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| {{About|reasoning and its study|other uses}}
| | The author's name is Christy. Distributing production is where her primary earnings arrives from. My spouse doesn't like it the way I do but what I truly like performing is caving but I don't have the time lately. For a while I've been in Alaska but I will have to transfer in a yr or two.<br><br>Feel free to surf to my web blog: [http://c045.danah.co.kr/home/index.php?document_srl=1356970&mid=qna accurate psychic predictions] |
| {{Philosophy sidebar}}
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| '''Logic''' (from the {{lang-grc|[[wikt:λογική|λογική]], ''logike''}})<ref name="argumentative"/> has two meanings: first, it describes the use of valid [[reasoning]] in some activity; second, it names the [[normative]] study of reasoning or a branch thereof.<ref name="PopkinStroll1993"/><ref name="jacquette2002"/> In the latter sense, it features most prominently in the subjects of [[philosophy]], [[mathematics]], and [[computer science]].
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| Logic was studied in several ancient civilizations, including [[India]],<ref name="syllogistic"/> [[China]],<ref name="mohist"/> [[Persia]] and [[Ancient Greece|Greece]]. In the West, logic was established as a formal discipline by [[Aristotle]], who gave it a fundamental place in philosophy. The study of logic was part of the classical [[trivium]], which also included grammar and rhetoric. Logic was further extended by [[Al-Farabi]] who categorized it into two separate groups (idea and proof). Later, [[Avicenna]] revived the study of logic and developed relationship between temporalis and the implication. In the East, logic was developed by [[Buddhism|Buddhists]] and [[Jainism|Jains]].
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| Logic is often divided into three parts; [[inductive reasoning]], [[abductive reasoning]], and [[deductive reasoning]].
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| ==The study of logic==
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| {{rquote|right|Upon this first, and in one sense this sole, rule of reason, that in order to learn you must desire to learn, and in so desiring not be satisfied with what you already incline to think, there follows one corollary which itself deserves to be inscribed upon every wall of the city of philosophy: Do not block the way of inquiry.|[[Charles Sanders Peirce]], "First Rule of Logic"}}
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| The concept of [[Argument form|logical form]] is central to logic, it being held that the validity of an argument is determined by its logical form, not by its content. Traditional [[syllogism|Aristotelian syllogistic logic]] and modern symbolic logic are examples of formal logics.
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| * '''[[Informal logic]]''' is the study of [[natural language]] [[Logical argument|arguments]]. The study of [[fallacies]] is an especially important branch of informal logic. The dialogues of [[Plato]]<ref name="The Portable Plato"/> are good examples of informal logic.
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| * '''[[Mathematical formalism|Formal logic]]''' is the study of [[inference]] with purely formal content. An inference possesses a ''purely formal content'' if it can be expressed as a particular application of a wholly abstract rule, that is, a rule that is not about any particular thing or property. The works of [[Aristotle]] contain the earliest known formal study of logic. Modern formal logic follows and expands on Aristotle.<ref name="The Basic Works"/> In many definitions of logic, [[logical consequence|logical inference]] and inference with purely formal content are the same. This does not render the notion of informal logic vacuous, because no formal logic captures all of the nuance of natural language.
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| * '''[[Symbolic logic]]''' is the study of symbolic abstractions that capture the formal features of logical inference.<ref name="Principia"/><ref name="Hamilton"/> Symbolic logic is often divided into two branches: [[propositional logic]] and [[predicate logic]].
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| * '''[[Mathematical logic]]''' is an extension of symbolic logic into other areas, in particular to the study of [[model theory]], [[proof theory]], [[set theory]], and [[recursion theory]].
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| ===Logical form===
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| {{main|Logical form}}
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| Logic is generally considered '''formal''' when it analyzes and represents the ''form'' of any valid argument type. The form of an argument is displayed by representing its sentences in the formal grammar and symbolism of a logical language to make its content usable in formal inference. If one considers the notion of form too philosophically loaded, one could say that formalizing simply means translating English sentences into the language of logic.
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| This is called showing the ''logical form'' of the argument. It is necessary because indicative sentences of ordinary language show a considerable variety of form and complexity that makes their use in inference impractical. It requires, first, ignoring those grammatical features irrelevant to logic (such as gender and declension, if the argument is in Latin), replacing conjunctions irrelevant to logic (such as 'but') with logical conjunctions like 'and' and replacing ambiguous, or alternative logical expressions ('any', 'every', etc.) with expressions of a standard type (such as 'all', or the universal quantifier ∀).
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| Second, certain parts of the sentence must be replaced with schematic letters. Thus, for example, the expression ''all As are Bs'' shows the logical form common to the sentences ''all men are mortals'', ''all cats are carnivores'', ''all Greeks are philosophers'' and so on.
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| That the concept of form is fundamental to logic was already recognized in ancient times. Aristotle uses variable letters to represent valid inferences in ''[[Prior Analytics]]'', leading [[Jan Łukasiewicz]] to say that the introduction of variables was 'one of Aristotle's greatest inventions'.<ref name="Aristotle's syllogistic from the standpoint of modern formal logic"/> According to the followers of Aristotle (such as [[Ammonius Saccas|Ammonius]]), only the logical principles stated in schematic terms belong to logic, not those given in concrete terms. The concrete terms ''man', 'mortal'', etc., are analogous to the substitution values of the schematic placeholders 'A', 'B', 'C', which were called the 'matter' (Greek 'hyle') of the inference.
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| The fundamental difference between modern formal logic and traditional, or Aristotelian logic, lies in their differing analysis of the logical form of the sentences they treat.
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| * In the traditional view, the form of the sentence consists of (1) a subject (e.g., 'man') plus a sign of quantity ('all' or 'some' or 'no'); (2) the [[Copula (linguistics)|copula]], which is of the form 'is' or 'is not'; (3) a predicate (e.g., 'mortal'). Thus: all men are mortal. The logical constants such as 'all', 'no' and so on, plus sentential connectives such as 'and' and 'or' were called 'syncategorematic' terms (from the Greek 'kategorei' – to predicate, and 'syn' – together with). This is a fixed scheme, where each judgment has an identified quantity and copula, determining the logical form of the sentence.
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| * According to the modern view, the fundamental form of a simple sentence is given by a recursive schema, involving [[logical connective]]s, such as a quantifier with its bound variable, which are joined by juxtaposition to other sentences, which in turn may have logical structure.
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| * The modern view is more complex, since a single judgement of Aristotle's system involves two or more logical connectives. For example, the sentence "All men are mortal" involves, in term logic, two non-logical terms "is a man" (here ''M'') and "is mortal" (here ''D''): the sentence is given by the judgement A(M,D). In [[predicate logic]], the sentence involves the same two non-logical concepts, here analyzed as <math>m(x)</math> and <math>d(x)</math>, and the sentence is given by <math>\forall x. (m(x) \rightarrow d(x))</math>, involving the logical connectives for [[universal quantification]] and [[material conditional|implication]].
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| * But equally, the modern view is more powerful. Medieval logicians recognized the [[problem of multiple generality]], where Aristotelian logic is unable to satisfactorily render such sentences as "Some guys have all the luck", because both quantities "all" and "some" may be relevant in an inference, but the fixed scheme that Aristotle used allows only one to govern the inference. Just as linguists recognize recursive structure in natural languages, it appears that logic needs recursive structure.
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| ===Deductive and inductive reasoning, and retroductive inference===
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| [[Deductive reasoning]] concerns what follows necessarily from given premises (if a, then b). However, [[inductive reasoning]]—the process of deriving a reliable generalization from observations—has sometimes been included in the study of logic. Similarly, it is important to distinguish deductive validity and inductive validity (called "[[cogency]]"). An inference is deductively valid [[if and only if]] there is no possible situation in which all the premises are true but the conclusion false. An inductive argument can be neither valid nor invalid; its premises give only some degree of probability, but not certainty, to its conclusion.
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| The notion of deductive validity can be rigorously stated for systems of formal logic in terms of the well-understood notions of [[semantics]]. Inductive validity on the other hand requires us to define a reliable generalization of some set of observations. The task of providing this definition may be approached in various ways, some less formal than others; some of these definitions may use [[mathematical model]]s of probability. For the most part this discussion of logic deals only with deductive logic.
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| Retroductive inference is a mode of reasoning that Peirce proposed as operating over and above induction and deduction to "open up new ground" in processes of theorizing (1911, p. 2). He defines retroduction as a logical inference that allows us to "render comprehensible" some observations/events we perceive, by relating these back to a posited state of affairs that would help to shed light on the observations
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| (Peirce, 1911, p. 2). He remarks that the "characteristic formula" of reasoning that he calls retroduction is that it involves reasoning from a [[consequent]] (any observed/experienced phenomena that confront us) to an [[antecedent (logic)|antecedent]] (that is, a posited state of things that helps us to render comprehensible the observed phenomenon). Or, as he otherwise puts it, it can be considered as "regressing from a consequent to a hypothetical antecedent" (1911, p. 4).<ref> See for instance, the discussion at: http://www.helsinki.fi/science/commens/dictionary.html</ref>
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| Some authors suggest that this mode of inference can be used within social theorizing to postulate social structures/mechanisms that explain the way that social outcomes arise in social life—and that in turn indicates that these structures or mechanisms are alterable with sufficient social will (and envisioning of alternatives). In other words, this logic is specifically liberative in that it can be used to point to transformative potential in our way of organizing our social existence by our re-examining/exploring the deep structures that generate outcomes (and life chances for people). In her book on New Racism (2010) Norma Romm offers an account of various interpretations of what can be said to be involved in retroduction as a form of inference and how this can also be seen to be linked to a style of theorizing (and caring) where processes of knowing (which she sees as dialogically rooted) are linked to social justice projects.<ref>http://www.springer.com/978-90-481-8727-0</ref>
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| ===Consistency, validity, soundness, and completeness===
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| Among the important properties that [[logical system]]s can have:
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| * '''[[Consistency proof|Consistency]]''', which means that no theorem of the system contradicts another.<ref name="Bergmann, Merrie 2009"/>
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| * '''[[Validity]]''', which means that the system's rules of proof never allow a false inference from true premises. A logical system has the property of [[soundness]] when the logical system has the property of validity and uses only premises that prove true (or, in the case of axioms, are true by definition).<ref name="Bergmann, Merrie 2009"/>
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| * '''[[Completeness]]''', of a logical system, which means that if a formula is true, it can be proven (if it is true, it is a theorem of the system).
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| * '''[[Soundness]]''', the term soundness has multiple separate meanings, which creates a bit of confusion throughout the literature. Most commonly, soundness refers to logical systems, which means that if some formula can be proven in a system, then it is true in the relevant model/structure (if A is a theorem, it is true). This is the converse of completeness. A distinct, peripheral use of soundness refers to arguments, which means that the premises of a valid argument are true in the actual world.
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| Some logical systems do not have all four properties. As an example, [[Kurt Gödel]]'s [[Gödel's incompleteness theorems|incompleteness theorems]] show that sufficiently complex formal systems of arithmetic cannot be consistent and complete;<ref name="Hamilton"/> however, first-order predicate logics not extended by specific axioms to be arithmetic formal systems with equality can be complete and consistent.<ref name="Introduction to Mathematical Logic"/>
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| ===Rival conceptions of logic===
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| {{Main|Definitions of logic}}
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| Logic arose (see below) from a concern with correctness of [[argumentation]]. Modern logicians usually wish to ensure that logic studies just those arguments that arise from appropriately general forms of inference. For example, Thomas Hofweber writes in the [[Stanford Encyclopedia of Philosophy]] that logic "does not, however, cover good reasoning as a whole. That is the job of the theory of [[rationality]]. Rather it deals with inferences whose validity can be traced back to the formal features of the representations that are involved in that inference, be they linguistic, mental, or other representations".<ref name="stanford-logic-onthology"/>
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| By contrast, [[Immanuel Kant]] argued that logic should be conceived as the science of judgement, an idea taken up in [[Gottlob Frege]]'s logical and philosophical work. But Frege's work is ambiguous in the sense that it is both concerned with the "laws of thought" as well as with the "laws of truth", i.e. it both treats logic in the context of a theory of the mind, and treats logic as the study of abstract formal structures.
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| ==History==
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| {{Main|History of logic}}
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| [[File:Aristotle Altemps Inv8575.jpg|thumb|150px|[[Aristotle]], 384–322 BC.]]
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| In Europe, logic was first developed by [[Aristotle]].<ref name="achievement"/> [[Aristotelian logic]] became widely accepted in science and mathematics and remained in wide use in the West until the early 19th century.<ref name="mtu"/> Aristotle's system of logic was responsible for the introduction of [[hypothetical syllogism]],<ref name="google"/> [[Temporal logic|temporal]] [[modal logic]],<ref name="google1"/><ref name="google2"/> and [[Inductive reasoning|inductive logic]],<ref name="google3"/> as well as influential terms such as [[terminology|terms]], [[predicables]], [[syllogisms]] and [[proposition]]s. In [[Europe]] during the later medieval period, major efforts were made to show that Aristotle's ideas were compatible with [[Christian]] faith. During the [[High Middle Ages]], logic became a main focus of philosophers, who would engage in critical logical analyses of philosophical arguments, often using variations of the methodology of [[scholasticism]]. In 1323, [[William of Ockham]]'s influential ''[[Sum of Logic|Summa Logicae]]'' was released. By the 18th century, the structured approach to arguments had degenerated and fallen out of favour, as depicted in [[Ludvig Holberg|Holberg]]'s satirical play ''[[Erasmus Montanus]]''.
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| The [[Logic in China|Chinese logical]] philosopher [[Gongsun Long]] (ca. 325–250 BC) proposed the paradox "One and one cannot become two, since neither becomes two."<ref name="propositions"/> In China, the tradition of scholarly investigation into logic, however, was repressed by the [[Qin dynasty]] following the legalist philosophy of [[Han Feizi]].
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| In India, innovations in the scholastic school, called [[Nyaya]], continued from ancient times into the early 18th century with the [[Navya-Nyaya]] school. By the 16th century, it developed theories resembling modern logic, such as [[Gottlob Frege]]'s "distinction between sense and reference of proper names" and his "definition of number," as well as the theory of "restrictive conditions for universals" anticipating some of the developments in modern [[set theory]].<ref name="Chakrabarti"/> Since 1824, Indian logic attracted the attention of many Western scholars, and has had an influence on important 19th-century logicians such as [[Charles Babbage]], [[Augustus De Morgan]], and [[George Boole]].<ref name="Indian logic: a reader"/> In the 20th century, Western philosophers like Stanislaw Schayer and Klaus Glashoff have explored Indian logic more extensively.
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| The [[Syllogism|syllogistic]] logic developed by Aristotle predominated in the West until the mid-19th century, when interest in the [[foundations of mathematics]] stimulated the development of symbolic logic (now called [[mathematical logic]]). In 1854, George Boole published ''[[The Laws of Thought|An Investigation of the Laws of Thought on Which are Founded the Mathematical Theories of Logic and Probabilities]]'', introducing symbolic logic and the principles of what is now known as [[Boolean logic]]. In 1879, Gottlob Frege published ''[[Begriffsschrift]]'', which inaugurated modern logic with the invention of [[Quantification|quantifier]] notation. From 1910 to 1913, [[Alfred North Whitehead]] and [[Bertrand Russell]] published ''[[Principia Mathematica]]''<ref name="Principia"/> on the foundations of mathematics, attempting to derive mathematical truths from [[axiom]]s and [[inference rule]]s in symbolic logic. In 1931, [[Gödel]] raised serious problems with the foundationalist program and logic ceased to focus on such issues.
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| The development of logic since Frege, Russell, and [[Wittgenstein]] had a profound influence on the practice of philosophy and the perceived nature of philosophical problems (see [[Analytic philosophy]]), and [[Philosophy of mathematics]]. Logic, especially sentential logic, is implemented in computer [[Digital electronics|logic circuits]] and is fundamental to [[computer science]]. Logic is commonly taught by university philosophy departments, often as a compulsory discipline.
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| ==Topics in logic==
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| ===Syllogistic logic===
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| {{Main|Aristotelian logic}}
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| The ''[[Organon]]'' was [[Aristotle]]'s body of work on logic, with the ''[[Prior Analytics]]'' constituting the first explicit work in formal logic, introducing the syllogistic.<ref name="Aristotle"/> The parts of syllogistic logic, also known by the name [[term logic]], are the analysis of the judgements into propositions consisting of two terms that are related by one of a fixed number of relations, and the expression of inferences by means of [[syllogism]]s that consist of two propositions sharing a common term as premise, and a conclusion that is a proposition involving the two unrelated terms from the premises.
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| Aristotle's work was regarded in classical times and from medieval times in Europe and the Middle East as the very picture of a fully worked out system. However, it was not alone: the [[Stoics]] proposed a system of [[propositional logic]] that was studied by medieval logicians. Also, the [[problem of multiple generality]] was recognised in medieval times. Nonetheless, problems with syllogistic logic were not seen as being in need of revolutionary solutions.
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| Today, some academics claim that Aristotle's system is generally seen as having little more than historical value (though there is some current interest in extending term logics), regarded as made obsolete by the advent of propositional logic and the [[predicate calculus]]. Others use Aristotle in [[argumentation theory]] to help develop and critically question argumentation schemes that are used in [[artificial intelligence]] and [[legal]] arguments.
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| ===Propositional logic (sentential logic)===
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| {{Main|Propositional calculus}}
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| A propositional calculus or logic (also a sentential calculus) is a formal system in which formulae representing propositions can be formed by combining [[atomic propositions]] using [[logical connectives]], and in which a system of formal proof rules establishes certain formulae as "theorems".
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| ===Predicate logic===
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| {{Main|Predicate logic}}
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| Predicate logic is the generic term for symbolic formal systems such as [[first-order logic]], [[second-order logic]], [[many-sorted logic]], and [[infinitary logic]].
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| Predicate logic provides an account of [[quantifiers]] general enough to express a wide set of arguments occurring in natural language. Aristotelian syllogistic logic specifies a small number of forms that the relevant part of the involved judgements may take. Predicate logic allows sentences to be analysed into subject and argument in several additional ways—allowing predicate logic to solve the [[problem of multiple generality]] that had perplexed medieval logicians.
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| The development of predicate logic is usually attributed to [[Gottlob Frege]], who is also credited as one of the founders of [[analytical philosophy]], but the formulation of predicate logic most often used today is the first-order logic presented in [[Principles of Mathematical Logic]] by [[David Hilbert]] and [[Wilhelm Ackermann]] in 1928. The analytical generality of predicate logic allowed the formalisation of mathematics, drove the investigation of [[set theory]], and allowed the development of [[Alfred Tarski]]'s approach to [[model theory]]. It provides the foundation of modern [[mathematical logic]].
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| Frege's original system of predicate logic was second-order, rather than first-order. [[Second-order logic]] is most prominently defended (against the criticism of [[Willard Van Orman Quine]] and others) by [[George Boolos]] and [[Stewart Shapiro]].
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| ===Modal logic===
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| {{Main|Modal logic}}
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| In languages, [[Linguistic modality|modality]] deals with the phenomenon that sub-parts of a sentence may have their semantics modified by special verbs or modal particles. For example, "''We go to the games''" can be modified to give "''We should go to the games''", and "''We can go to the games''"" and perhaps "''We will go to the games''". More abstractly, we might say that modality affects the circumstances in which we take an assertion to be satisfied.
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| [[Aristotle]]'s logic is in large parts concerned with the theory of non-modalized logic. Although, there are passages in his work, such as the famous [[problem of future contingents|sea-battle argument]] in ''[[De Interpretatione]]'' § 9, that are now seen as anticipations of modal logic and its connection with [[potentiality]] and time, the earliest formal system of modal logic was developed by [[Avicenna]], whom ultimately developed a theory of "[[Temporal logic|temporally]] [[Modal logic|modalized]]" syllogistic.<ref name=Britannica>{{cite web |url=http://www.britannica.com/ebc/article-65928 |title=History of logic: Arabic logic |publisher=[[Encyclopædia Britannica]]}}</ref>
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| While the study of necessity and possibility remained important to philosophers, little logical innovation happened until the landmark investigations of [[Clarence Irving Lewis]] in 1918, who formulated a family of rival axiomatizations of the alethic modalities. His work unleashed a torrent of new work on the topic, expanding the kinds of modality treated to include [[deontic logic]] and [[epistemic logic]]. The seminal work of [[Arthur Prior]] applied the same formal language to treat [[temporal logic]] and paved the way for the marriage of the two subjects. [[Saul Kripke]] discovered (contemporaneously with rivals) his theory of [[Kripke semantics|frame semantics]], which revolutionised the formal technology available to modal logicians and gave a new [[graph theory|graph-theoretic]] way of looking at modality that has driven many applications in [[computational linguistics]] and [[computer science]], such as [[dynamic logic (modal logic)|dynamic logic]].
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| ===Informal reasoning===
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| {{Main|Informal logic}}
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| The motivation for the study of logic in ancient times was clear: it is so that one may learn to distinguish good from bad arguments, and so become more effective in argument and oratory, and perhaps also to become a better person. Half of the works of Aristotle's [[Organon]] treat inference as it occurs in an informal setting, side by side with the development of the syllogistic, and in the Aristotelian school, these informal works on logic were seen as complementary to Aristotle's treatment of [[rhetoric]].
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| This ancient motivation is still alive, although it no longer takes centre stage in the picture of logic; typically [[dialectic]]al logic forms the heart of a course in [[critical thinking]], a compulsory course at many universities.
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| [[Argumentation theory]] is the study and research of informal logic, fallacies, and critical questions as they relate to every day and practical situations. Specific types of dialogue can be analyzed and questioned to reveal premises, conclusions, and fallacies. Argumentation theory is now applied in [[artificial intelligence]] and [[law]].
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| ===Mathematical logic===
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| {{Main|Mathematical logic}}
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| Mathematical logic really refers to two distinct areas of research: the first is the application of the techniques of formal logic to mathematics and mathematical reasoning, and the second, in the other direction, the application of mathematical techniques to the representation and analysis of formal logic.<ref name="Introduction to Elementary Mathematical Logic"/>
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| The earliest use of mathematics and [[geometry]] in relation to logic and philosophy goes back to the ancient Greeks such as [[Euclid]], [[Plato]], and [[Aristotle]].<ref name="The Cambridge Companion to Aristotle"/> Many other ancient and medieval philosophers applied mathematical ideas and methods to their philosophical claims.<ref name="Prior Analytics"/>
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| [[File:Kurt gödel.jpg|thumb|150px|[[Kurt Gödel]]]]
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| One of the boldest attempts to apply logic to mathematics was undoubtedly the [[logicism]] pioneered by philosopher-logicians such as [[Gottlob Frege]] and [[Bertrand Russell]]: the idea was that mathematical theories were logical [[tautology (logic)|tautologies]], and the programme was to show this by means to a reduction of mathematics to logic.<ref name="Principia"/> The various attempts to carry this out met with a series of failures, from the crippling of Frege's project in his ''Grundgesetze'' by [[Russell's paradox]], to the defeat of [[Hilbert's program]] by [[Gödel's incompleteness theorem]]s.
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| Both the statement of Hilbert's program and its refutation by Gödel depended upon their work establishing the second area of mathematical logic, the application of mathematics to logic in the form of [[proof theory]].<ref name="Introduction to Mathematical Logic5"/> Despite the negative nature of the incompleteness theorems, [[Gödel's completeness theorem]], a result in [[model theory]] and another application of mathematics to logic, can be understood as showing how close logicism came to being true: every rigorously defined mathematical theory can be exactly captured by a first-order logical theory; Frege's [[proof calculus]] is enough to ''describe'' the whole of mathematics, though not ''equivalent'' to it. Thus we see how complementary the two areas of mathematical logic have been.{{Citation needed|date=July 2007}}
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| If proof theory and model theory have been the foundation of mathematical logic, they have been but two of the four pillars of the subject. [[Set theory]] originated in the study of the infinite by [[Georg Cantor]], and it has been the source of many of the most challenging and important issues in mathematical logic, from [[Cantor's theorem]], through the status of the [[Axiom of Choice]] and the question of the independence of the [[continuum hypothesis]], to the modern debate on [[large cardinal]] axioms.
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| [[Recursion theory]] captures the idea of computation in logical and [[arithmetic]] terms; its most classical achievements are the undecidability of the [[Entscheidungsproblem]] by [[Alan Turing]], and his presentation of the [[Church-Turing thesis]].<ref name="Theory of computation: formal languages, automata, and complexity"/> Today recursion theory is mostly concerned with the more refined problem of [[complexity class]]es — when is a problem efficiently solvable? — and the classification of [[Turing degree|degrees of unsolvability]].<ref name="Theory of computation: formal languages, automata, and complexity6"/>
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| ===Philosophical logic===
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| {{Main|Philosophical logic}}
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| [[Philosophical logic]] deals with formal descriptions of natural language. Most philosophers assume that the bulk of "normal" proper reasoning can be captured by logic, if one can find the right method for translating ordinary language into that logic. Philosophical logic is essentially a continuation of the traditional discipline that was called "Logic" before the invention of mathematical logic. Philosophical logic has a much greater concern with the connection between natural language and logic. As a result, philosophical logicians have contributed a great deal to the development of non-standard logics (e.g., [[free logic]]s, [[tense logic]]s) as well as various extensions of [[classical logic]] (e.g., [[modal logic]]s), and non-standard semantics for such logics (e.g., [[Saul Kripke|Kripke]]'s technique of supervaluations in the semantics of logic).
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| Logic and the philosophy of language are closely related. Philosophy of language has to do with the study of how our language engages and interacts with our thinking. Logic has an immediate impact on other areas of study. Studying logic and the relationship between logic and ordinary speech can help a person better structure his own arguments and critique the arguments of others. Many popular arguments are filled with errors because so many people are untrained in logic and unaware of how to formulate an argument correctly.
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| ===Computational logic===
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| {{Main|Logic in computer science}}
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| Logic cut to the heart of computer science as it emerged as a discipline: [[Alan Turing]]'s work on the [[Entscheidungsproblem]] followed from [[Kurt Gödel]]'s work on the [[incompleteness theorems]], and the notion of general purpose computers that came from this work was of fundamental importance to the designers of the computer machinery in the 1940s.
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| In the 1950s and 1960s, researchers predicted that when human knowledge could be expressed using logic with [[mathematical notation]], it would be possible to create a machine that reasons, or artificial intelligence. This was more difficult than expected because of the complexity of human reasoning. In [[logic programming]], a program consists of a set of axioms and rules. Logic programming systems such as [[Prolog]] compute the consequences of the axioms and rules in order to answer a query.
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| Today, logic is extensively applied in the fields of [[Artificial Intelligence]], and [[Computer Science]], and these fields provide a rich source of problems in formal and informal logic. [[Argumentation theory]] is one good example of how logic is being applied to artificial intelligence. The [[ACM Computing Classification System]] in particular regards:
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| * Section F.3 on [[Logics and meanings of programs]] and F.4 on [[Mathematical logic and formal languages]] as part of the theory of computer science: this work covers [[formal semantics of programming languages]], as well as work of [[formal methods]] such as [[Hoare logic]]
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| * [[Boolean logic]] as fundamental to computer hardware: particularly, the system's section B.2 on [[Arithmetic and logic structures]], relating to operatives AND, NOT, and OR;
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| * Many fundamental logical formalisms are essential to section I.2 on artificial intelligence, for example [[modal logic]] and [[default logic]] in [[Knowledge representation formalisms and methods]], [[Horn clause]]s in logic programming, and [[description logic]].
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| Furthermore, computers can be used as tools for logicians. For example, in symbolic logic and mathematical logic, proofs by humans can be computer-assisted. Using [[automated theorem proving]] the machines can find and check proofs, as well as work with proofs too lengthy to write out by hand.
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| ===Bivalence and the law of the excluded middle===
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| {{Main|Principle of bivalence}}
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| The logics discussed above are all "[[Principle of bivalence|bivalent]]" or "two-valued"; that is, they are most naturally understood as dividing propositions into true and false propositions. [[Non-classical logic]]s are those systems that reject bivalence.
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| Hegel developed his own [[Hegelian dialectic|dialectic logic]] that extended [[Immanuel Kant|Kant]]'s transcendental logic but also brought it back to ground by assuring us that "neither in heaven nor in earth, neither in the world of mind nor of nature, is there anywhere such an abstract 'either–or' as the understanding maintains. Whatever exists is concrete, with difference and opposition in itself".<ref name="philosophical"/>
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| In 1910, [[Nicolai A. Vasiliev]] extended the law of excluded middle and the law of contradiction and proposed the law of excluded fourth and logic tolerant to contradiction.<ref name="Brenner2008"/> In the early 20th century [[Jan Łukasiewicz]] investigated the extension of the traditional true/false values to include a third value, "possible", so inventing [[ternary logic]], the first [[multi-valued logic]].{{Citation needed|date=October 2008}}
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| Logics such as [[fuzzy logic]] have since been devised with an infinite number of "degrees of truth", represented by a [[real number]] between 0 and 1.<ref name="stanford"/>
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| [[Intuitionistic logic]] was proposed by [[L.E.J. Brouwer]] as the correct logic for reasoning about mathematics, based upon his rejection of the [[law of the excluded middle]] as part of his [[intuitionism]]. Brouwer rejected formalisation in mathematics, but his student [[Arend Heyting]] studied intuitionistic logic formally, as did [[Gerhard Gentzen]]. Intuitionistic logic is of great interest to computer scientists, as it is a [[constructive logic]] and can be applied for extracting verified programs from proofs.
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| [[Modal logic]] is not truth conditional, and so it has often been proposed as a non-classical logic. However, modal logic is normally formalised with the principle of the excluded middle, and its [[relational semantics]] is bivalent, so this inclusion is disputable.
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| ==="Is logic empirical?"===
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| {{Main|Is logic empirical?}}
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| What is the [[Epistemology|epistemological]] status of the [[classical logic|laws of logic]]? What sort of argument is appropriate for criticizing purported principles of logic? In an influential paper entitled "Is logic empirical?"<ref name="Is Logic Empirical?"/> [[Hilary Putnam]], building on a suggestion of [[W.V. Quine]], argued that in general the facts of propositional logic have a similar epistemological status as facts about the physical universe, for example as the laws of [[mechanics]] or of [[general relativity]], and in particular that what physicists have learned about quantum mechanics provides a compelling case for abandoning certain familiar principles of classical logic: if we want to be [[philosophical realism|realists]] about the physical phenomena described by quantum theory, then we should abandon the [[principle of distributivity]], substituting for classical logic the [[quantum logic]] proposed by [[Garrett Birkhoff]] and [[John von Neumann]].<ref name="The Logic of Quantum Mechanics"/>
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| Another paper of the same name by [[Sir Michael Dummett]] argues that Putnam's desire for realism mandates the law of distributivity.<ref name="Truth and Other Enigmas"/> Distributivity of logic is essential for the realist's understanding of how propositions are true of the world in just the same way as he has argued the principle of bivalence is. In this way, the question, "Is logic empirical?" can be seen to lead naturally into the fundamental controversy in [[metaphysics]] on [[realism versus anti-realism]].
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| ===Implication: strict or material?===
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| {{Main|Paradox of entailment}}
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| It is obvious that the notion of implication formalised in classical logic does not comfortably translate into natural language by means of "if… then…", due to a number of
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| problems called the [[paradoxes of material implication]].
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| The first class of paradoxes involves counterfactuals, such as ''If the moon is made of green cheese, then 2+2=5'', which are puzzling because natural language does not support the [[principle of explosion]]. Eliminating this class of paradoxes was the reason for [[C. I. Lewis]]'s formulation of [[strict implication]], which eventually led to more radically revisionist logics such as [[relevance logic]].
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| The second class of paradoxes involves redundant premises, falsely suggesting that we know the succedent because of the antecedent: thus "if that man gets elected, granny will die" is materially true since granny is mortal, regardless of the man's election prospects. Such sentences violate the [[Gricean maxim]] of relevance, and can be modelled by logics that reject the principle of [[monotonicity of entailment]], such as relevance logic.
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| ===Tolerating the impossible===
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| {{Main|Paraconsistent logic}}
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| [[Hegel]] was deeply critical of any simplified notion of the [[Law of Non-Contradiction]]. It was based on [[Gottfried Wilhelm Leibniz|Leibniz]]'s idea that this law of logic also requires a sufficient ground to specify from what point of view (or time) one says that something cannot contradict itself. A building, for example, both moves and does not move; the ground for the first is our solar system and for the second the earth. In Hegelian dialectic, the law of non-contradiction, of identity, itself relies upon difference and so is not independently assertable.
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| Closely related to questions arising from the paradoxes of implication comes the suggestion that logic ought to tolerate [[inconsistency]]. [[Relevance logic]] and [[paraconsistent logic]] are the most important approaches here, though the concerns are different: a key consequence of [[classical logic]] and some of its rivals, such as [[intuitionistic logic]], is that they respect the [[principle of explosion]], which means that the logic collapses if it is capable of deriving a contradiction. [[Graham Priest]], the main proponent of [[dialetheism]], has argued for paraconsistency on the grounds that there are in fact, true contradictions.<ref name="stanford7"/>
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| ===Rejection of logical truth===
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| The philosophical vein of various kinds of skepticism contains many kinds of doubt and rejection of the various bases on which logic rests, such as the idea of logical form, correct inference, or meaning, typically leading to the conclusion that there are no [[logical truth]]s. Observe that this is opposite to the usual views in [[philosophical skepticism]], where logic directs skeptical enquiry to doubt received wisdoms, as in the work of [[Sextus Empiricus]].
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| [[Friedrich Nietzsche]] provides a strong example of the rejection of the usual basis of logic: his radical rejection of idealisation led him to reject truth as a "...mobile army of metaphors, metonyms, and anthropomorphisms—in short ... metaphors which are worn out and without sensuous power; coins which have lost their pictures and now matter only as metal, no longer as coins." <ref name="nietzsche"/> His rejection of truth did not lead him to reject the idea of either inference or logic completely, but rather suggested that "logic [came] into existence in man's head [out] of illogic, whose realm originally must have been immense. Innumerable beings who made inferences in a way different from ours perished".<ref name="nietzsche8"/> Thus there is the idea that logical inference has a use as a tool for human survival, but that its existence does not support the existence of truth, nor does it have a reality beyond the instrumental: "Logic, too, also rests on assumptions that do not correspond to anything in the real world".<ref name="nietzsche9"/>
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| This position held by Nietzsche however, has come under extreme scrutiny for several reasons. He fails to demonstrate the validity of his claims and merely asserts them rhetorically. Although, since he is criticising the established criteria of validity, this does not undermine his position for one could argue that the demonstration of validity provided in the name of logic was just as rhetorically based. Some philosophers, such as [[Jürgen Habermas]], claim his position is self-refuting—and accuse Nietzsche of not even having a coherent perspective, let alone a theory of knowledge.<ref name="nietzsche10"/> Again, it is unclear if this is a decisive critique for the criteria of coherency and consistent theory are exactly what is under question. [[Georg Lukács]], in his book ''The Destruction of Reason'', asserts that, "Were we to study Nietzsche's statements in this area from a logico-philosophical angle, we would be confronted by a dizzy chaos of the most lurid assertions, arbitrary and violently incompatible."<ref name="marxists"/> Still, in this respect his 'theory' would be a much better depicition of a confused and chaotic reality than any consistent and compatible theory. [[Bertrand Russell]] referred to Nietzsche's claims as "empty words" in his book ''A History of Western Philosophy''.<ref name="amazon"/>
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| ==See also==
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| {{Wikipedia books}}
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| {{Columns-list|2|
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| * [[Digital electronics]] (also known as ''[[digital logic]]'' or [[logic gate]]s)
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| * [[Fallacies]]
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| * [[List of logic journals]]
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| * [[Logic puzzle]]
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| * [[Logic symbols]]
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| * [[Mathematics]]
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| ** [[List of mathematics articles]]
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| ** [[Outline of mathematics]]
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| * [[Metalogic]]
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| * [[Outline of logic]]
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| * [[Philosophy]]
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| ** [[List of philosophy topics]]
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| ** [[Outline of philosophy]]
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| * [[Reason]]
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| * ''[[Straight and Crooked Thinking]]'' (book)
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| * [[Table of logic symbols]]
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| * [[Truth]]
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| * [[Vector logic]]
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| }}
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| ==Notes and references==
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| {{Reflist|30em|refs=
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| <ref name="achievement">E.g., Kline (1972, p.53) wrote "A major achievement of Aristotle was the founding of the science of logic".</ref>
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| <!-- | |
| <ref name="Advances in Argumentation Theory and Research">{{cite book | editor1-first = J. Robert | editor1-last = Cox | editor2-first = Charles Arthur | editor2-last = Willard | editor2-link = Charles Arthur Willard | title = Advances in Argumentation Theory and Research | publisher = [[Southern Illinois University Press]] | year = 1983 | isbn = 978-0-8093-1050-0}}</ref>-->
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| <ref name="amazon">{{cite book |first=Bertrand |last=Russell |title=A History of Western Philosophy }}</ref>
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| <ref name="argumentative">"possessed of reason, intellectual, dialectical, argumentative", also related to [[wiktionary:λόγος|λόγος]] (''logos''), "word, thought, idea, argument, account, reason, or principle" (Liddell & Scott 1999; Online Etymology Dictionary 2001).</ref>
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| <ref name="Aristotle's syllogistic from the standpoint of modern formal logic">{{cite book | title = Aristotle's syllogistic from the standpoint of modern formal logic | publisher = Oxford University Press | edition = 2nd | year = 1957 | page = 7 | first = Jan | last = Łukasiewicz | authorlink = Jan Łukasiewicz | isbn = 978-0-19-824144-7}}</ref>
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| <ref name="Aristotle">{{cite web | url = http://www.britannica.com/EBchecked/topic/346217/history-of-logic/65920/Aristotle | title = Aristotle | work = [[Encyclopædia Britannica]]}}</ref>
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| <ref name="Bergmann, Merrie 2009">{{cite book |last=Bergmann |first=Merrie |first2=James |last2=Moor |first3=Jack |last3=Nelson |title=The Logic Book |edition=Fifth |location=New York, NY |publisher=McGraw-Hill |year=2009 |isbn=978-0-07-353563-0 }}</ref>
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| <ref name="Brenner2008">{{cite book |author=Joseph E. Brenner|title=Logic in Reality|url=http://books.google.com/books?id=Jnj5E6C9UwsC&pg=PA28|accessdate=9 April 2012|date=3 August 2008|publisher=Springer|isbn=978-1-4020-8374-7|pages=28–30}}</ref>
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| <ref name="Chakrabarti">{{cite journal |title=Some Comparisons Between Frege's Logic and Navya-Nyaya Logic|author=Kisor Kumar Chakrabarti|journal=Philosophy and Phenomenological Research|volume=36|issue=4|date=June 1976|pages=554–563|publisher=International Phenomenological Society|jstor=2106873|quote=This paper consists of three parts. The first part deals with Frege's distinction between sense and reference of proper names and a similar distinction in Navya-Nyaya logic. In the second part we have compared Frege's definition of number to the Navya-Nyaya definition of number. In the third part we have shown how the study of the so-called 'restrictive conditions for universals' in Navya-Nyaya logic anticipated some of the developments of modern set theory.|doi=10.2307/2106873}}</ref>
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| <ref name="google">Jonathan Lear (1986). "''[http://books.google.com/books?id=lXI7AAAAIAAJ&pg=PA34&dq&hl=en#v=onepage&q=&f=false Aristotle and Logical Theory]''". [[Cambridge University Press]]. p.34. ISBN 0-521-31178-0</ref>
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| <ref name="google1">Simo Knuuttila (1981). "''[http://books.google.com/books?id=iCCUF_OtA8AC&pg=PA71&dq&hl=en#v=onepage&q=&f=false Reforging the great chain of being: studies of the history of modal theories]''". Springer Science & Business. p.71. ISBN 90-277-1125-9</ref>
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| <ref name="google2">Michael Fisher, Dov M. Gabbay, Lluís Vila (2005). "''[http://books.google.com/books?id=Ajsvo6jWNhgC&pg=PA119&dq&hl=en#v=onepage&q=&f=false Handbook of temporal reasoning in artificial intelligence]''". Elsevier. p.119. ISBN 0-444-51493-7</ref>
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| <ref name="google3">Harold Joseph Berman (1983). "''[http://books.google.com/books?id=9-8fIBVgCQYC&pg=PA133&dq&hl=en#v=onepage&q=&f=false Law and revolution: the formation of the Western legal tradition]''". [[Harvard University Press]]. p.133. ISBN 0-674-51776-8</ref>
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| <!-- Unused citation <ref name="google4">Peter Thomas Geach (1980). "''[http://books.google.com/books?id=XCUrx6WGgrwC&pg=PA316&dq&hl=en#v=onepage&q=&f=false Logic Matters]''". [[University of California Press]]. p.316. ISBN 0-520-03847-9</ref> -->
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| <ref name="Hamilton">For a more modern treatment, see {{cite book | first = A. G. | last = Hamilton | title = Logic for Mathematicians | publisher = Cambridge University Press | year = 1980 | isbn = 0-521-29291-3}}</ref>
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| <ref name="Indian logic: a reader">{{cite book |title=Indian logic: a reader|author=Jonardon Ganeri|publisher=[[Routledge]]|year=2001|isbn=0-7007-1306-9|pages=vii, 5, 7}}</ref>
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| <ref name="Introduction to Elementary Mathematical Logic">{{cite book | first = Abram A. | last = Stolyar | title = Introduction to Elementary Mathematical Logic | page = 3 | publisher = Dover Publications | year = 1983 | isbn = 0-486-64561-4}}</ref>
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| <ref name="Introduction to Mathematical Logic">{{cite book | first = Elliott | last = Mendelson | title = Introduction to Mathematical Logic | chapter = Quantification Theory: Completeness Theorems | year = 1964 | publisher = Van Nostrand | isbn = 0-412-80830-7}}</ref>
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| <ref name="Introduction to Mathematical Logic5">{{cite book | last = Mendelson | first = Elliott | year = 1964 | title = Introduction to Mathematical Logic | publisher = Wadsworth & Brooks/Cole Advanced Books & Software | location = Monterey, Calif. | oclc = 13580200 | chapter = Formal Number Theory: Gödel's Incompleteness Theorem}}</ref>
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| <ref name="Is Logic Empirical?">{{cite journal | last = Putnam | first = H. | authorlink = Hilary Putnam | year = 1969 | title = Is Logic Empirical? | journal = Boston Studies in the Philosophy of Science | volume = 5}}</ref>
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| <ref name="marxists">{{cite web|author=Georg Lukács |url=http://www.marxists.org/archive/lukacs/works/destruction-reason/ch03.htm |title=The Destruction of Reason by Georg Lukács 1952 |publisher=Marxists.org |date= |accessdate=2013-06-16}}</ref>
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| <ref name="mohist">[[Mohist]]s and the [[school of Names]] date back at 2200 years.</ref>
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| <ref name="mtu">"[http://chemistry.mtu.edu/%7Epcharles/SCIHISTORY/aristotle.html Aristotle]", MTU Department of Chemistry.</ref>
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| <ref name="nietzsche">Nietzsche, 1873, [[On Truth and Lies in a Nonmoral Sense]].</ref>
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| <ref name="nietzsche10">Babette Babich, Habermas, Nietzsche, and Critical Theory</ref>
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| <ref name="nietzsche8">Nietzsche, 1882, ''[[The Gay Science]]''.</ref>
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| <ref name="nietzsche9">Nietzsche, 1878, ''[[Human, All Too Human]]''</ref>
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| <ref name="philosophical">{{cite book | first = G. W. F | last = Hegel | authorlink = Georg Wilhelm Friedrich Hegel | title = [[Philosophy of Mind]] | series = Encyclopedia of the Philosophical Sciences | others = trans. [[William Wallace (Scottish philosopher)|William Wallace]] | location = Oxford | publisher = Clarendon Press | year = 1971 | page = 174 | origyear = 1817 | isbn = 0-19-875014-5}}</ref>
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| <ref name="PopkinStroll1993">{{cite book |author1=Richard Henry Popkin|author2=Avrum Stroll|title=Philosophy Made Simple|url=http://books.google.com/books?id=TWNo-4euyesC&pg=PR7|accessdate=5 March 2012|date=1 July 1993|publisher=Random House Digital, Inc|isbn=978-0-385-42533-9|page=238}}</ref>
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| <ref name="jacquette2002">{{cite book |author=Jacquette, D.|title=A Companion to Philosophical Logic|page=2|publisher=Wiley Online Library|year=2002}}</ref>
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| <ref name="Principia">{{cite book | first1 = Alfred North | last1 = Whitehead | authorlink1 = Alfred North Whitehead | first2 = Bertrand | last2 = Russell | authorlink2 = Bertrand Russell | title = Principia Mathematica to *56 | publisher = [[Cambridge University Press]] | year = 1967 | isbn = 0-521-62606-4}}</ref>
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| <ref name="Principia">{{cite book | first1 = Alfred North | last1 = Whitehead | authorlink1 = Alfred North Whitehead | first2 = Bertrand | last2 = Russell | authorlink2 = Bertrand Russell | title = [[Principia Mathematica| Principia Mathematica to *56]] | publisher = [[Cambridge University Press]] | year = 1967 | isbn = 0-521-62606-4 | chapter = Chapter I: Preliminary Explanations of Ideas and Notation}}</ref>
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| <ref name="Prior Analytics">{{cite book | author = Aristotle | authorlink = Aristotle | title = Prior Analytics | page = 115 | publisher = Hackett Publishing Co. | year = 1989 | isbn = 978-0-87220-064-7}}</ref>
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| <ref name="propositions">The four Catuṣkoṭi logical divisions are formally very close to the four opposed propositions of the Greek [[tetralemma]], which in turn are analogous to the four [[truth value]]s of modern [[relevance logic]] Cf. Belnap (1977); Jayatilleke, K. N., (1967, The logic of four alternatives, in ''Philosophy East and West'', University of Hawaii Press).</ref>
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| <ref name="stanford-logic-onthology">{{cite book | last = Hofweber | first = T. | year = 2004 | chapter = Logic and Ontology | title = Stanford Encyclopedia of Philosophy | editor-first = Edward N | editor-last = Zalta | editor-link = Edward N. Zalta | chapterurl = http://plato.stanford.edu/entries/logic-ontology}}</ref>
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| <ref name="stanford">{{cite book | last = Hájek | first = Petr | authorlink = Petr Hájek | year = 2006 | chapter = Fuzzy Logic | title = [[Stanford Encyclopedia of Philosophy]] | editor-first = Edward N. | editor-last = Zalta | editor-link = Edward N. Zalta | chapterurl = http://plato.stanford.edu/entries/logic-fuzzy/}}</ref>
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| <ref name="stanford7">{{cite book | last = Priest | first = Graham | authorlink = Graham Priest | year = 2008 | chapter = Dialetheism | title = [[Stanford Encyclopedia of Philosophy]] | editor-first = Edward N. | editor-last = Zalta | editor-link = Edward N. Zalta | chapterurl = http://plato.stanford.edu/entries/dialetheism}}</ref>
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| <ref name="syllogistic">For example, [[Nyaya]] (syllogistic recursion) dates back 1900 years.</ref>
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| <ref name="The Basic Works">{{cite book | author = Aristotle | authorlink = Aristotle | title = The Basic Works | editor-first = Richard | editor-last = Mckeon | publisher = Modern Library | year = 2001 | isbn = 0-375-75799-6 | chapter = [[Posterior Analytics]]}}</ref>
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| <ref name="The Cambridge Companion to Aristotle">{{cite book | first = Jonathan | last = Barnes | title = The Cambridge Companion to Aristotle | page = 27 | publisher = Cambridge University Press | year = 1995 | isbn = 0-521-42294-9}}</ref>
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| <ref name="The Logic of Quantum Mechanics">{{cite journal | last1 = Birkhoff | first1 = G. | authorlink1 = Garrett Birkhoff | last2 = von Neumann | first2 = J. | authorlink2 = John von Neumann | year = 1936 | title = The Logic of Quantum Mechanics | journal = [[Annals of Mathematics]] | volume = 37 | pages = 823–843 | doi = 10.2307/1968621 | jstor = 1968621 | issue = 4 | publisher = Annals of Mathematics}}</ref>
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| <ref name="The Portable Plato">{{cite book | author = Plato | authorlink = Plato | title = The Portable Plato | editor-first = Scott | editor-last = Buchanan | publisher = Penguin | year = 1976 | isbn = 0-14-015040-4}}</ref>
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| <ref name="Theory of computation: formal languages, automata, and complexity">{{cite book | last = Brookshear | first = J. Glenn | year = 1989 | title = Theory of computation: formal languages, automata, and complexity | publisher = Benjamin/Cummings Pub. Co. | location = Redwood City, Calif. | isbn = 0-8053-0143-7 | chapter = Computability: Foundations of Recursive Function Theory}}</ref>
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| <ref name="Theory of computation: formal languages, automata, and complexity6">{{cite book | last = Brookshear | first = J. Glenn | year = 1989 | title = Theory of computation: formal languages, automata, and complexity | publisher = Benjamin/Cummings Pub. Co. | location = Redwood City, Calif. | isbn = 0-8053-0143-7 | chapter = Complexity}}</ref>
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| <ref name="Truth and Other Enigmas">{{cite book | last = Dummett | first = M. | authorlink = Michael Dummett | year = 1978 | chapter = Is Logic Empirical? | title = Truth and Other Enigmas | isbn = 0-674-91076-1}}</ref>
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| }}
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| ==Bibliography==
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| {{Refbegin}}
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| * [[Nuel Belnap]], (1977). "A useful four-valued logic". In Dunn & Eppstein, ''Modern uses of multiple-valued logic''. Reidel: Boston.
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| * [[Józef Maria Bocheński]] (1959). ''A précis of [[mathematical logic]]''. Translated from the French and German editions by Otto Bird. D. Reidel, Dordrecht, South Holland.
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| * Józef Maria Bocheński, (1970). ''A history of [[formal logic]]''. 2nd Edition. Translated and edited from the German edition by Ivo Thomas. Chelsea Publishing, New York.
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| * {{cite book | last = Brookshear | first = J. Glenn | year = 1989 | title = Theory of computation: formal languages, automata, and complexity | publisher = Benjamin/Cummings Pub. Co. | location = Redwood City, Calif. | isbn = 0-8053-0143-7}}
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| * Cohen, R.S, and Wartofsky, M.W. (1974). ''Logical and Epistemological Studies in Contemporary Physics''. Boston Studies in the Philosophy of Science. D. Reidel Publishing Company: Dordrecht, Netherlands. ISBN 90-277-0377-9.
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| * Finkelstein, D. (1969). "Matter, Space, and Logic". in R.S. Cohen and M.W. Wartofsky (eds. 1974).
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| * [[Dov Gabbay|Gabbay, D.M.]], and Guenthner, F. (eds., 2001–2005). ''Handbook of Philosophical Logic''. 13 vols., 2nd edition. Kluwer Publishers: Dordrecht.
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| * [[David Hilbert|Hilbert, D.]], and [[Wilhelm Ackermann|Ackermann, W]], (1928). ''Grundzüge der theoretischen Logik'' (''[[Principles of Mathematical Logic]]''). Springer-Verlag. [http://worldcat.org/oclc/2085765 OCLC 2085765]
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| * [[Susan Haack]], (1996).'' Deviant Logic, Fuzzy Logic: Beyond the Formalism'', University of Chicago Press.
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| * [[Wilfred Hodges|Hodges, W.]], (2001). ''Logic. An introduction to Elementary Logic'', Penguin Books.
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| * Hofweber, T., (2004), [http://plato.stanford.edu/entries/logic-ontology/ Logic and Ontology]. ''[[Stanford Encyclopedia of Philosophy]]''. [[Edward N. Zalta]] (ed.).
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| * Hughes, R.I.G., (1993, ed.). ''A Philosophical Companion to First-Order Logic''. Hackett Publishing.
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| * {{cite book | first = Morris | last = Kline | title = Mathematical Thought From Ancient to Modern Times | publisher = Oxford University Press | year = 1972 | isbn = 0-19-506135-7}}
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| * [[William Kneale (logician)|Kneale, William]], and Kneale, Martha, (1962). ''The Development of Logic''. Oxford University Press, London, UK.
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| * {{cite web | url = http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2363716 | title = Logikos | first1 = Henry George | last1 = Liddell | authorlink1 = Henry Liddell | first2 = Robert | last2 = Scott | authorlink2 = Robert Scott (philologist) | work = [[A Greek-English Lexicon]] | publisher = [[Perseus Project]] | accessdate = 8 May 2009}}
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| * Mendelson, Elliott, (1964). ''Introduction to Mathematical Logic''. Wadsworth & Brooks/Cole Advanced Books & Software: Monterey, Calif. [http://worldcat.org/oclc/13580200 OCLC 13580200]
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| * {{cite web | first= Robert | last = Harper | url = http://www.etymonline.com/index.php?term=logic | title = Logic | work = [[Online Etymology Dictionary]] | year = 2001 | accessdate = 8 May 2009}}
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| * [[Barry Smith (ontologist)|Smith, B.]], (1989). "Logic and the Sachverhalt". ''The Monist'' 72(1):52–69.
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| * [[Alfred North Whitehead|Whitehead, Alfred North]] and [[Bertrand Russell]], (1910). ''[[Principia Mathematica]]''. Cambridge University Press: Cambridge, England. [http://worldcat.org/oclc/1041146 OCLC 1041146]
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| {{Refend}}
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| ==External links==
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| {{Sister project links|Logic}}
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| {{Library resources box}}
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| *{{PhilPapers|category|logic-and-philosophy-of-logic}}
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| *{{InPho|taxonomy|2245}}
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| *{{IEP|category/s-l-m/logic/}}
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| * {{springer|title=Logical calculus|id=p/l060690}}
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| * [http://logic-law.com/index.php?title=Verbal_Logic An Outline for Verbal Logic]
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| * Introductions and tutorials
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| ** [http://www.galilean-library.org/manuscript.php?postid=43782 An Introduction to Philosophical Logic], by Paul Newall, aimed at beginners.
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| ** [http://www.fecundity.com/logic/ forall x: an introduction to formal logic], by [[P.D. Magnus]], covers sentential and quantified logic.
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| ** [http://www.filozofia.uw.edu.pl/kpaprzycka/Publ/xLogicSelfTaught.html Logic Self-Taught: A Workbook] (originally prepared for on-line logic instruction).
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| *** [[Nicholas Rescher]]. (1964). ''Introduction to Logic'', St. Martin's Press.
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| * Essays
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| ** [http://durendal.org:8080/lcsl/ "Symbolic Logic"] and [http://www.gutenberg.org/etext/4763 "The Game of Logic"], [[Lewis Carroll]], 1896.
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| ** [http://etext.lib.virginia.edu/DicHist/analytic/anaVII.html Math & Logic: The history of formal mathematical, logical, linguistic and methodological ideas.] In ''The Dictionary of the History of Ideas.''
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| * Online Tools
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| ** [http://thefirstscience.org/syllogistic-machine/ Interactive Syllogistic Machine] A web based syllogistic machine for exploring fallacies, figures, terms, and modes of syllogisms.
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| * Reference material
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| ** [http://www.earlham.edu/~peters/courses/log/transtip.htm Translation Tips], by Peter Suber, for translating from English into logical notation.
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| ** [http://www.ontology.co/history-of-logic.htm Ontology and History of Logic. An Introduction] with an annotated bibliography.
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| * Reading lists
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| ** The [http://www.ucl.ac.uk/philosophy/LPSG/ London Philosophy Study Guide] offers many suggestions on what to read, depending on the student's familiarity with the subject:
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| *** [http://www.ucl.ac.uk/philosophy/LPSG/L&M.htm Logic & Metaphysics]
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| *** [http://www.ucl.ac.uk/philosophy/LPSG/SetTheory.htm Set Theory and Further Logic]
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| *** [http://www.ucl.ac.uk/philosophy/LPSG/MathLogic.htm Mathematical Logic]
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| {{Logic}}
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| [[Category:Formal sciences]]
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| [[Category:Logic| ]]
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