TITLE: "'Category' was defined in order to define 'functor', which was defined in order to define 'natural transformation'"
QUESTION [9 upvotes]: 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.

REPLY [17 votes]: 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)

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