… is from page 27 of the 1997 collection of some of Leland Yeager’s finest essays, The Fluttering Veil  (George Selgin, ed.); specifically, it’s from Leland’s 1971 essay “Monetarism”:
Recognizing that the world is complicated is not the same as knowing how to succeed with complicated manipulations.
(I had the great good fortune as a student to have had more than my share of outstanding professors – Israel Kirzner, Saul Levmore, and Fritz Machlup, to name only three in addition to Leland. None surpassed, and few matched, Leland’s skill in the classroom and mastery of the material.)