… is from page 294 of Vol. 19 (Ideas, Persons, and Events ) of The Collected Works of James M. Buchanan; specifically, it’s from Jim’s 1996 essay “Adam Smith as Inspiration” (emphasis added):
Despite the efforts of scientists over the two centuries since Adam Smith, the citizenry – and especially in its capacity to act collectively through the agency of politics – does not acknowledge the authority of economic science or of economists. The feasibility spaces that are laid out by the behavioral postulates that define human nature are not considered to be constraining. We believe and act, every day, on the romantic fiction of the schemer who would have us think that economic reality is what we choose to make it. The crudest of mercantilist fallacies against which Adam Smith directed his finest rhetoric remain alive and well as we end the twentieth century.