Courant 1937a, II, §9: "Here we remark merely in passing that it is possible to use this approximate representation of the increment by the linear expression to construct a logically satisfactory definition of a "differential", as was done by Cauchy in particular." Courant, Richard (1937a), Differential and integral calculus. Vol. I, Wiley Classics Library, New York: John Wiley & Sons (published 1988), ISBN978-0-471-60842-4, MR1009558.