On Sep 7, 2008, at 2:33 PM, R Brown wrote:
There is another curiosity about the axioms for a category, namely the infuence of the known axioms for a groupoid (Brandt, 1926). Bill Cockcroft told me that these axioms had influenced E-M. These axioms were well used in the algebra group at Chicago. However when I asked Sammy about this in 1985 he firmly said `no, and was why the notion of groupoid did not appear as an example in the E-M paper'!
Perhaps it was a case of forgetting the influence?
I certainly heard Saunders mention Brandt groupoids as examples. (Not very good examples, since all maps are invertible.) But, as everyone knows, it is not the definition of a category that is the key part, but seeing that functors and natural transformations are interesting.