(Edit #1 after Carlo's response)

It is often claimed that *the notion of natural transformations existed in mathematical vocabulary long before it had a definition*. In fact, I quoted the statement in *italic* from [1, p. 2]. As another example, in [2, p. 70] Ralf Kromer says: *The claim is that there was, at the time when [3] was written, a current informal parlance consisting in calling certain transformations natural and that Mac Lane and Eilenberg tried (and succeeded) to grasp this informal parlance mathematically*.

Three years after [3], Eilenberg and Maclane discovered the fact that this notion could be mathematically defined in [4].

However, Ralf Kromer casts doubt on the above mentioned claim, due to lack of evidence (see: [2, p. 70]).

(Edit #2 after Eric's comment) My question is, can you supply an evidence of use of the phrase "natural transformations" or its variants in mathematical literature prior to [3]? I must add that in [2] Kromer gives a number of examples of use of phrases such as "natural homomorphism" or "natural projection" in the literature prior to or around the same time as [3], but in each case they turn out to have different meanings. So I am asking for example(s) of use of the phrase "natural transformations", which are really natural transformations.

**References:**

- Peter Freyd:
**Abelian Categories**(1964). - Ralf Kromer:
**Tool and Object: a history and philosophy of category theory**(2007). - Samuel Eilenberg and Saunders Maclane:
*Group extensions and homology*, Annals Math. (2)**43**, p.p. 757–831 (1942). - Samuel Eilenberg and Saunders Maclane:
*General theory of natural transformations*, Trans. AMS,**58**, p.p.: 231-294 (1945).

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