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

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.Felix Klein

The greatest mathematicians, as Archimedes, Newton, and Gauss, always united theory and applications in equal measure.Felix Klein

Thus, in a sense, mathematics has been most advanced by those who distinguished themselves by intuition rather than by rigorous proofs.Felix Klein