The phrase "to fix ideas" shows up over 800 times in the complete JSTOR database. The earliest case is: Note on the Theory of the Alternate Current Dynamo John Hopkinson Proceedings of the Royal Society of London > Vol. 42 (1887), pp. 167-170 Stable URL: http://links.jstor.org/sici?sici=0370-1662%281887%2942%3C167%3ANOTTOT%3E2.0.CO%3B2-9 It seems to be used in a variety of sciences -- physics, mathematics, psychology, and anthropology, for example -- in the 19th century. Restricting attention to economics journals in JSTOR, the earliest case is: Bilateral Monopoly A. L. Bowley The Economic Journal > Vol. 38, No. 152 (Dec., 1928), pp. 651-659 Stable URL: http://links.jstor.org/sici?sici=0013-0133%28192812%2938%3A152%3C651%3ABM%3E2.0.CO%3B2-X I would imagine that it predates the journals archived in JSTOR. Kevin Hoover