A while ago I read in a book (or a paper?) that a very well-known mathematician (Saunders Maclane?) in his lectures used to mock the classical set-theoretical definition of natural numbers:
0 = {}, 1 = {{}}, 2 = {{}, {{}}}, ...
Who was that mathematician?
(Added after reading comments of Asaf Karagila):
Asaf, thank you for the references, but my question is not about von Neumann definition of ordinals, but (let me repeat again): Who was the famous mathematician, who in his lectures used to critisize the classical set-theoretical definition of natural numbers? Maybe, Saunders Maclane? And what is a reference on this critique?