about the theory of general relativity, in a letter dated November 24, 1919, to Albert Einstein.
Paul Ehrenfest: Cytaty po angielsku
“You will get your difficulties with the point electron.”
as quoted by Hendrik Casimir in an interview http://www.aip.org/history/ohilist/4550_1.html by Thomas Kuhn, Léon Rosenfeld, Aage Bohr and Erik Ruedinger at the Institute for Theoretical Physics, Copenhagen, 5 and 6 July, 1963.
Attributed