George Janedlize writes
Could you please explain this better... the Bourbaki group simply did not see the importance of category theory... However, there were three great category-theorists in that group... I have never heard of any joint work of Charles Ehresmann with any of the two others, Eilenberg and Grothendieck... ...the relation between Bourbaki Tractate and category theory should have been determined by their separate or joint influence and therefore also by their communication with each other (if any).
I'll try to explain why there is no contradiction. 1. Charles only participated actively to the Bourbaki group from 1935 to the mid forties, at a time he did not know category theory. In 1935 he had written a first version for the volume "Theorie des ensembles" where he introduced the notions of local structures and associated pseudogroups of transformations (not so far from groupoids!), but this version was not accepted and he did not like the final version published much later. After the war, he only participated irregularly because he felt that he was no more able to make himself heard, the decisions being taken by "those who spoke the more loudly" (as he said to me). 2. Around 1950 it was decided that active participation ended at 45 (the age Charles had then), lessening the influence of those (Eilenberg, Cartan, Chevalley and Dieudonne) who could have stressed the importance of categories. I don't know exactly when Grothendieck became a member, but it was much later, and I think he did not remain for long. Later on, disdain for category theory had developed in France... 3. As for the communication between Charles and the other category-theorists, he had no contact with Grothendieck who was much younger. He was friendly with Eilenberg but did not see him often. Before the war he lived in Paris and regularly met Henri Cartan, Dieudonne, and more specially, Chevalley (both had regular exchanges with the philosophers Cavailles and Lautman). But their communication almost ceased after the war when he developed all his activity in Strasbourg (up to 1955) and was out of France for a long part of the year. Anyway, before our joint work (from the mid sixties up to his death), Charles worked essentially alone and published no joint work at all, except 6 Notes on Topology or Geometry with some of his students. When he began to specialize in category theory in the sixties, it was not well understood by other French mathematicians, and his influence dwindled up to a real opposition in the seventies. Andree