… is from the final paragraph of my GMU colleague emeritus – and the 1986 winner of the Nobel Prize in Economics – James Buchanan ‘s 1978 essay “Natural and Artifactual Man” (emphasis in the original):
Man wants liberty to become the man he wants to become. He does so precisely because he does not know what man he will want to become in time.
This exquisite and profound essay is reprinted as Chapter 5 in James M. Buchanan, What Should Economists Do?  (H. Geoffrey Brennan & Robert D. Tollison, eds., Liberty Fund, 1979), pp. 93-112.