To Ehrenfest I owe a great deal. I studied physics at a time when a number of fascinating persons were there together. Ehrenfest would not instruct as such, as he preferred dialogue. Thanks to him I could participate in discussions with Albert Einstein. Also Kamerling Onnes, Lorentz and Zeeman were present. Being a student in the hands of such teachers, you are very fortunate indeed.

In: NRC-Handelsblad in 1987; as quoted in: Jan Tinbergen at MacTutor History of Mathematics, 2009: Quote about one of his teachers at the University of Leiden