Abstract
The theory of ordinal numbers is a natural and very powerful generalization of the order-theoretical properties of natural numbers. In particular it furnishes transfinite induction, a method for constructing rather complicated mathematical concepts and for proving properties valid beyond the natural numbers. Ordinal numbers can also serve as a basis for introducing cardinal numbers. The latter evaluate “how many elements” a set possesses, being thus a kind of “quantitative” generalization of natural numbers, widely used in mathematics.