While the various axiomatic systems of set theory saved mathematics from its immediate peril, they failed to satisfy a great many people. In particular, many who were sensitive to the elegance and universality of mathematics were quick to point out that the creations of Zermelo and von Neumann must be regarded as provisional solutions—as expedients to solve a temporary problem; they will have to be replaced, sooner or later, by a mathematical theory of broader scope, which treats the concept of “set” in its full, intuitive generality. This argument against axiomatic set theory—that it deals with an amputated version of our intuitive conception of a set—has important philosophical ramifications; it is part of a far wider debate, on the nature of mathematical “truth.” The debate centers around the following question: Are mathematical concepts creations (that is, inventions) of the human mind, or do they exist independently of us in a “platonic” realm of concepts, merely to be discovered by the mathematician? The latter opinion is often referred to as “platonic realism” and is the dominant viewpoint of classical mathematics. We illustrate these two opposing points of view by showing how they apply to a particular concept—the notion of natural numbers. From the viewpoint of platonic realism, the concepts “one,” “two,” “three,” and so on, exist in nature and existed before the first man began to count. If intelligent beings exist elsewhere in the universe, then, no matter how different they are from us, they have no doubt discovered the natural numbers and found them to have the same properties they have for us. On the other hand, according to the opposing point of view, while three cows, three stones, or three trees exist in nature, the natural number three is a creation of our minds; we have invented a procedure for constructing the natural numbers (by starting from zero and adding 1 each time, thus producing successively 1, 2, 3, etc.) and have in this manner fashioned a conceptual instrument of our own
making.