… is from page 56 of my late Nobel-laureate colleague Jim Buchanan ’s 1989 paper “Rational Choice Models in the Social Sciences,” as this paper is reprinted in Moral Science and Moral Order  (2001), Vol. 17 of The Collected Works of James M. Buchanan :
Only individuals choose; only individuals act. An understanding of any social interaction process must be based on an analysis of the choice behavior of persons who participate in that process. Results that are predicted or may be observed in social interaction must be factored down into the separate choices made by individuals.
As Buchanan goes on to explain, while individuals (of course) are influenced by each other, and while many choices are consciously made collectively, in human society choices are made only by individuals. Choices are not made by society, by the state, by the market, by voters, by ethnic groups, by sexes, or by any other collection of individuals as such.