Foundations of mathematics

<h2 id="ancient-greece">Ancient Greece</h2>
Further information: <a href="/facts/Greek_mathematics/ealke3z8">Greek mathematics</a> and <a href="/facts/Greek_philosophy/LWEkp8JX">Greek philosophy</a>
Most civilisations developed some mathematics, mainly for practical purposes, such as counting (merchants), <a href="/facts/Surveying/qchGcnRC">surveying</a> (delimitation of fields), <a href="/facts/Sanskrit_prosody/6Wjo0MEA">prosody</a>, <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>, and <a href="/facts/Astrology/kFP09KjH">astrology</a>. It seems that <a href="/facts/Ancient_Greek_philosophers/LWEkp8JX">ancient Greek philosophers</a> were the first to study the nature of mathematics and its relation with the real world.
<a href="/facts/Zeno_of_Elea/RJKVQWvh">Zeno of Elea</a> (c. 490 – c. 430 BC) produced several paradoxes he used to support his thesis that movement does not exist. These paradoxes involve <a href="/facts/Mathematical_infinity/cF506uD7">mathematical infinity</a>, a concept that was outside the mathematical foundations of that time and was not well understood before the end of the 19th century.
The <a href="/facts/Pythagoreanism/25BjMN2z">Pythagorean school of mathematics</a> originally insisted that the only numbers are natural numbers and ratios of natural numbers. The discovery (c. 5th century BC) that the ratio of the diagonal of a square to its side is not the ratio of two natural numbers was a shock to them which they only reluctantly accepted. A testimony of this is the modern terminology of <a href="/facts/Irrational_number/Psv19TET">irrational number</a> for referring to a number that is not the quotient of two integers, since "irrational" means originally "not reasonable" or "not accessible with reason".
The fact that length ratios are not represented by rational numbers was resolved by <a href="/facts/Eudoxus_of_Cnidus/FgPAy5L9">Eudoxus of Cnidus</a> (408–355 BC), a student of <a href="/facts/Plato/jGtT1UD5">Plato</a>, who reduced the comparison of two irrational ratios to comparisons of integer multiples of the magnitudes involved. His method anticipated that of <a href="/facts/Dedekind_cut/rqhbb24J">Dedekind cuts</a> in the modern definition of real numbers by <a href="/facts/Richard_Dedekind/CLkWDhY3">Richard Dedekind</a> (1831–1916);<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a> see <a href="/facts/Eudoxus_of_Cnidus/FgPAy5L9">Eudoxus of Cnidus § Eudoxus' proportions</a>.
In the <a href="/facts/Posterior_Analytics/D9prwxCp">Posterior Analytics</a>, <a href="/facts/Aristotle/U2bDlBFW">Aristotle</a> (384–322 BC) laid down the <a href="/facts/Logic/8JnpJLtt">logic</a> for organizing a field of knowledge by means of primitive concepts, axioms, postulates, definitions, and theorems. Aristotle took a majority of his examples for this from arithmetic and from geometry, and his logic served as the foundation of mathematics for centuries. This method resembles the modern <a href="/facts/Axiomatic_method/dwMz70d5">axiomatic method</a> but with a big philosophical difference: axioms and postulates were supposed to be true, being either self-evident or resulting from <a href="/facts/Experiment/esltM54f">experiments</a>, while no other truth than the correctness of the proof is involved in the axiomatic method. So, for Aristotle, a proved theorem is true, while in the axiomatic methods, the proof says only that the axioms imply the statement of the theorem.
Aristotle's logic reached its high point with <a href="/facts/Euclid/dHpVyL4R">Euclid</a>'s <a href="/facts/Euclid%27s_Elements/nPKvCIxs">Elements</a> (300 BC), a treatise on mathematics structured with very high standards of rigor: Euclid justifies each proposition by a demonstration in the form of chains of <a href="/facts/Syllogisms/cgpmd6jB">syllogisms</a> (though they do not always conform strictly to Aristotelian templates).
Aristotle's <a href="/facts/Syllogistic_logic/cgpmd6jB">syllogistic logic</a>, together with its exemplification by Euclid's Elements, are recognized as scientific achievements of ancient Greece, and remained as the foundations of mathematics for centuries.

<h2 id="before-infinitesimal-calculus">Before infinitesimal calculus</h2>
During <a href="/facts/Middle_Ages/RqzEKB9v">Middle Ages</a>, Euclid's Elements stood as a perfectly solid foundation for mathematics, and <a href="/facts/Philosophy_of_mathematics/z7xBcdWA">philosophy of mathematics</a> concentrated on the <a href="/facts/Ontology/QJKTsEIJ">ontological status</a> of mathematical concepts; the question was whether they exist independently of perception (<a href="/facts/Philosophical_realism/cPo2pK16">realism</a>) or within the mind only (<a href="/facts/Conceptualism/mQsRrgW9">conceptualism</a>); or even whether they are simply names of collection of individual objects (<a href="/facts/Nominalism/3FE99hPX">nominalism</a>). 
In Elements, the only numbers that are considered are <a href="/facts/Natural_number/u65og8JE">natural numbers</a> and ratios of lengths. This geometrical view of non-integer numbers remained dominant until the end of Middle Ages, although the rise of <a href="/facts/Algebra/nMdqczjo">algebra</a> led to consider them independently from geometry, which implies implicitly that there are foundational primitives of mathematics. For example, the transformations of equations introduced by <a href="/facts/Al-Khwarizmi/24KdxyEQ">Al-Khwarizmi</a> and the <a href="/facts/Cubic_formula/Eqm3HCMx">cubic</a> and <a href="/facts/Quartic_formula/lHuBhMk7">quartic</a> formulas discovered in the 16th century result from algebraic manipulations that have no geometric counterpart.
Nevertheless, this did not challenge the classical foundations of mathematics since all properties of numbers that were used can be deduced from their geometrical definition.
In 1637, <a href="/facts/Ren%C3%A9_Descartes/ohqj6DMf">René Descartes</a> published <a href="/facts/La_G%C3%A9om%C3%A9trie/ELdb057s">La Géométrie</a>, in which he showed that geometry can be reduced to algebra by means <a href="/facts/Cartesian_coordinate_system/lkTqvMmB">coordinates</a>, which are numbers determining the position of a point. This gives to the numbers that he called <a href="/facts/Real_number/R02gw5Pb">real numbers</a> a more foundational role (before him, numbers were defined as the ratio of two lengths). Descartes' book became famous after 1649 and paved the way to <a href="/facts/Infinitesimal_calculus/MEmUTYoz">infinitesimal calculus</a>.

<h2 id="infinitesimal-calculus">Infinitesimal calculus</h2>
<a href="/facts/Isaac_Newton/4L7gTncN">Isaac Newton</a> (1642–1727) in England and <a href="/facts/Gottfried_Wilhelm_Leibniz/cTc2er83">Leibniz</a> (1646–1716) in Germany independently developed the <a href="/facts/Infinitesimal_calculus/MEmUTYoz">infinitesimal calculus</a> for dealing with mobile points (such as planets in the sky) and variable quantities.
This needed the introduction of new concepts such as <a href="/facts/Continuous_function/sKbl02pB">continuous functions</a>, <a href="/facts/Derivative/7Ito70Dh">derivatives</a> and <a href="/facts/Limit_(mathematics)/j8JBhj4e">limits</a>. For dealing with these concepts in a logical way, they were defined in terms of <a href="/facts/Infinitesimal/JKcTMIgD">infinitesimals</a> that are hypothetical numbers that are infinitely close to zero. The strong implications of infinitesimal calculus on foundations of mathematics is illustrated by a pamphlet of the Protestant philosopher <a href="/facts/George_Berkeley/KKnYyDC2">George Berkeley</a> (1685–1753), who wrote "[Infinitesimals] are neither finite quantities, nor quantities infinitely small, nor yet nothing. May we not call them the ghosts of departed quantities?".<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>
Also, a lack of rigor has been frequently invoked, because infinitesimals and the associated concepts were not formally defined (<a href="/facts/Line_(geometry)/gJqsr4Gj">lines</a> and <a href="/facts/Plane_(geometry)/tAUtRFcn">planes</a> were not formally defined either, but people were more accustomed to them). Real numbers, continuous functions, derivatives were not formally defined before the 19th century, as well as <a href="/facts/Euclidean_geometry/V6cTcoCy">Euclidean geometry</a>. It is only in the 20th century that a formal definition of infinitesimals has been given, with the proof that the whole infinitesimal can be deduced from them.
Despite its lack of firm logical foundations, infinitesimal calculus was quickly adopted by mathematicians, and validated by its numerous applications; in particular the fact that the planet trajectories can be deduced from the <a href="/facts/Newton%27s_law_of_gravitation/F6qH2XC3">Newton's law of gravitation</a>.

<h2 id="19th-century">19th century</h2>
In the 19th century, mathematics developed quickly in many directions. Several of the problems that were considered led to questions on the foundations of mathematics. Frequently, the proposed solutions led to further questions that were often simultaneously of philosophical and mathematical nature. All these questions led, at the end of the 19th century and the beginning of the 20th century, to debates which have been called the foundational crisis of mathematics. The following subsections describe the main such foundational problems revealed during the 19th century.

<h3>Real analysis</h3>
See also: <a href="/facts/Mathematical_analysis/XXeR26Ih">Mathematical analysis § History</a>
<a href="/facts/Cauchy/aoTjgj6j">Cauchy</a> (1789–1857) started the project of giving rigorous bases to <a href="/facts/Infinitesimal_calculus/MEmUTYoz">infinitesimal calculus</a>. In particular, he rejected the heuristic principle that he called the <a href="/facts/Generality_of_algebra/avqGgAgD">generality of algebra</a>, which consisted to apply properties of <a href="/facts/Algebraic_operation/wl69LmeN">algebraic operations</a> to <a href="/facts/Infinite_sequences/gV2S4uqp">infinite sequences</a> without proper proofs. In his <a href="/facts/Cours_d%27Analyse/IUUvmV23">Cours d'Analyse</a> (1821), he considered very small quantities, which could presently be called "sufficiently small quantities"; that is, a sentence such that "if x is very small then ..." must be understood as "there is a (sufficiently large) <a href="/facts/Natural_number/u65og8JE">natural number</a> n such that |x| < 1/n". In the proofs he used this in a way that predated the modern <a href="/facts/(%CE%B5,_%CE%B4)-definition_of_limit/3Yd7BdvM">(ε, δ)-definition of limit</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a>
The modern <a href="/facts/(%CE%B5,_%CE%B4)-definition_of_limit/3Yd7BdvM">(ε, δ)-definition of limits</a> and <a href="/facts/Continuous_functions/sKbl02pB">continuous functions</a> was first developed by <a href="/facts/Bernard_Bolzano/BUbRVY4p">Bolzano</a> in 1817, but remained relatively unknown, and Cauchy probably did know Bolzano's work.
<a href="/facts/Karl_Weierstrass/4U8vdQzL">Karl Weierstrass</a> (1815–1897) formalized and popularized the (ε, δ)-definition of limits, and discovered some pathological functions that seemed paradoxical at this time, such as <a href="/facts/Weierstrass_function/m9jU4LE5">continuous, nowhere-differentiable functions</a>. Indeed, such functions contradict previous conceptions of a function as a rule for computation or a smooth graph. 
At this point, the program of <a href="/facts/Arithmetization_of_analysis/N4cw0P9d">arithmetization of analysis</a> (reduction of <a href="/facts/Mathematical_analysis/XXeR26Ih">mathematical analysis</a> to arithmetic and algebraic operations) advocated by Weierstrass was essentially completed, except for two points.
Firstly, a formal definition of real numbers was still lacking. Indeed, beginning with <a href="/facts/Richard_Dedekind/CLkWDhY3">Richard Dedekind</a> in 1858, several mathematicians worked on the definition of the real numbers, including <a href="/facts/Hermann_Hankel/rWmnbQC8">Hermann Hankel</a>, <a href="/facts/Charles_M%C3%A9ray/IM7zMUzy">Charles Méray</a>, and <a href="/facts/Eduard_Heine/FY8enC39">Eduard Heine</a>, but this is only in 1872 that two independent complete definitions of real numbers were published: one by Dedekind, by means of <a href="/facts/Dedekind_cut/rqhbb24J">Dedekind cuts</a>; the other one by <a href="/facts/Georg_Cantor/k5v1DvW4">Georg Cantor</a> as equivalence classes of <a href="/facts/Cauchy_sequences/7Q5c8TuF">Cauchy sequences</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> 
Several problems were left open by these definitions, which contributed to the <a href="/facts/Foundational_crisis_of_mathematics/yjpbbUkt">foundational crisis of mathematics</a>. Firstly both definitions suppose that <a href="/facts/Rational_number/VJQmyY05">rational numbers</a> and thus <a href="/facts/Natural_number/u65og8JE">natural numbers</a> are rigorously defined; this was done a few years later with <a href="/facts/Peano_axioms/DhhDnNug">Peano axioms</a>. Secondly, both definitions involve <a href="/facts/Infinite_set/9rTN0xGH">infinite sets</a> (Dedekind cuts and sets of the elements of a Cauchy sequence), and Cantor's <a href="/facts/Set_theory/1ptfsvUP">set theory</a> was published several years later.
The third problem is more subtle: and is related to the foundations of logic: classical logic is a <a href="/facts/First-order_logic/lcISqWIg">first-order logic</a>; that is, <a href="/facts/Quantifier_(logic)/bgmvwxoo">quantifiers</a> apply to variables representing individual elements, not to variables representing (infinite) sets of elements. The basic property of the <a href="/facts/Completeness_of_the_real_numbers/UQIn0qDw">completeness of the real numbers</a> that is required for defining and using real numbers involves a quantification on infinite sets. Indeed, this property may be expressed either as for every infinite sequence of real numbers, if it is a <a href="/facts/Cauchy_sequence/7Q5c8TuF">Cauchy sequence</a>, it has a limit that is a real number, or as every subset of the real numbers that is <a href="/facts/Bounded_set/yCldjtoy">bounded</a> has a <a href="/facts/Least_upper_bound/oXCKVopz">least upper bound</a> that is a real number. This need of quantification over infinite sets is one of the motivation of the development of <a href="/facts/Higher-order_logic/hdUhry6m">higher-order logics</a> during the first half of the 20th century.

<h3>Non-Euclidean geometries</h3>
See also: <a href="/facts/Non-Euclidean_geometry/efKa7De2">Non-Euclidean geometry § History</a>
Before the 19th century, there were many failed attempts to derive the <a href="/facts/Parallel_postulate/xhSxUX4u">parallel postulate</a> from other axioms of geometry. In an attempt to prove that its negation leads to a contradiction, <a href="/facts/Johann_Heinrich_Lambert/CG3IHVB9">Johann Heinrich Lambert</a> (1728–1777) started to build <a href="/facts/Hyperbolic_geometry/DjTMKdgy">hyperbolic geometry</a> and introduced the <a href="/facts/Hyperbolic_functions/Y4Cb5S35">hyperbolic functions</a> and computed the area of a <a href="/facts/Hyperbolic_triangle/hTQldn7s">hyperbolic triangle</a> (where the sum of angles is less than 180°). 
Continuing the construction of this new geometry, several mathematicians proved independently that if it is <a href="/facts/Inconsistent/IHkdnA9M">inconsistent</a>, then <a href="/facts/Euclidean_geometry/V6cTcoCy">Euclidean geometry</a> is also inconsistent and thus that the parallel postulate cannot be proved. This was proved by <a href="/facts/Nikolai_Lobachevsky/0fe8z6jY">Nikolai Lobachevsky</a> in 1826, <a href="/facts/J%C3%A1nos_Bolyai/JeXBranr">János Bolyai</a> (1802–1860) in 1832 and <a href="/facts/Carl_Friedrich_Gauss/f4eSMBD7">Carl Friedrich Gauss</a> (unpublished).
Later in the 19th century, the German mathematician <a href="/facts/Bernhard_Riemann/oV2fL0k8">Bernhard Riemann</a> developed <a href="/facts/Elliptic_geometry/oMnFAoVb">Elliptic geometry</a>, another <a href="/facts/Non-Euclidean_geometry/efKa7De2">non-Euclidean geometry</a> where no parallel can be found and the sum of angles in a triangle is more than 180°. It was proved consistent by defining points as pairs of <a href="/facts/Antipodal_point/2V3228jA">antipodal points</a> on a sphere (or <a href="/facts/Hypersphere/bFu2S7E2">hypersphere</a>), and lines as <a href="/facts/Great_circle/x4fq6eNm">great circles</a> on the sphere. 
These proofs of unprovability of the parallel postulate lead to several philosophical problems, the main one being that before this discovery, the parallel postulate and all its consequences were considered as true. So, the non-Euclidean geometries challenged the concept of <a href="/facts/Mathematical_truth/NyQmBtUC">mathematical truth</a>.

<h3>Synthetic vs. analytic geometry</h3>
Since the introduction of <a href="/facts/Analytic_geometry/3qWP0kM0">analytic geometry</a> by <a href="/facts/Ren%C3%A9_Descartes/ohqj6DMf">René Descartes</a> in the 17th century, there were two approaches to geometry, the old one called <a href="/facts/Synthetic_geometry/IRL0B4K7">synthetic geometry</a>, and the new one, where everything is specified in terms of real numbers called <a href="/facts/Coordinates/Mtd0q4SP">coordinates</a>.
Mathematicians did not worry much about the contradiction between these two approaches before the mid-nineteenth century, where there was "an acrimonious controversy between the proponents of synthetic and analytic methods in <a href="/facts/Projective_geometry/rHKSaEbx">projective geometry</a>, the two sides accusing each other of mixing projective and metric concepts".<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> Indeed, there is no concept of distance in a <a href="/facts/Projective_space/7MSpA9cM">projective space</a>, and the <a href="/facts/Cross-ratio/tj4DK3Vk">cross-ratio</a>, which is a number, is a basic concept of synthetic projective geometry. 
<a href="/facts/Karl_von_Staudt/LjB7QkDM">Karl von Staudt</a> developed a purely geometric approach to this problem by introducing "throws" that form what is presently called a <a href="/facts/Field_(mathematics)/xAjAS4ko">field</a>, in which the cross ratio can be expressed.
Apparently, the problem of the equivalence between analytic and synthetic approach was completely solved only with <a href="/facts/Emil_Artin/rI3vIFo2">Emil Artin</a>'s book <a href="/facts/Geometric_Algebra_(book)/KNF4SNa9">Geometric Algebra</a> published in 1957. It was well known that, given a <a href="/facts/Field_(mathematics)/xAjAS4ko">field</a> k, one may define <a href="/facts/Affine_space/tlvI3oX1">affine</a> and projective spaces over k in terms of k-<a href="/facts/Vector_space/M1pxMLx2">vector spaces</a>. In these spaces, the <a href="/facts/Pappus_hexagon_theorem/t0Hb33dG">Pappus hexagon theorem</a> holds. Conversely, if the Pappus hexagon theorem is included in the axioms of a plane geometry, then one can define a field k such that the geometry is the same as the affine or projective geometry over k.

<h3>Natural numbers</h3>
Main article: <a href="/facts/Peano_arithmetic/DhhDnNug">Peano arithmetic</a>
The work of making rigorous real analysis and the definition of real numbers, consisted of reducing everything to <a href="/facts/Rational_number/VJQmyY05">rational numbers</a> and thus to <a href="/facts/Natural_number/u65og8JE">natural numbers</a>, since positive rational numbers are fractions of natural numbers. There was therefore a need of a formal definition of natural numbers, which imply as <a href="/facts/Axiomatic_theory/dwMz70d5">axiomatic theory</a> of <a href="/facts/Arithmetic/hvHwkbMV">arithmetic</a>. This was started with <a href="/facts/Charles_Sanders_Peirce/z76dNqhM">Charles Sanders Peirce</a> in 1881 and <a href="/facts/Richard_Dedekind/CLkWDhY3">Richard Dedekind</a> in 1888, who defined a natural numbers as the <a href="/facts/Cardinality/m6VPojwT">cardinality</a> of a <a href="/facts/Finite_set/za1newlP">finite set</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> However, this involves <a href="/facts/Set_theory/1ptfsvUP">set theory</a>, which was not formalized at this time.
<a href="/facts/Giuseppe_Peano/VT2Wsmu7">Giuseppe Peano</a> provided in 1888 a complete axiomatisation based on the <a href="/facts/Ordinal_numeral/Lg6cCHuN">ordinal</a> property of the natural numbers. The last Peano's axiom is the only one that induces logical difficulties, as it begin with either "if S is a set then" or "if 
 
 
 
 φ
 
 
 {\displaystyle \varphi }
 
 is a <a href="/facts/Predicate_(mathematical_logic)/5WazAvKF">predicate</a> then". So, Peano's axioms induce a <a href="/facts/Quantification_(logic)/bgmvwxoo">quantification</a> on infinite sets, and this means that Peano arithmetic is what is presently called a <a href="/facts/Second-order_logic/C138cGXQ">Second-order logic</a>.
This was not well understood at that times, but the fact that <a href="/facts/Infinity/cF506uD7">infinity</a> occurred in the definition of the natural numbers was a problem for many mathematicians of this time. For example, <a href="/facts/Henri_Poincar%C3%A9/AazL2j4X">Henri Poincaré</a> stated that axioms can only be demonstrated in their finite application, and concluded that it is "the power of the mind" which allows conceiving of the indefinite repetition of the same act.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> This applies in particular to the use of the last Peano axiom for showing that the <a href="/facts/Successor_function/R1hhkSEG">successor function</a> generates all natural numbers. Also, <a href="/facts/Leopold_Kronecker/T0lMQofb">Leopold Kronecker</a> said "God made the integers, all else is the work of man".<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> This may be interpreted as "the integers cannot be mathematically defined".

<h3>Infinite sets</h3>
Before the second half of the 19th century, <a href="/facts/Infinity/cF506uD7">infinity</a> was a philosophical concept that did not belong to mathematics. However, with the rise of <a href="/facts/Infinitesimal_calculus/MEmUTYoz">infinitesimal calculus</a>, mathematicians became accustomed to infinity, mainly through <a href="/facts/Potential_infinity/ebBSkEbV">potential infinity</a>, that is, as the result of an endless process, such as the definition of an <a href="/facts/Infinite_sequence/gV2S4uqp">infinite sequence</a>, an <a href="/facts/Infinite_series/yDnlsgIe">infinite series</a> or a <a href="/facts/Limit_(mathematics)/j8JBhj4e">limit</a>. The possibility of an <a href="/facts/Actual_infinity/ebBSkEbV">actual infinity</a> was the subject of many philosophical disputes.
<a href="/facts/Set_(mathematics)/BfucfHMq">Sets</a>, and more specially <a href="/facts/Infinite_set/9rTN0xGH">infinite sets</a> were not considered as a mathematical concept; in particular, there was no fixed term for them. A dramatic change arose with the work of <a href="/facts/Georg_Cantor/k5v1DvW4">Georg Cantor</a> who was the first mathematician to systematically study infinite sets. In particular, he introduced <a href="/facts/Cardinal_number/ccoKwU4R">cardinal numbers</a> that measure the size of infinite sets, and <a href="/facts/Ordinal_number/GelFLjJt">ordinal numbers</a> that, roughly speaking, allow one to continue to count after having reach infinity. One of his major results is the discovery that there are strictly more real numbers than natural numbers (the cardinal of the <a href="/facts/Continuum_(set_theory)/ErGtLklG">continuum</a> of the real numbers is greater than that of the natural numbers).
These results were rejected by many mathematicians and philosophers, and led to debates that are a part of the foundational crisis of mathematics.
The crisis was amplified with the <a href="/facts/Russel%27s_paradox/XupqWGYt">Russel's paradox</a> that asserts that the phrase "the set of all sets" is self-contradictory. This condradiction introduced a doubt on the <a href="/facts/Consistency/IHkdnA9M">consistency</a> of all mathematics.
With the introduction of the <a href="/facts/Zermelo%E2%80%93Fraenkel_set_theory/UYIsPbXw">Zermelo–Fraenkel set theory</a> (c. 1925) and its adoption by the mathematical community, the doubt about the consistency was essentially removed, although consistency of set theory cannot be proved because of <a href="/facts/G%C3%B6del%27s_incompleteness_theorem/du8PRu8g">Gödel's incompleteness theorem</a>.

<h3>Mathematical logic</h3>
In 1847, <a href="/facts/Augustus_De_Morgan/6fQ1KTPb">De Morgan</a> published his <a href="/facts/De_Morgan%27s_laws/dj3F8xpt">laws</a> and <a href="/facts/George_Boole/fWJT2rek">George Boole</a> devised an algebra, now called <a href="/facts/Boolean_algebra/VkCToAmg">Boolean algebra</a>, that allows expressing <a href="/facts/Aristotle%27s/U2bDlBFW">Aristotle's</a> logic in terms of formulas and <a href="/facts/Algebraic_operation/wl69LmeN">algebraic operations</a>. Boolean algebra is the starting point of mathematization <a href="/facts/Logic/8JnpJLtt">logic</a> and the basis of <a href="/facts/Propositional_calculus/MNt5v31V">propositional calculus</a>
Independently, in the 1870's, <a href="/facts/Charles_Sanders_Peirce/z76dNqhM">Charles Sanders Peirce</a> and <a href="/facts/Gottlob_Frege/FXyStVYj">Gottlob Frege</a> extended propositional calculus by introducing <a href="/facts/Quantifier_(logic)/bgmvwxoo">quantifiers</a>, for building <a href="/facts/Predicate_logic/lcISqWIg">predicate logic</a>.
Frege pointed out three desired properties of a logical theory:<a href="/facts/Consistency/IHkdnA9M">consistency</a> (impossibility of proving contradictory statements), <a href="/facts/Completeness_(logic)/GzK9zLnu">completeness</a> (any statement is either provable or refutable; that is, its negation is provable), and <a href="/facts/Decidability_(logic)/17SHPMo0">decidability</a> (there is a decision procedure to test every statement).
By near the turn of the century, <a href="/facts/Bertrand_Russell/SRpwNX2U">Bertrand Russell</a> popularized Frege's work and discovered <a href="/facts/Russel%27s_paradox/XupqWGYt">Russel's paradox</a> which implies that the phrase "the set of all sets" is self-contradictory. This paradox seemed to make the whole mathematics inconsistent and is one of the major causes of the foundational crisis of mathematics.

<h2 id="foundational-crisis">Foundational crisis</h2>
The foundational crisis of mathematics
arose at the end of the 19th century and the beginning of the 20th century with the discovery of several <a href="/facts/Paradox/3VLGjuWf">paradoxes</a> or counter-intuitive results. 
The first one was the proof that the <a href="/facts/Parallel_postulate/xhSxUX4u">parallel postulate</a> cannot be proved. This results from a construction of a <a href="/facts/Non-Euclidean_geometry/efKa7De2">non-Euclidean geometry</a> inside <a href="/facts/Euclidean_geometry/V6cTcoCy">Euclidean geometry</a>, whose <a href="/facts/Inconsistency/IHkdnA9M">inconsistency</a> would imply the inconsistency of Euclidean geometry. A well known paradox is <a href="/facts/Russell%27s_paradox/XupqWGYt">Russell's paradox</a>, which shows that the phrase "the set of all sets that do not contain themselves" is self-contradictory. Other philosophical problems were the proof of the existence of <a href="/facts/Mathematical_object/Juhyow9M">mathematical objects</a> that cannot be computed or explicitly described, and the proof of the existence of theorems of <a href="/facts/Arithmetic/hvHwkbMV">arithmetic</a> that cannot be proved with <a href="/facts/Peano_arithmetic/DhhDnNug">Peano arithmetic</a>. 
Several schools of <a href="/facts/Philosophy_of_mathematics/z7xBcdWA">philosophy of mathematics</a> were challenged with these problems in the 20th century, and are described below. 
These problems were also studied by mathematicians, and this led to establish <a href="/facts/Mathematical_logic/SaI7Y3wN">mathematical logic</a> as a new area of mathematics, consisting of providing mathematical definitions to logics (sets of <a href="/facts/Inference_rules/QV5NklQ9">inference rules</a>), mathematical and logical theories, theorems, and proofs, and of using mathematical methods to prove theorems about these concepts. 
This led to unexpected results, such as <a href="/facts/G%C3%B6del%27s_incompleteness_theorems/du8PRu8g">Gödel's incompleteness theorems</a>, which, roughly speaking, assert that, if a theory contains the standard arithmetic, it cannot be used to prove that it itself is not <a href="/facts/Consistent_theory/IHkdnA9M">self-contradictory</a>; and, if it is not self-contradictory, there are theorems that cannot be proved inside the theory, but are nevertheless true in some technical sense. 
<a href="/facts/Zermelo%E2%80%93Fraenkel_set_theory/UYIsPbXw">Zermelo–Fraenkel set theory</a> with the <a href="/facts/Axiom_of_choice/1Ura2HTh">axiom of choice</a> (ZFC) is a logical theory established by <a href="/facts/Ernst_Zermelo/uvW8gDq0">Ernst Zermelo</a> and <a href="/facts/Abraham_Fraenkel/Hc8IBfmY">Abraham Fraenkel</a>. It became the standard foundation of modern mathematics, and, unless the contrary is explicitly specified, it is used in all modern mathematical texts, generally implicitly.
Simultaneously, the <a href="/facts/Axiomatic_method/dwMz70d5">axiomatic method</a> became a de facto standard: the proof of a theorem must result from explicit <a href="/facts/Axiom/mLjP14Mc">axioms</a> and previously proved theorems by the application of clearly defined inference rules. The axioms need not correspond to some reality. Nevertheless, it is an open philosophical problem to explain why the axiom systems that lead to rich and useful theories are those resulting from abstraction from the physical reality or other mathematical theory.
In summary, the foundational crisis is essentially resolved, and this opens new philosophical problems. In particular, it cannot be proved that the new foundation (ZFC) is not self-contradictory. It is a general consensus that, if this would happen, the problem could be solved by a mild modification of ZFC. 

<h3>Philosophical views</h3>
Main article: <a href="/facts/Philosophy_of_mathematics/z7xBcdWA">Philosophy of mathematics</a>
When the foundational crisis arose, there was much debate among mathematicians and logicians about what should be done for restoring confidence in mathematics. This involved philosophical questions about <a href="/facts/Mathematical_truth/NyQmBtUC">mathematical truth</a>, the relationship of mathematics with <a href="/facts/Reality/oC1xokDB">reality</a>, the reality of <a href="/facts/Mathematical_object/Juhyow9M">mathematical objects</a>, and the nature of mathematics.
For the problem of foundations, there was two main options for trying to avoid paradoxes. The first one led to <a href="/facts/Intuitionism/1gTeosv9">intuitionism</a> and <a href="/facts/Constructivism_(mathematics)/1VtNuLwT">constructivism</a>, and consisted to restrict the logical rules for remaining closer to intuition, while the second, which has been called <a href="/facts/Formalism_(mathematics)/g0690AVL">formalism</a>, considers that a theorem is true if it can be deduced from <a href="/facts/Axiom/mLjP14Mc">axioms</a> by applying inference rules (<a href="/facts/Formal_proof/6nEhV4cw">formal proof</a>), and that no "trueness" of the axioms is needed for the validity of a theorem.

<h4>Formalism</h4>
Main article: <a href="/facts/Formalism_(mathematics)/g0690AVL">Formalism (mathematics)</a>
It has been claimed[by whom?] that formalists, such as <a href="/facts/David_Hilbert/1vhlHn4b">David Hilbert</a> (1862–1943), hold that mathematics is only a language and a series of games. Hilbert insisted that formalism, called "formula game" by him, is a fundamental part of mathematics, but that mathematics must not be reduced to formalism. Indeed, he used the words "formula game" in his 1927 response to <a href="/facts/L._E._J._Brouwer/6q6ULf3b">L. E. J. Brouwer</a>'s criticisms:

<blockquote>And to what extent has the formula game thus made possible been successful? This formula game enables us to express the entire thought-content of the science of mathematics in a uniform manner and develop it in such a way that, at the same time, the interconnections between the individual propositions and facts become clear ... The formula game that Brouwer so deprecates has, besides its mathematical value, an important general philosophical significance. For this formula game is carried out according to certain definite rules, in which the technique of our thinking is expressed. These rules form a closed system that can be discovered and definitively stated.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a></blockquote>
Thus Hilbert is insisting that mathematics is not an arbitrary game with arbitrary rules; rather it must agree with how our thinking, and then our speaking and writing, proceeds.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>

<blockquote>We are not speaking here of arbitrariness in any sense. Mathematics is not like a game whose tasks are determined by arbitrarily stipulated rules. Rather, it is a conceptual system possessing internal necessity that can only be so and by no means otherwise.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a></blockquote>
The foundational philosophy of formalism, as exemplified by <a href="/facts/David_Hilbert/1vhlHn4b">David Hilbert</a>, is a response to the paradoxes of <a href="/facts/Set_theory/1ptfsvUP">set theory</a>, and is based on <a href="/facts/Formal_logic/SaI7Y3wN">formal logic</a>. Virtually all mathematical <a href="/facts/Theorem/f2Pe1FMD">theorems</a> today can be formulated as theorems of set theory. The truth of a mathematical statement, in this view, is represented by the fact that the statement can be derived from the <a href="/facts/Zermelo%E2%80%93Fraenkel_set_theory/UYIsPbXw">axioms of set theory</a> using the rules of formal logic.
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 "true" mathematical statements (e.g., the <a href="/facts/Peano_axioms/DhhDnNug">laws of arithmetic</a>) appear to be true, and so on. <a href="/facts/Hermann_Weyl/4C5HhyGU">Hermann Weyl</a> posed these very questions to Hilbert:

<blockquote>What "truth" or objectivity can be ascribed to this theoretic construction of the world, which presses far beyond the given, is a profound philosophical problem. It is closely connected with the further question: what impels us to take as a basis precisely the particular axiom system developed by Hilbert? Consistency is indeed a necessary but not a sufficient condition. For the time being we probably cannot answer this question ...<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a></blockquote>
In some cases these questions may be sufficiently answered through the study of formal theories, in disciplines such as <a href="/facts/Reverse_mathematics/nHLm6JUQ">reverse mathematics</a> and <a href="/facts/Computational_complexity_theory/etHuVF3U">computational complexity theory</a>. As noted by Weyl, <a href="/facts/Formal_logical_system/KaeDAjK7">formal logical systems</a> also run the risk of <a href="/facts/Consistency_proof/IHkdnA9M">inconsistency</a>; in <a href="/facts/Peano_axioms/DhhDnNug">Peano arithmetic</a>, this arguably has already been settled with several proofs of <a href="/facts/Consistency_proof/IHkdnA9M">consistency</a>, but there is debate over whether or not they are sufficiently <a href="/facts/Finitism/UudNtdnd">finitary</a> to be meaningful. <a href="/facts/G%C3%B6del%27s_incompleteness_theorem/du8PRu8g">Gödel's second incompleteness theorem</a> establishes that logical systems of arithmetic can never contain a valid proof of their own <a href="/facts/Consistency_proof/IHkdnA9M">consistency</a>. 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.

<h4>Intuitionism</h4>
Main articles: <a href="/facts/Intuitionism/1gTeosv9">Intuitionism</a> and <a href="/facts/Constructivism_(mathematics)/1VtNuLwT">Constructivism (mathematics)</a>
Intuitionists, such as <a href="/facts/L._E._J._Brouwer/6q6ULf3b">L. E. J. Brouwer</a> (1882–1966), hold that mathematics is a creation of the human mind. Numbers, like fairy tale characters, are merely mental entities, which would not exist if there were never any human minds to think about them.
The foundational philosophy of <a href="/facts/Intuitionism/1gTeosv9">intuitionism</a> or <a href="/facts/Constructivism_(mathematics)/1VtNuLwT">constructivism</a>, as exemplified in the extreme by <a href="/facts/Luitzen_Egbertus_Jan_Brouwer/6q6ULf3b">Brouwer</a> and <a href="/facts/Stephen_Kleene/sKSax2se">Stephen Kleene</a>, 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 <a href="/facts/Reductio_ad_absurdum/5AJUtVmL">reductio ad absurdum</a> is suspect.
Some modern <a href="/facts/Theory/9vrNS1Rr">theories</a> in the philosophy of mathematics deny the existence of foundations in the original sense. Some theories tend to focus on <a href="/facts/Mathematical_practice/wFICd4rc">mathematical practice</a>, and aim to describe and analyze the actual working of mathematicians as a <a href="/facts/Social_group/L0Ny3Nv6">social group</a>. Others try to create a <a href="/facts/Cognitive_science_of_mathematics/8AozQ5Qj">cognitive science of mathematics</a>, 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.

<h4>Logicism</h4>
Main article: <a href="/facts/Logicism/YvHs6X3E">Logicism</a>
<a href="/facts/Logicism/YvHs6X3E">Logicism</a> is a school of thought, and research programme, in the philosophy of mathematics, based on the thesis that mathematics is an extension of logic or that some or all mathematics may be derived in a suitable formal system whose axioms and rules of inference are 'logical' in nature. <a href="/facts/Bertrand_Russell/SRpwNX2U">Bertrand Russell</a> and <a href="/facts/Alfred_North_Whitehead/7KuylmP3">Alfred North Whitehead</a> championed this theory initiated by <a href="/facts/Gottlob_Frege/FXyStVYj">Gottlob Frege</a> and influenced by <a href="/facts/Richard_Dedekind/CLkWDhY3">Richard Dedekind</a>.

<h4>Set-theoretic Platonism</h4>
Main article: <a href="/facts/Set-theoretic_Platonism/z7xBcdWA">Set-theoretic Platonism</a>
Many researchers in <a href="/facts/Axiomatic_set_theory/1ptfsvUP">axiomatic set theory</a> have subscribed to what is known as set-theoretic <a href="/facts/Platonism/0mvr2x2p">Platonism</a>, exemplified by <a href="/facts/Kurt_G%C3%B6del/P14DqKqK">Kurt Gödel</a>.
Several set theorists followed this approach and actively searched for axioms that may be considered as true for heuristic reasons and that would decide the <a href="/facts/Continuum_hypothesis/R0yGqHND">continuum hypothesis</a>. Many <a href="/facts/Large_cardinal/jkSeV92D">large cardinal</a> axioms were studied, but the hypothesis always remained <a href="/facts/Independence_(mathematical_logic)/xW9tCrGB">independent</a> from them and it is now considered unlikely that CH can be resolved by a new large cardinal axiom. Other types of axioms were considered, but none of them has reached consensus on the continuum hypothesis yet. Recent work by <a href="/facts/Joel_David_Hamkins/Cd525uY3">Hamkins</a> proposes a more flexible alternative: a set-theoretic <a href="/facts/Multiverse/GUCDxvPg">multiverse</a> allowing free passage between set-theoretic universes that satisfy the continuum hypothesis and other universes that do not.

<h4>Indispensability argument for realism</h4>
Main article: <a href="/facts/Quine%E2%80%93Putnam_indispensability_argument/z7xBcdWA">Quine–Putnam indispensability argument</a>
This <a href="/facts/Quine%E2%80%93Putnam_indispensability_thesis/z7xBcdWA">argument</a> by <a href="/facts/Willard_Quine/2RrhATxa">Willard Quine</a> and <a href="/facts/Hilary_Putnam/ujwAh4Ry">Hilary Putnam</a> says (in Putnam's shorter words),

<blockquote>... quantification over mathematical entities is indispensable for science ... therefore we should accept such quantification; but this commits us to accepting the existence of the mathematical entities in question.</blockquote>
However, Putnam was not a Platonist.

<h4>Rough-and-ready realism</h4>
Few mathematicians are typically concerned on a daily, working basis over logicism, formalism or any other philosophical position. Instead, their primary concern is that the mathematical enterprise as a whole always remains productive. Typically, they see this as ensured by remaining open-minded, practical and busy; as potentially threatened by becoming overly-ideological, fanatically reductionistic or lazy.
Such a view has also been expressed by some well-known physicists.
For example, the Physics Nobel Prize laureate <a href="/facts/Richard_Feynman/32vGFrtB">Richard Feynman</a> said

<blockquote>People say to me, "Are you looking for the ultimate laws of physics?" No, I'm not ... If it turns out there is a simple ultimate law which explains everything, so be it – that would be very nice to discover. If it turns out it's like an onion with millions of layers ... then that's the way it is. But either way there's Nature and she's going to come out the way She is. So therefore when we go to investigate we shouldn't predecide what it is we're looking for only to find out more about it.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a></blockquote>
And <a href="/facts/Steven_Weinberg/G61D6kJw">Steven Weinberg</a>:<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>

<blockquote>The insights of philosophers have occasionally benefited physicists, but generally in a negative fashion – by protecting them from the preconceptions of other philosophers. ... without some guidance from our preconceptions one could do nothing at all. It is just that philosophical principles have not generally provided us with the right preconceptions.</blockquote>
Weinberg believed that any undecidability in mathematics, such as the continuum hypothesis, could be potentially resolved despite the incompleteness theorem, by finding suitable further axioms to add to set theory.

<h4>Philosophical consequences of Gödel's completeness theorem</h4>
Main article: <a href="/facts/G%C3%B6del%27s_completeness_theorem/yq02MUyw">Gödel's completeness theorem</a>
Gödel's completeness theorem establishes an equivalence in first-order logic between the formal provability of a formula and its truth in all possible models. Precisely, for any consistent first-order theory it gives an "explicit construction" of a model described by the theory; this model will be countable if the language of the theory is countable. However this "explicit construction" is not algorithmic. It is based on an iterative process of completion of the theory, where each step of the iteration consists in adding a formula to the axioms if it keeps the theory consistent; but this consistency question is only semi-decidable (an algorithm is available to find any contradiction but if there is none this consistency fact can remain unprovable).

<h3>More paradoxes</h3>
See also: <a href="/facts/List_of_statements_independent_of_ZFC/rdDFlEBp">List of statements independent of ZFC</a> and <a href="/facts/List_of_paradoxes/Npc3Wl2o">List of paradoxes</a>
The following lists some notable results in metamathematics. <a href="/facts/Zermelo%E2%80%93Fraenkel_set_theory/UYIsPbXw">Zermelo–Fraenkel set theory</a> is the most widely studied axiomatization of set theory. It is abbreviated ZFC when it includes the <a href="/facts/Axiom_of_choice/1Ura2HTh">axiom of choice</a> and ZF when the axiom of choice is excluded.

<ul><li>1920: <a href="/facts/Thoralf_Skolem/M7t2mfGx">Thoralf Skolem</a> corrected <a href="/facts/Leopold_L%C3%B6wenheim/H4xDPMK1">Leopold Löwenheim</a>'s proof of what is now called the <a href="/facts/Downward_L%C3%B6wenheim%E2%80%93Skolem_theorem/pb72J40H">downward Löwenheim–Skolem theorem</a>, leading to <a href="/facts/Skolem%27s_paradox/Dsu3Jun5">Skolem's paradox</a> discussed in 1922, namely the existence of countable models of ZF, making infinite cardinalities a relative property.</li>
<li>1922: Proof by <a href="/facts/Abraham_Fraenkel/Hc8IBfmY">Abraham Fraenkel</a> that the axiom of choice cannot be proved from the axioms of <a href="/facts/Zermelo_set_theory/o3fzd9nl">Zermelo set theory</a> with <a href="/facts/Urelement/YLbuKp37">urelements</a>.</li>
<li>1931: Publication of <a href="/facts/G%C3%B6del%27s_incompleteness_theorems/du8PRu8g">Gödel's incompleteness theorems</a>, showing that essential aspects of Hilbert's program could not be attained. It showed how to construct, for any sufficiently powerful and consistent recursively axiomatizable system – such as necessary to axiomatize the elementary theory of <a href="/facts/Arithmetic/hvHwkbMV">arithmetic</a> on the (infinite) set of natural numbers – a statement that formally expresses its own unprovability, which he then proved equivalent to the claim of consistency of the theory; so that (assuming the consistency as true), the system is not powerful enough for proving its own consistency, let alone that a simpler system could do the job. It thus became clear that the notion of mathematical truth cannot be completely determined and reduced to a purely <a href="/facts/Formal_system/KaeDAjK7">formal system</a> as envisaged in Hilbert's program. 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).</li>
<li>1936: <a href="/facts/Alfred_Tarski/7h6NySAj">Alfred Tarski</a> proved his <a href="/facts/Tarski%27s_undefinability_theorem/T9LgFkVx">truth undefinability theorem</a>.</li>
<li>1936: <a href="/facts/Alan_Turing/klov9b6g">Alan Turing</a> proved that a general algorithm to solve the <a href="/facts/Halting_problem/Xz1bac40">halting problem</a> for all possible program-input pairs cannot exist.</li>
<li>1938: Gödel proved the <a href="/facts/Constructible_universe/pxU0cmL3">consistency of the axiom of choice and of the generalized continuum hypothesis</a>.</li>
<li>1936–1937: <a href="/facts/Alonzo_Church/b7D52Itv">Alonzo Church</a> and <a href="/facts/Alan_Turing/klov9b6g">Alan Turing</a>, respectively, published independent papers showing that a general solution to the <a href="/facts/Entscheidungsproblem/y8LDal2X">Entscheidungsproblem</a> is impossible: the universal validity of statements in first-order logic is not decidable (it is only <a href="/facts/Semi-decidable/QBxmXBbY">semi-decidable</a> as given by the <a href="/facts/Completeness_theorem/yq02MUyw">completeness theorem</a>).</li>
<li>1955: <a href="/facts/Pyotr_Novikov/nI15TQVe">Pyotr Novikov</a> showed that there exists a <a href="/facts/Finitely_presented_group/f9emPUoI">finitely presented group</a> G such that the <a href="/facts/Word_problem_for_groups/eKBrkBPv">word problem</a> for G is undecidable.</li>
<li>1963: <a href="/facts/Paul_Cohen_(mathematician)/wMLaE3Vz">Paul Cohen</a> showed that the Continuum Hypothesis is unprovable from <a href="/facts/Zermelo%E2%80%93Fraenkel_set_theory/UYIsPbXw">ZFC</a>. Cohen's proof developed the method of <a href="/facts/Forcing_(mathematics)/3vMqqKcq">forcing</a>, which is now an important tool for establishing <a href="/facts/Independence_(mathematical_logic)/xW9tCrGB">independence</a> results in set theory.</li>
<li>1964: Inspired by the fundamental randomness in physics, <a href="/facts/Gregory_Chaitin/EQLL0gSt">Gregory Chaitin</a> starts publishing results on <a href="/facts/Algorithmic_information_theory/Tltn3I0Y">algorithmic information theory</a> (measuring incompleteness and randomness in mathematics).<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a></li>
<li>1966: Paul Cohen showed that the axiom of choice is unprovable in ZF even without <a href="/facts/Urelements/YLbuKp37">urelements</a>.</li>
<li>1970: <a href="/facts/Hilbert%27s_tenth_problem/EaGPklEc">Hilbert's tenth problem</a> is proven unsolvable: there is no recursive solution to decide whether a <a href="/facts/Diophantine_equation/mYaHp7oX">Diophantine equation</a> (multivariable polynomial equation) has a solution in integers.</li>
<li>1971: <a href="/facts/Suslin%27s_problem/ddnehNel">Suslin's problem</a> is proven to be independent from ZFC.</li></ul>
<h2 id="toward-resolution-of-the-crisis">Toward resolution of the crisis</h2>
Starting in 1935, the <a href="/facts/Nicolas_Bourbaki/qRJhs9nr">Bourbaki</a> group of French mathematicians started publishing a series of books to formalize many areas of mathematics on the new foundation of set theory.
The intuitionistic school did not attract many adherents, and it was not until <a href="/facts/Errett_Bishop/NJ6DQoqc">Bishop</a>'s work in 1967 that <a href="/facts/Constructivism_(mathematics)/1VtNuLwT">constructive mathematics</a> was placed on a sounder footing.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a>
One may consider that <a href="/facts/Hilbert%27s_program/9VnGkMbf">Hilbert's program has been partially completed</a>, so that the crisis is essentially resolved, satisfying ourselves with lower requirements than Hilbert's original ambitions. His ambitions were expressed in a time when nothing was clear: it was not clear whether mathematics could have a rigorous foundation at all.
There are many possible variants of set theory, which differ in consistency strength, where stronger versions (postulating higher types of infinities) contain formal proofs of the consistency of weaker versions, but none contains a formal proof of its own consistency. Thus the only thing we do not have is a formal proof of consistency of whatever version of set theory we may prefer, such as ZF.
In practice, most mathematicians either do not work from axiomatic systems, or if they do, do not doubt the consistency of <a href="/facts/ZFC/UYIsPbXw">ZFC</a>, generally their preferred axiomatic system. In most of mathematics as it is practiced, the incompleteness and paradoxes of the underlying formal theories never played a role anyway, and in those branches in which they do or whose formalization attempts would run the risk of forming inconsistent theories (such as logic and category theory), they may be treated carefully.
The development of <a href="/facts/Category_theory/6zS3DXPa">category theory</a> in the middle of the 20th century showed the usefulness of set theories guaranteeing the existence of larger classes than does ZFC, such as <a href="/facts/Von_Neumann%E2%80%93Bernays%E2%80%93G%C3%B6del_set_theory/5wLhAIMz">Von Neumann–Bernays–Gödel set theory</a> or <a href="/facts/Tarski%E2%80%93Grothendieck_set_theory/Y52Kk3aP">Tarski–Grothendieck set theory</a>, albeit that in very many cases the use of large cardinal axioms or Grothendieck universes is formally eliminable.
One goal of the <a href="/facts/Reverse_Mathematics/nHLm6JUQ">reverse mathematics</a> program is to identify whether there are areas of "core mathematics" in which foundational issues may again provoke a crisis.

<h2 id="see-also">See also</h2>
<ul>
<li>Mathematics portal</li><li>History of science portal</li><li>Philosophy portal</li></ul>
<ul><li><a href="/facts/Aristotelian_realist_philosophy_of_mathematics/3au3NJsl">Aristotelian realist philosophy of mathematics</a></li>
<li><a href="/facts/Mathematical_logic/SaI7Y3wN">Mathematical logic</a></li>
<li><a href="/facts/Brouwer%E2%80%93Hilbert_controversy/IM8dH1Jg">Brouwer–Hilbert controversy</a></li>
<li><a href="/facts/Church%E2%80%93Turing_thesis/hDHn641f">Church–Turing thesis</a></li>
<li><a href="/facts/Controversy_over_Cantor%27s_theory/7dHrQ1M6">Controversy over Cantor's theory</a></li>
<li><a href="/facts/Epistemology/NsDwpcYH">Epistemology</a></li>
<li><a href="/facts/Euclid%27s_Elements/nPKvCIxs">Euclid's Elements</a></li>
<li><a href="/facts/Hilbert%27s_problems/HaQWIxvV">Hilbert's problems</a></li>
<li><a href="/facts/Implementation_of_mathematics_in_set_theory/5tNFa5DW">Implementation of mathematics in set theory</a></li>
<li><a href="/facts/Liar_paradox/YUdbUtJl">Liar paradox</a></li>
<li><a href="/facts/New_Foundations/jT6vAvj5">New Foundations</a></li>
<li><a href="/facts/Philosophy_of_mathematics/z7xBcdWA">Philosophy of mathematics</a></li>
<li><a href="/facts/Principia_Mathematica/7ufeceV4">Principia Mathematica</a></li>
<li><a href="/facts/Quasi-empiricism_in_mathematics/WR4IBAW8">Quasi-empiricism in mathematics</a></li>
<li><a href="/facts/Charles_Sanders_Peirce/z76dNqhM">Mathematical thought of Charles Peirce</a></li></ul>
<h2 id="notes">Notes</h2>

<ul><li><a href="/facts/Jeremy_Avigad/rItWA34V">Avigad, Jeremy</a> (2003) <a href="https://www.andrew.cmu.edu/user/avigad/Papers/elementary.pdf">Number theory and elementary arithmetic</a>, Philosophia Mathematica Vol. 11, pp. 257–284</li>
<li><a href="/facts/Howard_Eves/DbNLVbPp">Eves, Howard</a> (1990), Foundations and Fundamental Concepts of Mathematics Third Edition, Dover Publications, INC, Mineola NY, <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-486-69609-X (pbk.) cf §9.5 Philosophies of Mathematics pp. 266–271. Eves lists the three with short descriptions prefaced by a brief introduction.</li>
<li>Goodman, N.D. (1979), "<a href="https://www.jstor.org/stable/2320581">Mathematics as an Objective Science</a>", in Tymoczko (ed., 1986).</li>
<li><a href="/facts/Wilbur_Dyre_Hart/2XDcVxXI">Hart, W.D.</a> (ed., 1996), The Philosophy of Mathematics, Oxford University Press, Oxford, UK.</li>
<li><a href="/facts/Reuben_Hersh/tvg8Sxe6">Hersh, R.</a> (1979), "Some Proposals for Reviving the Philosophy of Mathematics", in (Tymoczko 1986).</li>
<li><a href="/facts/David_Hilbert/1vhlHn4b">Hilbert, D.</a> (1922), "Neubegründung der Mathematik. Erste Mitteilung", Hamburger Mathematische Seminarabhandlungen 1, 157–177. Translated, "The New Grounding of Mathematics. First Report", in (Mancosu 1998).</li>
<li>Katz, Robert (1964), Axiomatic Analysis, D. C. Heath and Company.</li>
<li><a href="/facts/Stephen_Kleene/sKSax2se">Kleene, Stephen C.</a> (1991) [1952]. Introduction to Meta-Mathematics (Tenth impression 1991 ed.). Amsterdam NY: North-Holland Pub. Co. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-7204-2103-9.</li></ul>
In Chapter III A Critique of Mathematic Reasoning, §11. The paradoxes, Kleene discusses <a href="/facts/Intuitionism/1gTeosv9">Intuitionism</a> and <a href="/facts/Formalism_(mathematics)/g0690AVL">Formalism</a> in depth. Throughout the rest of the book he treats, and compares, both Formalist (classical) and Intuitionist logics with an emphasis on the former. Extraordinary writing by an extraordinary mathematician.
<ul><li>Mancosu, P. (ed., 1998), From Hilbert to Brouwer. The Debate on the Foundations of Mathematics in the 1920s, Oxford University Press, Oxford, UK.</li>
<li><a href="/facts/Hilary_Putnam/ujwAh4Ry">Putnam, Hilary</a> (1967), "Mathematics Without Foundations", Journal of Philosophy 64/1, 5–22. Reprinted, pp. 168–184 in W.D. Hart (ed., 1996).</li>
<li>—, "What is Mathematical Truth?", in Tymoczko (ed., 1986).</li>
<li>Sudac, Olivier (Apr 2001). "The prime number theorem is PRA-provable". Theoretical Computer Science. 257 (1–2): 185–239. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1016%2FS0304-3975%2800%2900116-X">10.1016/S0304-3975(00)00116-X</a>.</li>
<li><a href="/facts/Anne_Sjerp_Troelstra/94dULETx">Troelstra, A. S.</a> (no date but later than 1990), <a href="https://web.archive.org/web/20060209210015/http://staff.science.uva.nl/~anne/hhhist.pdf">"A History of Constructivism in the 20th Century"</a>, A detailed survey for specialists: §1 Introduction, §2 Finitism & §2.2 Actualism, §3 Predicativism and Semi-Intuitionism, §4 Brouwerian Intuitionism, §5 Intuitionistic Logic and Arithmetic, §6 Intuitionistic Analysis and Stronger Theories, §7 Constructive Recursive Mathematics, §8 Bishop's Constructivism, §9 Concluding Remarks. Approximately 80 references.</li>
<li><a href="/facts/Thomas_Tymoczko/sGdzpRwr">Tymoczko, T.</a> (1986), "Challenging Foundations", in Tymoczko (ed., 1986).</li>
<li>—,(ed., 1986), <a href="https://books.google.com/books?id=a-M9DwAAQBAJ&dq=%22New+Directions+in+the+Philosophy+of+Mathematics%22&pg=PR9">New Directions in the Philosophy of Mathematics</a>, 1986. Revised edition, 1998.</li>
<li>van Dalen D. (2008), "Brouwer, Luitzen Egbertus Jan (1881–1966)", in Biografisch Woordenboek van Nederland. URL:<a href="http://www.inghist.nl/Onderzoek/Projecten/BWN/lemmata/bwn2/brouwerle">http://www.inghist.nl/Onderzoek/Projecten/BWN/lemmata/bwn2/brouwerle</a> [2008-03-13]</li>
<li><a href="/facts/Hermann_Weyl/4C5HhyGU">Weyl, H.</a> (1921), "Über die neue Grundlagenkrise der Mathematik", Mathematische Zeitschrift 10, 39–79. Translated, "On the New Foundational Crisis of Mathematics", in (Mancosu 1998).</li>
<li><a href="/facts/Raymond_Louis_Wilder/E9ubx33G">Wilder, Raymond L.</a> (1952), Introduction to the Foundations of Mathematics, John Wiley and Sons, New York, NY.</li></ul>
<h2 id="external-links">External links</h2>

Wikiquote has quotations related to Foundations of mathematics.

<ul><li> Media related to Foundations of mathematics at Wikimedia Commons</li>
<li><a href="http://www.iep.utm.edu/category/s-l-m/math/">"Philosophy of mathematics"</a>. <a href="/facts/Internet_Encyclopedia_of_Philosophy/tmS0Sylv">Internet Encyclopedia of Philosophy</a>.</li>
<li><a href="http://www.personal.psu.edu/t20/papers/philmath/">Logic and Mathematics</a></li>
<li>Harvey M. Friedman, <a href="https://cpb-us-w2.wpmucdn.com/u.osu.edu/dist/1/1952/files/2014/01/fom12pt5.31.00-1jkl4df.pdf">Foundations of Mathematics: past, present, and future</a>, May 31, 2000, 8 pages.</li>
<li><a href="https://arxiv.org/html/nlin/0004007">A Century of Controversy over the Foundations of Mathematics</a> by Gregory Chaitin.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Joachim Lambek (2007), "Foundations of mathematics", Encyclopædia Britannica <a href="https://www.britannica.com/EBchecked/topic/369221/foundations-of-mathematics" target="_blank">https://www.britannica.com/EBchecked/topic/369221/foundations-of-mathematics</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">The thirteen books of Euclid's Elements, edited by Sir Thomas Heath. Vol. 2 (Book V). Translated by Heiberg. New York: Dover Publications. 1956. pp. 124–126. ISBN 0-486-60089-0. <a href="0-486-60089-0" target="_blank">0-486-60089-0</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">The Analyst, A Discourse Addressed to an Infidel Mathematician <a href="/wiki/The_Analyst" target="_blank">/wiki/The_Analyst</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Grabiner, Judith V. (1983), "Who Gave You the Epsilon? Cauchy and the Origins of Rigorous Calculus", American Mathematical Monthly, 90 (3): 185–194, doi:10.2307/2975545, JSTOR 2975545, collected in Who Gave You the Epsilon?, ISBN 978-0-88385-569-0 pp. 5–13. Also available at: http://www.maa.org/pubs/Calc_articles/ma002.pdf <a href="/wiki/American_Mathematical_Monthly" target="_blank">/wiki/American_Mathematical_Monthly</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">O'Connor, John J.; Robertson, Edmund F. (October 2005), "The real numbers: Stevin to Hilbert", MacTutor History of Mathematics Archive, University of St Andrews <a href="/wiki/Edmund_F._Robertson" target="_blank">/wiki/Edmund_F._Robertson</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Laptev, B.L. & B.A. Rozenfel'd (1996) Mathematics of the 19th Century: Geometry, page 40, Birkhäuser ISBN 3-7643-5048-2 <a href="/wiki/Springer_Science%2BBusiness_Media" target="_blank">/wiki/Springer_Science%2BBusiness_Media</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Dedekind, Richard. What Are and What Should the Numbers Be? Continuity and Irrational Numbers. Springer. ISBN 978-3-662-70059-4. <a href="978-3-662-70059-4" target="_blank">978-3-662-70059-4</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Poincaré, Henri (1905) [1902]. "On the nature of mathematical reasoning". La Science et l'hypothèse [Science and Hypothesis]. Translated by Greenstreet, William John. VI. <a href="https://en.wikisource.org/wiki/Science_and_Hypothesis/Chapter_1" target="_blank">https://en.wikisource.org/wiki/Science_and_Hypothesis/Chapter_1</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">The English translation is from Gray. In a footnote, Gray attributes the German quote to: "Weber 1891–1892, 19, quoting from a lecture of Kronecker's of 1886."[9][10] <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Hilbert 1927 The Foundations of Mathematics in van Heijenoort 1967:475 <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Hilbert 1927 The Foundations of Mathematics in van Heijenoort 1967:475 <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">p. 14 in Hilbert, D. (1919–20), Natur und Mathematisches Erkennen: Vorlesungen, gehalten 1919–1920 in Göttingen. Nach der Ausarbeitung von Paul Bernays (Edited and with an English introduction by David E. Rowe), Basel, Birkhauser (1992). <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Weyl 1927 Comments on Hilbert's second lecture on the foundations of mathematics in van Heijenoort 1967:484. Although Weyl the intuitionist believed that "Hilbert's view" would ultimately prevail, this would come with a significant loss to philosophy: "I see in this a decisive defeat of the philosophical attitude of pure phenomenology, which thus proves to be insufficient for the understanding of creative science even in the area of cognition that is most primal and most readily open to evidence – mathematics" (ibid). <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Richard Feynman, The Pleasure of Finding Things Out p. 23 <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Steven Weinberg, chapter Against Philosophy wrote, in Dreams of a final theory <a href="http://libcom.org/library/unexpected-uselessness-philosophy" target="_blank">http://libcom.org/library/unexpected-uselessness-philosophy</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Chaitin, Gregory (2006), "The Limits Of Reason" (PDF), Scientific American, 294 (3): 74–81, Bibcode:2006SciAm.294c..74C, doi:10.1038/scientificamerican0306-74, PMID 16502614, archived from the original (PDF) on 2016-03-04, retrieved 2016-02-22 <a href="/wiki/Gregory_Chaitin" target="_blank">/wiki/Gregory_Chaitin</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Andrej Bauer (2017), "Five stages of accepting constructive mathematics", Bull. Amer. Math. Soc., 54 (3): 485, doi:10.1090/bull/1556 <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
</ol>

Foundations of mathematics open-in-new

Foundations of mathematics