Editing Metamathematics

{{short description|Study of mathematics itself}}
{{refimprove|date=November 2018}}[[File:Russell, Whitehead - Principia Mathematica to 56.jpg|thumb|The title page of the [[Principia Mathematica]] (shortened version), an important work of metamathematics.]]

'''Metamathematics''' is the study of [[mathematics]] itself using mathematical methods. This study produces [[metatheory|metatheories]], which are [[Mathematical theory|mathematical theories]] about other mathematical theories. Emphasis on metamathematics (and perhaps the creation of the term itself) owes itself to [[David Hilbert]]'s [[Hilbert program|attempt]] to secure the [[foundations of mathematics]] in the early part of the 20th century. Metamathematics provides "a rigorous mathematical technique for investigating a great variety of foundation problems for mathematics and [[logic]]" (Kleene 1952, p.&nbsp;59). An important feature of metamathematics is its emphasis on differentiating between reasoning from inside a system and from outside a system. An informal illustration of this is categorizing the proposition "2+2=4" as belonging to mathematics while categorizing the proposition "'2+2=4' is valid" as belonging to metamathematics.

== History ==
Metamathematical [[metatheorem]]s about mathematics itself were originally differentiated from ordinary [[mathematical theorem]]s in the 19th century to focus on what was then called the [[foundational crisis of mathematics]]. [[Richard's paradox]] (Richard 1905) concerning certain 'definitions' of real numbers in the English language is an example of the sort of contradictions that can easily occur if one fails to distinguish between mathematics and metamathematics. Something similar can be said around the well-known [[Russell's paradox]] (Does the set of all those sets that do not contain themselves contain itself?).

Metamathematics was intimately connected to [[mathematical logic]], so that the early histories of the two fields, during the late 19th and early 20th centuries, largely overlap. More recently, mathematical logic has often included the study of new pure mathematics, such as [[set theory]], [[category theory]], [[recursion theory]] and pure [[model theory]].

Serious metamathematical reflection began with the work of [[Gottlob Frege]], especially his ''[[Begriffsschrift]]'', published in 1879.

[[David Hilbert]] was the first to invoke the term "metamathematics" with regularity (see [[Hilbert's program]]), in the early 20th century. In his hands, it meant something akin to contemporary [[proof theory]], in which finitary methods are used to study various axiomatized mathematical theorems (Kleene 1952, p.&nbsp;55).

Other prominent figures in the field include [[Bertrand Russell]], [[Thoralf Skolem]], [[Emil Post]], [[Alonzo Church]], [[Alan Turing]], [[Stephen Kleene]], [[Willard Quine]], [[Paul Benacerraf]], [[Hilary Putnam]], [[Gregory Chaitin]], [[Alfred Tarski]], [[Paul Cohen]] and [[Kurt Gödel]].

Today, [[metalogic]] and metamathematics broadly overlap, and both have been substantially subsumed by mathematical logic in academia.

== Milestones ==
=== The discovery of hyperbolic geometry ===
{{See also|Non-Euclidean geometry#History |Hyperbolic geometry#History}}

The discovery of [[hyperbolic geometry]] had important [[philosophy|philosophical]] consequences for metamathematics. Before its discovery there was just one geometry and mathematics; the idea that another geometry existed was considered improbable.

When [[Gauss]] discovered hyperbolic geometry, it is said that he did not publish anything about it out of fear of the "uproar of the [[Boeotia#Pejorative term|Boeotians]]", which would ruin his status as ''princeps mathematicorum'' (Latin, "the Prince of Mathematicians").<ref>{{cite book|last1=Torretti|first1=Roberto|title=Philosophy of Geometry from Riemann to Poincare|date=1978|publisher=Reidel|location=Dordrecht Holland|page=255}}</ref> 
The "uproar of the Boeotians" came and went, and gave an impetus to metamathematics and great improvements in [[mathematical rigour]], [[analytical philosophy]] and [[logic]].

=== ''Begriffsschrift'' ===
{{main|Begriffsschrift}}
''[[Begriffsschrift]]'' (German for, roughly, "concept-script") is a book on [[logic]] by [[Gottlob Frege]], published in 1879, and the [[formal system]] set out in that book.

''Begriffsschrift'' is usually translated as ''concept writing'' or ''concept notation''; the full title of the book identifies it as "a [[formula]] [[language]], modeled on that of [[arithmetic]], of pure [[thought]]." Frege's motivation for developing his formal approach to logic resembled [[Gottfried Wilhelm Leibniz|Leibniz]]'s motivation for his [[calculus ratiocinator]] (despite that, in his ''Foreword'' Frege clearly denies that he reached this aim, and also that his main aim would be constructing an ideal language like Leibniz's, what Frege declares to be quite hard and idealistic, however, not impossible task). Frege went on to employ his logical calculus in his research on the [[foundations of mathematics]], carried out over the next quarter century.

=== ''Principia Mathematica'' ===
{{main|Principia Mathematica}}
''Principia Mathematica'', or "PM" as it is often abbreviated, was an attempt to describe a set of [[axiom]]s and [[inference rule]]s in [[Mathematical logic|symbolic logic]] from which all mathematical truths could in principle be proven. As such, this ambitious project is of great importance in the history of mathematics and philosophy,<ref name="SEP">{{cite web
|url=http://plato.stanford.edu/entries/principia-mathematica/#SOPM
|title=Principia Mathematica (Stanford Encyclopedia of Philosophy)
|last=Irvine
|first=Andrew D.
|date=1 May 2003
|publisher=Metaphysics Research Lab, CSLI, Stanford University
|access-date=5 August 2009}}</ref> being one of the foremost products of the belief that such an undertaking may be achievable. However, in 1931, [[Gödel's incompleteness theorem]] proved definitively that PM, and in fact any other attempt, could never achieve this goal; that is, for any set of axioms and inference rules proposed to encapsulate mathematics, there would in fact be some truths of mathematics which could not be deduced from them.

One of the main inspirations and motivations for ''PM'' was the earlier work of [[Gottlob Frege]] on logic, which Russell discovered allowed for the construction of [[Russell's paradox|paradoxical sets]]. ''PM'' sought to avoid this problem by ruling out the unrestricted creation of arbitrary sets. This was achieved by replacing the notion of a general set with notion of a hierarchy of sets of different '[[system of types|types]]', a set of a certain type only allowed to contain sets of strictly lower types. Contemporary mathematics, however, avoids paradoxes such as Russell's in less unwieldy ways, such as the system of [[Zermelo–Fraenkel set theory]].

===Gödel's incompleteness theorem ===
{{main|Gödel's incompleteness theorem}}
Gödel's incompleteness theorems are two [[theorem]]s of [[mathematical logic]] that establish inherent limitations of all but the most trivial [[axiomatic system]]s capable of doing [[arithmetic]].  The theorems, proven by [[Kurt Gödel]] in 1931, are important both in mathematical logic and in the [[philosophy of mathematics]]. The two results are widely, but not universally, interpreted as showing that [[Hilbert's program]] to find a complete and consistent set of [[axiom]]s for all [[mathematics]] is impossible, giving a negative answer to [[Hilbert's second problem]].

The first incompleteness theorem states that no consistent system of axioms whose theorems can be listed by an "[[effective procedure]]" (e.g., a computer program, but it could be any sort of algorithm) is capable of proving all truths about the relations of the [[natural numbers]] ([[arithmetic]]).  For any such system, there will always be statements about the natural numbers that are true, but that are unprovable within the system. The second incompleteness theorem, an extension of the first, shows that such a system cannot demonstrate its own consistency.

===Tarski's definition of model-theoretic satisfaction===
{{main|T-schema}}
The T-schema or truth [[schema (logic)|schema]] (not to be confused with '[[Semantic theory of truth#Convention T|Convention T]]') is used to give an [[Recursive definition|inductive definition]] of truth which lies at the heart of any realisation of [[Alfred Tarski]]'s [[semantic theory of truth]]. Some authors refer to it as the "Equivalence Schema", a synonym introduced by [[Michael Dummett]].<ref name="Künne2005">{{cite book|author=Wolfgang Künne|title=Conceptions of truth|url=https://archive.org/details/conceptionsoftru0000kunn|url-access=registration|year=2003|publisher=Clarendon Press|isbn=978-0-19-928019-3|page=[https://archive.org/details/conceptionsoftru0000kunn/page/18 18]}}</ref>

The T-schema is often expressed in [[natural language]], but it can be formalized in [[Predicate logic|many-sorted predicate logic]] or [[modal logic]]; such a formalisation is called a '''''T-theory'''''.  T-theories form the basis of much fundamental work in [[philosophical logic]], where they are applied in several important controversies in [[analytic philosophy]].

As expressed in semi-natural language (where 'S' is the name of the sentence abbreviated to S):
'S' is true [[if and only if]] S

Example: 'snow is white' is true if and only if snow is white.

=== The undecidability of the ''Entscheidungsproblem'' ===
{{main|Entscheidungsproblem}}
The {{lang|de|Entscheidungsproblem}} ([[German language|German]] for '[[decision problem]]') is a challenge posed by [[David Hilbert]] in 1928.<ref>Hilbert and Ackermann</ref> The {{lang|de|Entscheidungsproblem}} asks for an [[algorithm]] that takes as input a statement of a [[first-order logic]] (possibly with a finite number of [[axiom]]s beyond the usual axioms of first-order logic) and answers "Yes" or "No" according to whether the statement is ''universally valid'', i.e., valid in every structure satisfying the axioms. By [[Gödel's completeness theorem|the completeness theorem of first-order logic]], a statement is universally valid if and only if it can be deduced from the axioms, so the {{lang|de|Entscheidungsproblem}} can also be viewed as asking for an algorithm to decide whether a given statement is provable from the axioms using the rules of logic.

In 1936, [[Alonzo Church]] and [[Alan Turing]] published independent papers<ref>Church's paper was presented to the American Mathematical Society on 19 April 1935 and published on 15 April 1936. Turing, who had made substantial progress in writing up his own results, was disappointed to learn of Church's proof upon its publication (see correspondence between [[Max Newman]] and Church in [http://diglib.princeton.edu/ead/getEad?eadid=C0948&kw= Alonzo Church papers] {{webarchive|url=https://web.archive.org/web/20100607132453/http://diglib.princeton.edu/ead/getEad?eadid=C0948&kw= |date=2010-06-07 }}). Turing quickly completed his paper and rushed it to publication; it was received by the ''Proceedings of the London Mathematical Society'' on 28 May 1936, read on 12 November 1936, and published in series 2, volume 42 (1936-7); it appeared in two sections: in Part 3 (pages 230-240), issued on Nov 30, 1936 and in Part 4 (pages 241-265), issued on Dec 23, 1936; Turing added corrections in volume 43(1937) pp. 544–546. See the footnote at the end of Soare:1996.</ref> showing that a general solution to the ''Entscheidungsproblem'' is impossible, assuming that the intuitive notation of "[[effectively calculable]]" is captured by the functions computable by a [[Turing machine]] (or equivalently, by those expressible in the [[lambda calculus]]). This assumption is now known as the [[Church–Turing thesis]].

== See also ==
{{portal|Mathematics}}
*[[Meta (prefix)|Meta]]
*[[Metalogic]]
*[[Model theory]]
*[[Philosophy of mathematics]]
*[[Proof theory]]

==References==
{{reflist}}

== Further reading ==
* W. J. Blok and Don Pigozzi, "[https://www.cambridge.org/core/journals/journal-of-symbolic-logic/article/alfred-tarskis-work-on-general-metamathematics/681F62DFAFE093561104304BDCF86C92 Alfred Tarski's Work on General Metamathematics]", ''The Journal of Symbolic Logic'', v. 53, No. 1 (Mar., 1988), pp.&nbsp;36&ndash;50.
* I. J. Good. "A Note on Richard's Paradox". ''Mind'', New Series, Vol. 75, No. 299 (Jul., 1966), p.&nbsp;431. [https://www.jstor.org/stable/2251836 JStor]
* [[Douglas Hofstadter]], 1980. ''[[Gödel, Escher, Bach]]''. Vintage Books. Aimed at laypeople.
* [[Stephen Cole Kleene]], 1952. ''Introduction to Metamathematics''. North Holland. Aimed at mathematicians.
* Jules Richard, ''Les Principes des Mathématiques et le Problème des Ensembles'', ''Revue Générale des Sciences Pures et Appliquées'' (1905); translated in Heijenoort J. van (ed.), ''Source Book in Mathematical Logic 1879-1931'' (Cambridge, Massachusetts, 1964).
* [[Alfred North Whitehead]], and [[Bertrand Russell]]. ''[[Principia Mathematica]]'', 3 vols, Cambridge University Press, 1910, 1912, and 1913. Second edition, 1925 (Vol. 1), 1927 (Vols 2, 3). Abridged as ''Principia Mathematica to *56'', Cambridge University Press, 1962.

{{Metalogic}}
{{Areas of mathematics |collapsed}}
{{Logic}}

{{Authority control}}

[[Category:Mathematical logic]]
[[Category:Logic]]
[[Category:Metatheory]]