The oral tradition at the University of Chicago attributed the observation that “sloppy writing reflects sloppy thinking” to Milton Friedman. Of course, it echoes George Orwell’s claim that “the slovenliness of our language makes it easier for us to have foolish thoughts.” Neither Friedman’s word “reflects” nor Orwell’s phrase “makes it easier” go far enough. The right verb is “produces.” Clear writing produces clearer thoughts. Sloppy writing produces sloppier thoughts. This is a natural consequence of the fact that anything stored in connections between neurons is part of a biochemical and electrical dynamic feedback loop.

To me, one of the ways in which my 1990 paper, Endogenous Technological Change, was a step forward relative to the first round models of endogenous growth was the explicit distinction that it allowed between the stock of human capital H and the stock of knowledge A. To be sure, this was a very small step. In the model, they interacted the simplest possible way. Human capital H was an input that could be used to produce new knowledge A.

Joshua Gans has a generous post that notes the 25th anniversary of the publication of my 1990 JPE article. I could not agree more with his observation that “there is more to be done …” in understanding the economics of ideas. His post helped me see how to respond to a conversation I had this summer. I’ll use the excuse of the anniversary to focus for the month on such basics as the meaning of the phrase nonrival good.

In an op ed last week, Steve Hilton, a former advisor to the government in the UK, boiled the policy dilemma in Europe down to its essence: Policy paralysis over the refugee crisis is convulsing Europe: Of course we want to help, but if we’re too generous, more will come. To understand what Hilton means, it helps to use a model, an abstract representation that captures the essence of a complicated situation.

In a talk at the World Bank that I gave last week, I repeated a riff that I’ve used before. Suppose your internist told you: The x-ray shows a mass that is probably cancer, but we don’t have any good randomized clinical trials showing that your surgeon’s recommendation, operating to remove it, actually causes the remission that tends to follow. However, we do have an extremely clever clinical trial showing conclusively that Botox will make you look younger.