Full Idea
Physical processes present us with phenomena in which the purely mathematical has to be separated out by abstraction.
Gist of Idea
We have to separate the mathematical from physical phenomena by abstraction
Source
C.I. Lewis (A Pragmatic Conception of the A Priori [1923], p.367)
Book Reference
Peirce,James,Dewey etc: 'Pragmatism - The Classic Writings', ed/tr. Thayer,H.S. [Hackett 1982], p.367
A Reaction
This is the father of modal logic endorsing traditional abstractionism, it seems. He is also, though, endorsing the view that a priori knowledge is created by us, with pragmatic ends in view.