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.
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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: 

