Formal system

<h2 id="concepts">Concepts</h2>

A formal system has the following:<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a><a class="footnote-ref" id="fnref:4" href="#fn:4">4</a><a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>

<ul><li><a href="/facts/Formal_language/crDTyP8q">Formal language</a>, which is a set of <a href="/facts/Well-formed_formula/YpTVlzuj">well-formed formulas</a>, which are strings of <a href="/facts/Symbol_(formal)/NS2oUqEk">symbols</a> from an <a href="/facts/Alphabet_(formal_languages)/6KW9qYRW">alphabet</a>, formed by a <a href="/facts/Formal_grammar/QLxaQNAz">formal grammar</a> (consisting of <a href="/facts/Production_(computer_science)/J7E2FXKs">production rules</a> or <a href="/facts/Formation_rule/8GUJp3hI">formation rules</a>).</li>
<li><a href="/facts/Deductive_system/KaeDAjK7">Deductive system</a>, deductive apparatus, or <a href="/facts/Proof_calculus/oXTtCnhC">proof system</a>, which has <a href="/facts/Rule_of_inference/QV5NklQ9">rules of inference</a> that take <a href="/facts/Axiom/mLjP14Mc">axioms</a> and infers <a href="/facts/Theorem/f2Pe1FMD">theorems</a>, both of which are part of the formal language.</li></ul>
A formal system is said to be <a href="/facts/Recursive_set/rsnT9cLn">recursive</a> (i.e. effective) or recursively enumerable if the set of axioms and the set of inference rules are <a href="/facts/Decidable_set/rsnT9cLn">decidable sets</a> or <a href="/facts/Recursively_enumerable_set/lpzr8jHn">semidecidable sets</a>, respectively.

<h3>Formal language</h3>

Main articles: <a href="/facts/Formal_language/crDTyP8q">Formal language</a>, <a href="/facts/Formal_grammar/QLxaQNAz">Formal grammar</a>, <a href="/facts/Syntax_(logic)/WrLeuvgp">Syntax (logic)</a>, and <a href="/facts/Logical_form/foVBtMX4">Logical form</a>
A <a href="/facts/Formal_language/crDTyP8q">formal language</a> is a language uses a set of strings whose symbols are taken from a specific alphabet, and operations used to form sentences from them.
. Like languages in <a href="/facts/Linguistics/oQuamvf2">linguistics</a>, formal languages generally have two aspects:

<ul><li>the <a href="/facts/Syntax_(logic)/WrLeuvgp">syntax</a> is what the language looks like (more formally: the set of possible expressions that are valid utterances in the language)</li>
<li>the <a href="/facts/Semantics_of_logic/VdAKHszK">semantics</a> are what the utterances of the language mean (which is formalized in various ways, depending on the type of language in question)</li></ul>
Usually only the <a href="/facts/Syntax_(logic)/WrLeuvgp">syntax</a> of a formal language is considered via the notion of a <a href="/facts/Formal_grammar/QLxaQNAz">formal grammar</a>. The two main categories of formal grammar are that of <a href="/facts/Generative_grammar/Pm1AMX5U">generative grammars</a>, which are sets of rules for how strings in a language can be written, and that of <a href="/facts/Analytic_grammar/QLxaQNAz">analytic grammars</a> (or reductive grammar<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>[unreliable source?]<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>), which are sets of rules for how a string can be analyzed to determine whether it is a member of the language.

<h3>Deductive system</h3>
Main articles: <a href="/facts/Inference/RBbe95bP">Inference</a>, <a href="/facts/Logical_consequence/AyDXp4dn">Logical consequence</a>, and <a href="/facts/Deductive_reasoning/HadA1zWS">Deductive reasoning</a>

A deductive system, also called a deductive apparatus,<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> consists of the <a href="/facts/Axiom/mLjP14Mc">axioms</a> (or <a href="/facts/Axiom_schema/vKNTz5IT">axiom schemata</a>) and <a href="/facts/Rules_of_inference/QV5NklQ9">rules of inference</a> that can be used to <a href="/facts/Formal_proof/6nEhV4cw">derive</a> <a href="/facts/Theorem/f2Pe1FMD">theorems</a> of the system.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a>
In order to sustain its deductive integrity, a deductive apparatus must be definable without reference to any <a href="/facts/Intended_interpretation/goPmjjQG">intended interpretation</a> of the language. The aim is to ensure that each line of a <a href="/facts/Mathematical_proof/7ECDrU80">derivation</a> is merely a <a href="/facts/Logical_consequence/AyDXp4dn">logical consequence</a> of the lines that precede it. There should be no element of any <a href="/facts/Interpretation_(logic)/goPmjjQG">interpretation</a> of the language that gets involved with the deductive nature of the system.
The <a href="/facts/Logical_consequence/AyDXp4dn">logical consequence</a> (or entailment) of the system by its logical foundation is what distinguishes a formal system from others which may have some basis in an abstract model. Often the formal system will be the basis for or even identified with a larger <a href="/facts/Theory/9vrNS1Rr">theory</a> or field (e.g. <a href="/facts/Euclidean_geometry/V6cTcoCy">Euclidean geometry</a>) consistent with the usage in modern mathematics such as <a href="/facts/Model_theory/vLWVV8sQ">model theory</a>.
An example of a deductive system would be the rules of inference and <a href="/facts/First-order_logic/lcISqWIg">axioms regarding equality</a> used in <a href="/facts/First-order_logic/lcISqWIg">first order logic</a>.
The two main types of deductive systems are proof systems and formal semantics.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a><a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>

<h4>Proof system</h4>
Main articles: <a href="/facts/Proof_system/oXTtCnhC">Proof system</a> and <a href="/facts/Formal_proof/6nEhV4cw">Formal proof</a>
Formal proofs are sequences of <a href="/facts/Well-formed_formula/YpTVlzuj">well-formed formulas</a> (or WFF for short) that might either be an <a href="/facts/Axiom/mLjP14Mc">axiom</a> or be the product of applying an inference rule on previous WFFs in the proof sequence. The last WFF in the sequence is recognized as a <a href="/facts/Theorem/f2Pe1FMD">theorem</a>.
Once a formal system is given, one can define the set of theorems which can be proved inside the formal system. This set consists of all WFFs for which there is a proof. Thus all axioms are considered theorems. Unlike the grammar for WFFs, there is no guarantee that there will be a <a href="/facts/Decidability_(logic)/17SHPMo0">decision procedure</a> for deciding whether a given WFF is a theorem or not. 
The point of view that generating formal proofs is all there is to mathematics is often called <a href="/facts/Formalism_(philosophy_of_mathematics)/g0690AVL">formalism</a>. <a href="/facts/David_Hilbert/1vhlHn4b">David Hilbert</a> founded <a href="/facts/Metamathematics/yeIm8vqC">metamathematics</a> as a discipline for discussing formal systems. Any language that one uses to talk about a formal system is called a <a href="/facts/Metalanguage/peNXFNUV">metalanguage</a>. The metalanguage may be a natural language, or it may be partially formalized itself, but it is generally less completely formalized than the formal language component of the formal system under examination, which is then called the object language, that is, the object of the discussion in question. The notion of theorem just defined should not be confused with theorems about the formal system, which, in order to avoid confusion, are usually called <a href="/facts/Metatheorem/Aj5hPekN">metatheorems</a>.

<h4>Formal semantics of logical system</h4>
Main articles: <a href="/facts/Semantics_of_logic/VdAKHszK">Semantics of logic</a>, <a href="/facts/Interpretation_(logic)/goPmjjQG">Interpretation (logic)</a>, and <a href="/facts/Model_theory/vLWVV8sQ">Model theory</a>
A logical system is a deductive system (most commonly <a href="/facts/First-order_logic/lcISqWIg">first order logic</a>) together with additional <a href="/facts/Non-logical_axioms/mLjP14Mc">non-logical axioms</a>. According to <a href="/facts/Model_theory/vLWVV8sQ">model theory</a>, a logical system may be given <a href="/facts/Interpretation_(logic)/goPmjjQG">interpretations</a> which describe whether a given <a href="/facts/Structure_(mathematical_logic)/GsMx1fEb">structure</a> - the mapping of formulas to a particular meaning - satisfies a well-formed formula. A structure that satisfies all the axioms of the formal system is known as a <a href="/facts/Model_(model_theory)/GsMx1fEb">model</a> of the logical system. 
A logical system is:

<ul><li><a href="/facts/Soundness/ndpamd2A">Sound</a>, if each well-formed formula that can be inferred from the axioms is satisfied by every model of the logical system.</li>
<li><a href="/facts/Completeness_(logic)/GzK9zLnu">Semantically complete</a>, if each well-formed formula that is satisfied by every model of the logical system can be inferred from the axioms.</li></ul>
An example of a logical system is <a href="/facts/Peano_arithmetic/DhhDnNug">Peano arithmetic</a>. The standard model of arithmetic sets the <a href="/facts/Domain_of_discourse/T1WRQxx6">domain of discourse</a> to be the <a href="/facts/Nonnegative_integer/u65og8JE">nonnegative integers</a> and gives the symbols their usual meaning.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> There are also <a href="/facts/Non-standard_model_of_arithmetic/e3EktWqv">non-standard models of arithmetic</a>.

<h2 id="history">History</h2>
Main articles: <a href="/facts/Formalism_(philosophy_of_mathematics)/g0690AVL">Formalism (philosophy of mathematics)</a> and <a href="/facts/Formal_logical_systems/SaI7Y3wN">Formal logical systems</a>
Early logic systems includes Indian logic of <a href="/facts/P%25C4%2581%25E1%25B9%2587ini/C2brRvXB">Pāṇini</a>, syllogistic logic of Aristotle, propositional logic of Stoicism, and Chinese logic of <a href="/facts/Gongsun_Long/Sr2TB4ga">Gongsun Long</a> (c. 325–250 BCE). In more recent times, contributors include <a href="/facts/George_Boole/fWJT2rek">George Boole</a>, <a href="/facts/Augustus_De_Morgan/6fQ1KTPb">Augustus De Morgan</a>, and <a href="/facts/Gottlob_Frege/FXyStVYj">Gottlob Frege</a>. <a href="/facts/Mathematical_logic/SaI7Y3wN">Mathematical logic</a> was developed in 19th century <a href="/facts/Europe/DTAtrKfh">Europe</a>.
<a href="/facts/David_Hilbert/1vhlHn4b">David Hilbert</a> instigated a <a href="/facts/Formalism_(philosophy_of_mathematics)/g0690AVL">formalist</a> movement called <a href="/facts/Hilbert%2527s_program/9VnGkMbf">Hilbert’s program</a> as a proposed solution to the <a href="/facts/Foundational_crisis_of_mathematics/yjpbbUkt">foundational crisis of mathematics</a>, that was eventually tempered by <a href="/facts/G%25C3%25B6del%2527s_incompleteness_theorems/du8PRu8g">Gödel's incompleteness theorems</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> The <a href="/facts/QED_manifesto/jhWAuaGJ">QED manifesto</a> represented a subsequent, as yet unsuccessful, effort at formalization of known mathematics.

<h2 id="see-also">See also</h2>
<ul>
<li>Systems science portal</li><li>Philosophy portal</li></ul>
<ul><li><a href="/facts/List_of_formal_systems/LS2pc9Qm">List of formal systems</a></li>
<li><a href="/facts/Formal_method/tw4FCWIb">Formal method</a> – Mathematical program specificationsPages displaying short descriptions of redirect targets</li>
<li><a href="/facts/Formal_science/U0MaCPB0">Formal science</a> – Study of abstract structures described by formal systems</li>
<li><a href="/facts/Logic_translation/TopIkPxe">Logic translation</a> – Translation of a text into a logical system</li>
<li><a href="/facts/Rewriting_system/9PHlnnnJ">Rewriting system</a> – Replacing subterm in a formula with another termPages displaying short descriptions of redirect targets</li>
<li><a href="/facts/Substitution_instance/xMGFYd1k">Substitution instance</a> – Concept in logicPages displaying short descriptions of redirect targets</li>
<li><a href="/facts/Theory_(mathematical_logic)/mYe5gQpI">Theory (mathematical logic)</a> – Set of sentences in a formal language</li></ul>

<h2 id="sources">Sources</h2>
<ul><li><a href="/facts/Geoffrey_Hunter_(logician)/SBTYATog">Hunter, Geoffrey</a> (1996) [1971]. Metalogic: An Introduction to the Metatheory of Standard First-Order Logic. University of California Press (published 1973). <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9780520023567. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/36312727">36312727</a>. (<a href="https://archive.org/details/metalogicintrodu0000hunt">accessible to patrons with print disabilities</a>)</li></ul>
<h2 id="further-reading">Further reading</h2>
<ul><li><a href="/facts/Douglas_Hofstadter/LxUaf01J">Hofstadter, Douglas</a>, 1979. <a href="/facts/G%25C3%25B6del%2c_Escher%2c_Bach%3a_An_Eternal_Golden_Braid/5VCxjXvk">Gödel, Escher, Bach: An Eternal Golden Braid</a> <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-465-02656-2. 777 pages.</li>
<li><a href="/facts/Stephen_Cole_Kleene/sKSax2se">Kleene, Stephen C.</a>, 1967. Mathematical Logic Reprinted by Dover, 2002. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-486-42533-9</li>
<li><a href="/facts/Raymond_M._Smullyan/UMEI6TgX">Smullyan, Raymond M.</a>, 1961. Theory of Formal Systems: Annals of Mathematics Studies, Princeton University Press (April 1, 1961) 156 pages <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-691-08047-X</li></ul>
<h2 id="external-links">External links</h2>

Look up formalisation in Wiktionary, the free dictionary.

<ul><li> Media related to Formal systems at Wikimedia Commons</li>
<li>Encyclopædia Britannica, <a href="https://www.britannica.com/eb/article-9034889/formal-system">Formal system</a> definition, 2007.</li>
<li>Daniel Richardson, <a href="http://cs.bath.ac.uk/pb/EMCL/DS/DS-Ref-2011/c19.pdf">Formal systems, logic and semantics</a></li>
<li><a href="https://planetmath.org/formalsystem">Formal System</a> at <a href="/facts/PlanetMath/wphPCtLf">PlanetMath</a>.</li>
<li>Encyclopedia of Mathematics, <a href="https://encyclopediaofmath.org/wiki/Formal_system">Formal system</a></li>
<li>Peter Suber, <a href="http://www.earlham.edu/~peters/courses/logsys/machines.htm">Formal Systems and Machines: An Isomorphism</a> <a href="https://web.archive.org/web/20110524103726/http://www.earlham.edu/~peters/courses/logsys/machines.htm">Archived</a> 2011-05-24 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>, 1997.</li>
<li>Ray Taol, <a href="https://cs.lmu.edu/~ray/notes/formalsystems/">Formal Systems</a></li>
<li><a href="http://www.cs.indiana.edu/~port/teach/641/formal.sys.haug.html">What is a Formal System?</a>: Some quotes from John Haugeland's `Artificial Intelligence: The Very Idea' (1985), pp. 48–64.</li></ul>

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

<ol>
<li id="fn:1">Hunter 1996, p. 7. - Hunter, Geoffrey (1996) [1971]. Metalogic: An Introduction to the Metatheory of Standard First-Order Logic. University of California Press (published 1973). ISBN 9780520023567. OCLC 36312727. <a href="https://search.worldcat.org/oclc/36312727" target="_blank">https://search.worldcat.org/oclc/36312727</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Zach, Richard (31 July 2003). "Hilbert's Program". Hilbert's Program, Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, Stanford University. <a href="https://plato.stanford.edu/archives/spr2016/entries/hilbert-program" target="_blank">https://plato.stanford.edu/archives/spr2016/entries/hilbert-program</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"Formal System". PlanetMath. <a href="https://planetmath.org/formalsystem" target="_blank">https://planetmath.org/formalsystem</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Rapaport, William J. (25 March 2010). "Syntax & Semantics of Formal Systems". University of Buffalo. <a href="https://cse.buffalo.edu/~rapaport/formalsystems" target="_blank">https://cse.buffalo.edu/~rapaport/formalsystems</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">"Formal System". Pr
 
 
 
 ∞
 
 
 {\displaystyle \infty }
 
fWiki. <a href="https://proofwiki.org/Definition:Formal_System" target="_blank">https://proofwiki.org/Definition:Formal_System</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"Reductive grammar". Dictionary of Scientific and Technical Terms (6th ed.). McGraw-Hill. Reductive grammar: (computer science) A set of syntactic rules for the analysis of strings to determine whether the strings exist in a language. <a href="http://encyclopedia2.thefreedictionary.com/reductive+grammar" target="_blank">http://encyclopedia2.thefreedictionary.com/reductive+grammar</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Rulifson, Johns F. (April 1968). "A Tree Meta for the XDS 940" (PDF). Augmentation Research Center. Retrieved 30 November 2024. There are two classes of formal-language definition compiler-writing schemes. The productive grammar approach is the most common. A productive grammar consists primarrly of a set of rules that describe a method of generating all possible strings of the language. The reductive or analytical grammar technique states a set of rules that describe a method of analyzing any string of characters and deciding whether that string is in the language. <a href="/wiki/Jeff_Rulifson" target="_blank">/wiki/Jeff_Rulifson</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">"Deductive Apparatus". Pr
 
 
 
 ∞
 
 
 {\displaystyle \infty }
 
fWiki. Retrieved 30 November 2024. <a href="https://proofwiki.org/Definition:Deductive_Apparatus" target="_blank">https://proofwiki.org/Definition:Deductive_Apparatus</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Hunter 1996, p. 7. - Hunter, Geoffrey (1996) [1971]. Metalogic: An Introduction to the Metatheory of Standard First-Order Logic. University of California Press (published 1973). ISBN 9780520023567. OCLC 36312727. <a href="https://search.worldcat.org/oclc/36312727" target="_blank">https://search.worldcat.org/oclc/36312727</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">"Deductive Apparatus". Pr
 
 
 
 ∞
 
 
 {\displaystyle \infty }
 
fWiki. Retrieved 30 November 2024. <a href="https://proofwiki.org/Definition:Deductive_Apparatus" target="_blank">https://proofwiki.org/Definition:Deductive_Apparatus</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">van Fraassen, Bas C. (2016) [1971]. Formal Semantics and Logic (PDF). Nousoul Digital Publishers. p. 12. Metalogic can in turn be roughly divided into two parts: proof theory and formal semantics... The division is not exact; many questions have been dealt with from both points of view, and some proof-theoretic methods and results are indispensable in semantics. <a href="/wiki/Bas_van_Fraassen" target="_blank">/wiki/Bas_van_Fraassen</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Kaye, Richard (1991). "1. The Standard Model". Models of Peano arithmetic. Oxford: Clarendon Press. p. 10. ISBN 9780198532132. <a href="9780198532132" target="_blank">9780198532132</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Zach, Richard (31 July 2003). "Hilbert's Program". Hilbert's Program, Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, Stanford University. <a href="https://plato.stanford.edu/archives/spr2016/entries/hilbert-program" target="_blank">https://plato.stanford.edu/archives/spr2016/entries/hilbert-program</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
</ol>

Formal system open-in-new

Formal system