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