… is from page 201 of my colleague Richard Wagner’s valuable 1987 essay “Liability Rules, Fiscal Institutions and Debt,” which is chapter 11 in the important 1987 collection Deficits  (James M. Buchanan, Charles K. Rowley, & Robert D. Tollison, eds):
The claims of Keynesian macroeconomic theory and policy may or may not be correct; I for one think they are quite false. But political survivability may be quite different from such matters of truth or falsity. Even if the Keynesian presuppositions about economics were true, and so the economy were unstable and government could alter its budgetary policy to offset private sources of instability, it does not follow that those policy outcomes would dominate alternative policies within existing democratic regimes. Moreover, even if those presuppositions were false, policy prescriptions based on those presuppositions could nonetheless have survival value within existing democratic institutions.
From this reality I conclude that it’s professional malpractice for a social scientist who pays little attention to the realities of how how collective-choice and government-decision-making processes actually work – namely, by carefully studying public choice  – to advocate that government implement his or her theories. Policy advocacy under such circumstances is professional malpractice regardless of the strength of the conviction with which the social scientist holds his or her theories to be true (and, indeed, regardless of the actual validity of those theories within their own theoretical domains).
Policy advocacy by persons insufficiently familiar with public-choice research is akin to, say, an automotive engineer designing an automobile with all sorts of (what are at least believed to be) internally consistent working parts, all of which conform well to known laws of physics, but none of which was designed with much knowledge at all of the actual features of the likely human beings who will drive the car – features such as people’s size, vision, the number of limbs they possess, and the purposes for which they likely are to use the car.