Ernst Zermelo, who created a system of axioms for set theory,
was a Privatdozent at Goettingen when Herr Geheimrat Felix Klein
held sway over the fabled mathematics department. As Pauli told it,
"Zermelo taught a course on mathematical logic and stunned his
students by posing the following question: All mathematicians in
Goettingen belong to one of two classes. In the first class belong
those mathematicians who do what Felix Klein likes, but what they
dislike. In the second class are those mathematicians who do what
Felix Klein likes, but what they dislike. To what class does Felix
Klein belong?"
Jordan, having listened intently, broke into roaring laughter. Pauli
paused, took a sip of wine and said disapprovingly, "Herr Jordan, you
have laughed too soon". He continued: "None of the awed students
could solve this blasphemous problem. Zermelo then crowed in his
highpitched voice, 'But, meine Herren, it's very simple. Felix
Klein isn't a mathematician.'" Jordan laughed again. Pauli
drained his wine glass approvingly and concluded with
"Zermelo was not offered a professorship at Goettingen".
 E.L. Schucking, in 'Jordan, Pauli,Politics,
Brecht and a variable gravitational constant'
Physics Today, Oct. 1999
