… is from Milton Friedman; specifically, it’s from page 41 of the transcript of a May 2004 conversation between Milton Friedman and Thomas Sowell at Stanford University’s Hoover Institution; this transcript is printed in the superb 2009 volume, Peter Bauer and the Economics of Prosperity, James A. Dorn & Barun S. Mitra, eds. (New Delhi: Academic Foundation):
Well, there are fewer limits on what you can promise than on what you can deliver.
Politicians specialize in exploring the outer boundaries of what can be promised; good economists (such as Friedman, Sowell, and Bauer) specialize in exploring the reality of what can be delivered. Reality itself – indifferent to the romantic fantasies of humans who act as if reality is optional – specializes in enforcing itself.