good at the maths, and at illustrating by examples; in that, he’s firmly Marshallean. I expect that Krugman, and most of modern econ, is happy using maths as an engine of inquiry as well. But if you can’t translate it into English that can readily be illustrated, there’s reasonable risk you’ve produced nonsense.