… is from page 7 of my late colleague Jim Buchanan’s 1971 paper “The Bases for Collective Action,” as this paper is reprinted in James M. Buchanan, Externalities and Public Expenditure Theory (2001), which is volume 15 of The Collected Works of James M. Buchanan:
In a setting where the sole function of government consists in the definition and enforcement of private personal and property rights, which include the enforcement of voluntary contracts and the policing against fraud and deceit, an orderly economic process would emerge in the natural course of events. It is important that this be recognized. So long as men differ from one another in tastes and/or basic endowments, and so long as they are free to negotiate enforceable contracts with one another, trades will be made. Individuals will find it in their own interest to exchange goods and services (including productive services such as their own labor) with each other. “In their own interest” here means simply that individuals can increase their ultimate command over real goods by trade. Exchange becomes essentially equivalent to production; it provides an efficient means of transforming less desired into more desired goods and services.