I am looking for the source (and original version) of the above oftrepeated quotation. Mac Lane mentions it in Categories for the Working Mathematician, attributing it to EilenbergMac Lane; however, I didn't see it while briefly skimming their paper General Theory of Natural Equivalences.
CW since some of the recent posts on MO have required little more than googling. Prior to the book you mentioned, MacLane attributed this saying to Peter Freyd in: MacLane, S. (1965). Categorical algebra. Bulletin of the American Mathematical Society, 71(1), 40106. Relevant excerpt: (p. 48) With regard to the original language, Eric Wofsey points out that Freyd's Abelian Categories (1964) begins with this description: 

