… is from page 307 of Milton Friedman‘s 1953 collection, Essays in Positive Economics; specifically, it’s from his brilliant review – entitled “Lerner on the Economics of Control,” and first published in the October 1947 issue of the Journal of Political Economy – of Abba Lerner’s The Economics of Control:
Under private enterprise profits are not only an incentive but also a criterion of performance and determine the entrepreneur’s ability to get command over resources. They cannot serve these other functions in the collectivist sector….
