A biography for Felix Kleinhasn’t been written yet.
“Everyone knows what a curve is, until he has studied enough mathematics to become confused through the countless number of possible exceptions.”
“Thus, in a sense, mathematics has been most advanced by those who distinguished themselves by intuition rather than by rigorous proofs.”
“The developing science departs at the same time more and more from its original scope and purpose and threatens to sacrifice its earlier unity and split into diverse branches.”
“The greatest mathematicians, as Archimedes, Newton, and Gauss, always united theory and applications in equal measure.”