MATEMATİĞİN TEMELLERİ
(Foundations of Mathematics)

"Matematiğin temelleri" olarak bilinen matematik dalı matematiğin tümü için geçerli olan en temel kavramları ve mantıksal yapıları inceler. Sayı, küme, fonksiyon, matematiksel tanıt, matematiksel tanım, matematiksel aksiyom, algoritma vb. gibi kavramlar Matematiksel mantık, Aksiyomatik Küme Teorisi, Tanıtlama Teorisi, Model Teorisi, Hesaplama teorisi,Kategori Teorisi gibi yine matematiğim temelleri olarak anılan alanlarda incelenir. Bununla birlikte matematiğin temellerinin araştırılması matematik felsefesinin ana konularından biridir. Bu daldaki can alıcı soru matematiksel önermelerin hangi nihai esaslara göre "doğru" ya da "gerçek" kabul edilebileceğidir.

Geçerli baskın matematiksel paradigma aksiyomatik küme kuramı ve formel mantık üzerine kurulmuştur. Günümüzde neredeyse bütün matematik teoremleri küme kuramının teoremlerişeklinde ifade edilebilmektedir. Bu bakış açısına göre matematiksel bir önermenin doğruluğu (gerçekliği) önermenin formel mantık yoluyla küme kuramının aksiyomlarından türetilebildiği iddiasından başka bir şey değildir. Bununla birlikte bu formel yaklaşım bazı konuları aydınlatmakta yeterisz kalır: Neden kullandığımız aksiyomlar yerine başka aksiyomlar kullanmayalım? Neden kullandığımız mantık kuralları yerine başka mantık kuralları kullanmayalım? Neden "doğru" matematiksel önermeler (örneğin aritmetik yasaları) fiziksel dünyada doğruymuş gibi görünür? Bu sorunsal Eugene Wigner tarafından (1960) "en:The unreasonable effectiveness of mathematics in the physical sciences" (Matematiğin doğa bilimlerindeki anlaşılmaz etkililiği) adlı çalışmasında ayrıntılı olarak işlenmiştir.

Yukarıda belirtilen formel gerçeklik nosyonunun hiçbir manası da olmayabilir. Başka bir deyişle tüm önermelerin, hatta paradoksların, küme kuramı aksiyomlarından türetilmesi olanaklı olabilir. Bunun ötesinde Gödel'in ikinci teoreminin sonucu olarak bunun böyle olmadığından hiçbir zaman emin olamayız.

Matematiksel gerçekçilikte (Platonizm olarak da bilinir), insanlardan bağımsız olan bir matematiksel nesneler dünyasının var olduğu öne sürülür. Matematiksel nesnelere ilişkin doğrular insanlar tarafından keşfedilir. Bu görüşe göre doğanın yasaları ve matematiğin yasaları benzer bir statüdedir ve matematik yasaların doğadaki etkililiğinin mantıksız olduğu savı geçerliliğini yitirir. Aksiyomlarımız değil, matematiksel nesnelerin elle tutulabilir gerçek dünyası matematiğin temellerini oluşturur. Bu noktada doğal olarak beliren soru, (Bu matematiksel dünyaya nasıl erişlebilir?) sorusudur.

Matematik felsefesinde bazı modern kuramlar, özgün anlamıyla, temellerin var olduğunu reddeder. Bazıları matematiksel uygulama üzerinde yoğunlaşır ve matematikçilerin bir sosyal grup olarak somut çalışmalarını betimlemeyi ve çözümlemeyi amaçlar. Yine başkaları, matematiğin 'gerçek dünyaya' uygulandığında güvenilirliği konusunda insanın bilişseliğine yoğunlaşarak matematiği bilişsel bilim olarak oluşturmaya çalışır. Bu kuramlarda temeller yalnızca insan düşüncesinde bulunur ve 'nesnel' dış yapıda yoktur. Bu konu hala çözüme kavuşturulamamıştır.

Foundations of mathematics is a term sometimes used for certain fields of mathematics, such as mathematical logic, axiomatic set theory, proof theory, model theory, and recursion theory. The search for foundations of mathematics is also a central question of the philosophy of mathematics: On what ultimate basis can mathematical statements be called true?

The foundational philosophy of Platonist mathematical realism, as exemplified by mathematician Kurt Gödel, proposes the existence of a world of mathematical objects independent of humans; the truths about these objects are discovered by humans. In this view, the laws of nature and the laws of mathematics have a similar status, and the effectivenessceases to be unreasonable. Not our axioms, but the very real world of mathematical objects forms the foundation. The obvious question, then, is: how do we access this world? [2]

The foundational philosophy of formalism, as exemplified by David Hilbert, is based onaxiomatic set theory and formal logic. Virtually all mathematical theorems today can be formulated as theorems of set theory. The truth of a mathematical statement, in this view, is then nothing but the claim that the statement can be derived from the axioms of set theory using the rules of formal logic. [2]

Merely the use of formalism alone does not explain several issues: why we should use the axioms we do and not some others, why we should employ the logical rules we do and not some others, why do "true" mathematical statements (e.g., the laws of arithmetic) appear to be true, and so on. In some cases these may be sufficiently answered through the study of formal theories, in disciplines such as reverse mathematics and computational complexity theory.Formal logical systems also run the risk of inconsistency; in Peano arithmetic, this arguably has already been settled with several proofs of consistency, but there is debate over whether or not they are sufficiently finitary to be meaningful. Gödel's second incompleteness theoremestablishes that logical systems of arithmetic can never contain a valid proof of their ownconsistency. What Hilbert wanted to do was prove a logical system S was consistent, based on principles P that only made up a small part of S. But Gödel proved that the principles P could not even prove P to be consistent, let alone S!

The foundational philosophy of intuitionism or constructivism, as exemplified in the extreme byBrouwer and more coherently by Stephen Kleene, requires proofs to be "constructive" in nature – the existence of an object must be demonstrated rather than inferred from a demonstration of the impossibility of its non-existence. For example, as a consequence of this the form of proof known as reductio ad absurdum is suspect. [2]

Some modern theories in the philosophy of mathematics deny the existence of foundations in the original sense. Some theories tend to focus on mathematical practice, and aim to describe and analyze the actual working of mathematicians as a social group. Others try to create acognitive science of mathematics, focusing on human cognition as the origin of the reliability of mathematics when applied to the real world. These theories would propose to find foundations only in human thought, not in any objective outside construct. The matter remains controversial.

The foundational crisis of mathematics (in German: Grundlagenkrise der Mathematik) was the early 20th century's term for the search for proper foundations of mathematics.

After several schools of the philosophy of mathematics ran into difficulties one after the other in the 20th century, the assumption that mathematics had any foundation that could be stated within mathematics itself began to be heavily challenged.

One attempt after another to provide unassailable foundations for mathematics was found to suffer from various paradoxes (such as Russell's paradox) and to be inconsistent: an undesirable situation in which every mathematical statement that can be formulated in a proposed system (such as 2 + 2 = 5) can also be proved in the system.

Various schools of thought on the right approach to the foundations of mathematics were fiercely opposing 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 bymetamathematical 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 (van Dalen, 2008). The fight was acrimonious. In 1920 Hilbert succeeded in having Brouwer, whom he considered a threat to mathematics, removed from the editorial board of Mathematische Annalen, the leading mathematical journal of the time.

Gödel's incompleteness theorems, proved in 1931, showed that essential aspects of Hilbert's program could not be attained. In Gödel's first result he showed how to construct, for any sufficiently powerful and consistent finitely axiomatizable system – such as necessary to axiomatize the elementary theory of arithmetic – a statement that can be shown to be true, but that does not follow from the rules of the system. It thus became clear that the notion of mathematical truth can not be reduced to a purely formal system as envisaged in Hilbert's program. In a next result Gödel showed that such a system was not powerful enough for proving its own consistency, let alone that a simpler system could do the job. This dealt a final blow to the heart of Hilbert's program, the hope that consistency could be established by finitistic means (it was never made clear exactly what axioms were the "finitistic" ones, but whatever axiomatic system was being referred to, it was a 'weaker' system than the system whose consistency it was supposed to prove). Meanwhile, the intuitionistic school had failed to attract adherents among working mathematicians, and floundered due to the difficulties of doing mathematics under the constraint of constructivism.

In a sense, the crisis has not been resolved, but faded away: most mathematicians either do not work from axiomatic systems, or if they do, do not doubt the consistency of ZFC, generally their preferred axiomatic system. In most of mathematics as it is practiced, the various logical paradoxes never played a role anyway, and in those branches in which they do (such as logicand category theory), they may be avoided.