Cf. https://golem.ph.utexas.edu/category/2013/07/the_first_commutative_diagram.html
Is this the first ever commutative diagram? By commutative diagram I mean anything that shows an equality (or coherency, but higher morphisms doesn’t show up until much later (another great question for this site: the first natural transformation)) of two morphisms, so a triangle is a commutative diagram, but a string isn’t.
