On March 3, 1845, German mathematician Georg Cantor, creator of the set theory was born Set Theory is considered the fundamental theory of mathematics. He also proved that the real numbers are "more numerous" than the natural numbers, which was quite shocking for his contemporaries that there should be different numbers of infinity.
F. Arzarello, M. Bussi, and O. Robutti. Proceedings of the 28th International Conference of the International Group for the Psychology of Mathematics Education, 4, page 89-96. (2004)