> For the complete documentation index, see [llms.txt](https://mandober.gitbook.io/math-debrief/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mandober.gitbook.io/math-debrief/100-fundamentals/120-foundations-of-mathematics/foundational-crisis.md). # Foundational crisis of mathematics The foundational crisis of mathematics (german: *Grundlagenkrise der Mathematik*) was the early 20th century's term for the search for proper foundations of mathematics. Several schools of the philosophy of mathematics ran into difficulties, one after the other, in the 20th century, as the assumption that mathematics had any foundation that could be consistently stated within mathematics itself was heavily challenged by the discovery of various paradoxes. The term "paradox" should not be confused with contradiction. A *contradiction* in a *formal theory* is a *formal proof* of *absurdity* inside the theory (e.g. 2+2=5), showing that this theory is *inconsistent* and must be rejected. However, a paradox may either be a surprising, but true, result in a given formal theory, or, an informal argument leading to a contradiction (so that a candidate theory, if it is to be formalized, must disallow at least one of its steps; in this case the problem is to find a satisfying theory without contradiction). Both meanings may apply if the formalized version of the argument forms the proof of a surprising truth. For instance, Russell's paradox may be expressed as "there is no set of all sets" (except in some marginal axiomatic set theories). Various schools of thought opposed each other. The leading school was that of the *formalist* approach, of which David Hilbert was the foremost proponent, culminating in what is known as *Hilbert's program*, which thought to ground mathematics on a small basis of a logical system proved sound by metamathematical finitistic means. The main opponent was the *intuitionist* school, led by L. E. J. Brouwer, which resolutely discarded formalism as a meaningless game with symbols. The fight was acrimonious. In 1920 Hilbert managed to remove Brouwer, whom he considered a threat to mathematics, from the editorial board of "Mathematische Annalen", the leading mathematical journal of the time. Philosophical views Main article: [Philosophy of mathematics](/math-debrief/100-fundamentals/120-foundations-of-mathematics/foundational-crisis.md)