… is from page 483 of James Buchanan ’s and Richard Wagner’s August 1978 Journal of Monetary Economics paper, “Dialogues Concerning Fiscal Religion,” as this paper is reprinted in Debt and Taxes  (2000), which is volume 14 of the The Collected Works of James M. Buchanan :
[P]oliticians, not economists, make economic policy.
DBx: Indisputable, of course. Yet too many economists continue to write, lecture, and opine as if they believe themselves to be offering policy advice to apolitical power-holders interested in nothing but selflessly and faithfully pursuing the public good as revealed to them by scribbling, yammering, and opinionated economists.
In fact, this belief by policy-offering economists is as realistic as is the belief that tigers can be convinced to bark like beagles and to dine exclusively on lettuce and sunflower seeds. Such fantastical performances can be imagined, but imagination is not reality.
Politicians will no more follow economists’ advice to pursue the public interest if doing so runs counter to politicians’ own interests than will tigers become vegetarians if lectured at by vegans.