… is from pages 325-326 of of Vol. 19 (Ideas, Persons, and Events  ) of The Collected Works of James M. Buchanan; specifically, it’s from Jim’s 1986 article “Ideas, Institutions, and Political Economy” – an article in which Jim spells out a wise case for abolishing the Council of Economic Advisors:
The whole Keynesian edifice was constructed on the preposterous supposition that economic advice is offered to a genuinely benevolent despot, an entity devoid of its own interests, and presumably willing and able to implement, without resistance, the advice offered to it. The early monetarist challenge was directed to the Keynesian analysis and, in itself, did not question the implicit political supposition….
Effective authority lodged with an hereditary monarch might represent the closest historical parallel to the implicitly presumed Keynesian model of politics.
Please join me in wishing Jim  a happy 93rd birthday today.