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I am looking for the source (and original version) of the above oft-repeated quotation. Mac Lane mentions it in Categories for the Working Mathematician, attributing it to Eilenberg-Mac Lane; however, I didn't see it while briefly skimming their paper General Theory of Natural Equivalences.

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up vote 17 down vote accepted

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), 40-106.

Relevant excerpt: (p. 48)

enter image description here

With regard to the original language, Eric Wofsey points out that Freyd's Abelian Categories (1964) begins with this description:

enter image description here

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Freyd makes this statement on this first page of his book Abelian categories: "It is not too misleading, at least historically, to say that categories are what one must define in order to define functors, and that functors are what one must define in order to define natural transformations." – Eric Wofsey Oct 2 '13 at 4:47
Great; I've added in an image from page 1 of Freyd's book. – Benjamin Dickman Oct 2 '13 at 4:56
Is it no true that the "serious disservice" to which Peter Freyd refers is committed in a number of text books? – Ronnie Brown Oct 2 '13 at 14:45

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