2010 Reprint of 1915 Edition. Georg Ferdinand Ludwig Philipp Cantor was a German mathematician, best known as the inventor of set theory, which has become a fundamental theory in mathematics. Cantor established the importance of one-to-one correspondence between sets, defined infinite and well-ordered sets, and proved that the real numbers are more numerous than the natural numbers. In fact, Cantor's theorem implies the existence of an infinity of infinities . He defined the cardinal and ordinal numbers and their arithmetic. Cantor's work is of great philosophical interest, a fact of which he was well aware. In 1895-97 Cantor fully propounded his view of continuity and the infinite, including infinite ordinals and cardinals, in his best known work, Contributions to the Founding of the Theory of Transfinite Numbers . This work contains his conception of transfinite numbers, to which he was led by his demonstration that an infinite set may be placed in a one-to-one correspondence with one of its subsets.