Everyone knows what a curve is, until he has studied enough mathematics to become confused through the countless number of possible exceptions.

Erster Band; Die gruppentheoretischen Grundlagen (in German) Leipzig: B. Annalen Bd. In the 1890s Klein turned to mathematical physics a subject from which he had never strayed far writing on the gyroscope with Arnold Sommerfeld.

Christian Felix Klein (25 April 1849 – 22 June 1925) was a German mathematician known for his work in group theory complex analysis non-Euclidean geometry and on the connections between geometry and group theory.