Felix Klein — German Mathematician born on April 25, 1849, died on June 22, 1925

Christian Felix Klein was a German mathematician and mathematics educator, known for his work in group theory, complex analysis, non-Euclidean geometry, and on the connections between geometry and group theory. His 1872 Erlangen Program, classifying geometries by their underlying symmetry groups, was a hugely influential synthesis of much of the mathematics of the day... (wikipedia)

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.