Set theory developed with surprising suddenness in the late nineteenth century. The theory was not preceded by the kind of long, evolutionary period that is generally associated with big mathematical breakthroughs. To mention two familiar examples, the calculus and non-Euclidean geometry developed over a period of many years and had contributions from a large number of people. The early development of set theory, by contrast, was very largely due to one man, Georg Cantor (1845-1918).