Georg Cantor

Georg Ferdinand Ludwig Philipp Cantor was a mathematician from Germany who is often credited with creating set theory. He was the first to study infinite sets and the cardinality of sets. He showed that the set of rational numbers and the set of integers are in a 1 to 1 correspondence. He also showed that the set of real numbers is greater than the set of rationals. He first introduced the idea of cardinality of infinite sets using the symbol $\aleph$ to represent the size. Because his work was so radical for his time, he encountered much resistance, mathematically, philosophically, and even theologically.