Everyone knows what a curve is, until he has studied enough mathematics to become confused through the countless number of possible exceptions.
Felix KleinThus, in a sense, mathematics has been most advanced by those who distinguished themselves by intuition rather than by rigorous proofs.
Felix KleinThe teacher manages to get along still with the cumbersome algebraic analysis, in spite of its difficulties and imperfections, and avoids the smooth infinitesimal calculus, although the eighteenth century shyness toward it had long lost all point.
Felix KleinIt is well known that the central problem of the whole of modern mathematics is the study of transcendental functions defined by differential equations.
Felix Klein