… is from page 145 of my GMU Econ colleague Larry White’s excellent paper “The Conflict Between Constitutionally Constraining the State and Empowering the State to Provide Public Goods,” which is chapter 7 in Richard E. Wagner, ed., James M. Buchanan: A Theorist of Political Economy and Social Philosophy  (2019):
It is rather that we should not forget, in moving our attention from blackboard constructions to the phenomena of the world outside the classroom, that the benefits of any proposed public good are no more than hypothetical. They are, by the premises of the theory, unobservable. Public goods theory thus does not provide an objective standard to guide public policy-making. It is epistemically impractical.