… is from page 322 of my late Nobel laureate colleague Jim Buchanan‘s 1982 paper “Cultural Evolution and Institutional Reform,” as this paper is reprinted in James M. Buchanan, Federalism, Liberty, and Law (2001), which is volume 18 of the Collected Works of James M. Buchanan:
“[E]fficiency is defined as “that which tends to emerge from the voluntary agreement among persons in the relevant group.” This definition becomes the only one possible unless it is presumed that the subjective evaluation of individuals is objectively known to external observers or that the evaluations relevant to efficiency are to be divorced from individual evaluations altogether.