1.deductive reasoning in which a conclusion is derived from two premises
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SyllogismSyl"lo*gism (?), n. [OE. silogisme, OF. silogime, sillogisme, F. syllogisme, L. syllogismus, Gr. syllogismo`s a reckoning all together, a reasoning, syllogism, fr. syllogi`zesqai to reckon all together, to bring at once before the mind, to infer, conclude; sy`n with, together + logi`zesqai to reckon, to conclude by reasoning. See Syn, and Logistic, Logic.] (Logic) The regular logical form of every argument, consisting of three propositions, of which the first two are called the premises, and the last, the conclusion. The conclusion necessarily follows from the premises; so that, if these are true, the conclusion must be true, and the argument amounts to demonstration; as in the following example:
☞ If the premises are not true and the syllogism is regular, the reasoning is valid, and the conclusion, whether true or false, is correctly derived.
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definition of Wikipedia
Disjunctive syllogism • Hypothetical syllogism • Multipremise syllogism • Politician's syllogism • Practical syllogism • Prosleptic syllogism • Quasisyllogism • Statistical syllogism
argument; reasoning; logical thinking; abstract thought[Classe]
(syllogism)[Thème]
mathematics[Domaine]
Reasoning[Domaine]
abstract thought, argument, logical thinking, reasoning  conclude, reason, reason out  logician, logistician[Hyper.]
deduce, deduce from, deduct, derive, extrapolate from, infer, infer from[Nominalisation]
synthesise, synthesize  deduce, infer  synthetic, synthetical  synthetic, synthetical  syllogism  syllogiser, syllogist, syllogizer  syllogise, syllogize[Dérivé]
deduction, deductive reasoning, synthesis[Hyper.]
syllogise, syllogize  syllogiser, syllogist, syllogizer  syllogistic[Dérivé]
syllogism (n.)↕
A syllogism (Greek: συλλογισμός – syllogismos – "conclusion," "inference") is a kind of logical argument in which one proposition (the conclusion) is inferred from two or more others (the premises) of a certain form. In antiquity, there were two rival theories of the syllogism: Aristotelian syllogistic and Stoic syllogistic.^{[1]}
In the Prior Analytics, Aristotle defines the syllogism as "a discourse in which, certain things having been supposed, something different from the things supposed results of necessity because these things are so." (24b18–20)
Despite this very general definition, in the Prior Analytics, Aristotle limits himself to categorical syllogisms, which consist of three categorical propositions.^{[2]} These included categorical modal syllogisms.^{[3]} From the Middle Ages onwards, "categorical syllogism" and "syllogism" were mostly used interchangeably, and the present article is concerned with this traditional use of "syllogism" only. The syllogism was at the core of traditional deductive reasoning, where facts are determined by combining existing statements, in contrast to inductive reasoning where facts are determined by repeated observations.
The syllogism was superseded by firstorder predicate logic following the work of Gottlob Frege, in particular his Begriffsschrift (Concept Script) (1879), but syllogisms remain useful.
Contents 
A categorical syllogism consists of three parts: the major premise, the minor premise and the conclusion.
Each part is a categorical proposition, and each categorical proposition contains two categorical terms.^{[4]} In Aristotle, each of the premises is in the form "All A are B," "Some A are B", "No A are B" or "Some A are not B", where "A" is one term and "B" is another. "All A are B," and "No A are B" are termed universal propositions; "Some A are B" and "Some A are not B" are termed particular propositions. More modern logicians allow some variation. Each of the premises has one term in common with the conclusion: in a major premise, this is the major term (i.e., the predicate of the conclusion); in a minor premise, it is the minor term (the subject) of the conclusion. For example:
Each of the three distinct terms represents a category. In the above example, "men", "mortal", and "Greeks". "Mortal" is the major term, "Greeks" the minor term. The premises also have one term in common with each other, which is known as the middle term; in this example, "men". Both of the premises are universal, as is the conclusion.
Here, the major term is "die", the minor term is "men", and the middle term is "mortals". The major premise is universal; the minor premise and the conclusion are particular.
A sorites is a form of argument in which a series of incomplete syllogisms is so arranged that the predicate of each premise forms the subject of the next until the subject of the first is joined with the predicate of the last in the conclusion. For example, if one argues that a given number of grains of sand does not make a heap and that an additional grain does not either, then to conclude that no additional amount of sand will make a heap is to construct a sorites argument.
Although there are infinitely many possible syllogisms, there are only a finite number of logically distinct types. We shall classify and enumerate them below. Note that the syllogism above has the abstract form:
(Note: M – Middle, S – subject, P – predicate. See below for more detailed explanation.)
The premises and conclusion of a syllogism can be any of four types, which are labeled by letters^{[5]} as follows. The meaning of the letters is given by the table:
code  quantifier  subject  copula  predicate  type  example  
a  All  S  are  P  universal affirmatives  All humans are mortal.  
e  No  S  are  P  universal negatives  No humans are perfect.  
i  Some  S  are  P  particular affirmatives  Some humans are healthy.  
o  Some  S  are not  P  particular negatives  Some humans are not clever. 
In Analytics, Aristotle mostly uses the letters A, B and C (actually, the Greek letters alpha, beta and gamma) as term place holders, rather than giving concrete examples, an innovation at the time. It is traditional to use is rather than are as the copula, hence All A is B rather than All As are Bs. It is traditional and convenient practice to use a, e, i, o as infix operators to enable the categorical statements to be written succinctly thus:
Form  Shorthand 

All A are B  AaB 
No A is B  AeB 
Some A are B  AiB 
Some A are not B  AoB 
This particular syllogistic form is dubbed BARBARA (see below) and can be written neatly as BaC,AaB > AaC.
The letter S is the subject of the conclusion, P is the predicate of the conclusion, and M is the middle term. The major premise links M with P and the minor premise links M with S. However, the middle term can be either the subject or the predicate of each premise where it appears. The differing positions of the major, minor, and middle terms gives rise to another classification of syllogisms known as the figure. Given that in each case the conclusion is SP, the four figures are:
Figure 1  Figure 2  Figure 3  Figure 4  
Major premise:  M–P  P–M  M–P  P–M  
Minor premise:  S–M  S–M  M–S  M–S 
Putting it all together, there are 256 possible types of syllogisms (or 512 if the order of the major and minor premises is changed, although this makes no difference logically). Each premise and the conclusion can be of type A, E, I or O, and the syllogism can be any of the four figures. A syllogism can be described briefly by giving the letters for the premises and conclusion followed by the number for the figure. For example, the syllogism BARBARA above is AAA1, or "AAA in the first figure".
The vast majority of the 256 possible forms of syllogism are invalid (the conclusion does not follow logically from the premises). The table below shows the valid forms. Even some of these are sometimes considered to commit the existential fallacy, meaning they are invalid if they mention an empty category. These controversial patterns are marked in italics.
Figure 1  Figure 2  Figure 3  Figure 4 
Barbara  Cesare  Datisi  Calemes 
Celarent  Camestres  Disamis  Dimatis 
Darii  Festino  Ferison  Fresison 
Ferio  Baroco  Bocardo  Calemos 
Barbari  Cesaro  Felapton  Fesapo 
Celaront  Camestros  Darapti  Bamalip 
The letters A, E, I, O have been used since the medieval Schools to form mnemonic names for the forms as follows: 'Barbara' stands for AAA, 'Celarent' for EAE, etc.
Next to each premise and conclusion is a shorthand description of the sentence. So in AAI3, the premise "All squares are rectangles" becomes "MaP"; the symbols mean that the first term ("square") is the middle term, the second term ("rectangle") is the predicate of the conclusion, and the relationship between the two terms is labeled "a" (All M are P).
The following table shows all syllogisms that are essentially different. The similar syllogisms share actually the same premises, just written in a different way. For example "Some pets are kittens" (SiM in Darii) could also be written as "Some kittens are pets" (MiS is Datisi).
In the Venn diagrams, the black areas indicate no elements, and the red areas indicate at least one element.
Similar: Cesare (EAE2)
Calemes (AEE4) 

Calemes is like Celarent with S and P exchanged.

Similar: Datisi (AII3)
Dimatis (IAI4) 

Dimatis is like Darii with S and P exchanged.

Similar: Festino (EIO2), Ferison (EIO3), Fresison (EIO4)
Bamalip (AAI4) 

Bamalip is like Barbari with S and P exchanged:

Similar: Cesaro (EAO2)
Similar: Calemos (AEO4)
Similar: Fesapo (EAO4)
This table shows all 24 valid syllogisms, represented by Venn diagrams.
(9 of them, on the right side of the table, require that one category must not be empty.)
Syllogisms of the same type are in the same row, and very similar syllogisms are in the same column.
1  Barbara 
Celarent 
Darii 
Ferio 
Barbari 
Celaront 

2  Cesare 
Camestres 
Festino 
Baroco 
Cesaro 
Camestros 

3  Datisi 
Disamis 
Ferison 
Bocardo 
Felapton 
Darapti 

4  Calemes 
Dimatis 
Fresison 
Calemos 
Fesapo 
Bamalip 
We may, with Aristotle, distinguish singular terms such as Socrates and general terms such as Greeks. Aristotle further distinguished (a) terms that could be the subject of predication, and (b) terms that could be predicated of others by the use of the copula (is are). (Such a predication is known as a distributive as opposed to nondistributive as in Greeks are numerous. It is clear that Aristotle's syllogism works only for distributive predication for we cannot reason All Greeks are animals, animals are numerous, therefore All Greeks are numerous.) In Aristotle’s view singular terms were of type (a) and general terms of type (b). Thus Men can be predicated of Socrates but Socrates cannot be predicated of anything. Therefore to enable a term to be interchangeable — that is to be either in the subject or predicate position of a proposition in a syllogism — the terms must be general terms, or categorical terms as they came to be called. Consequently the propositions of a syllogism should be categorical propositions (both terms general) and syllogisms employing just categorical terms came to be called categorical syllogisms.
It is clear that nothing would prevent a singular term occurring in a syllogism — so long as it was always in the subject position — however such a syllogism, even if valid, would not be a categorical syllogism. An example of such would be Socrates is a man, All men are mortal, therefore Socrates is mortal. Intuitively this is as valid as All Greeks are men, all men are mortal therefore all Greeks are mortals. To argue that its validity can be explained by the theory of syllogism it would be necessary to show that Socrates is a man is the equivalent of a categorical proposition. It can be argued Socrates is a man is equivalent to All that are identical to Socrates are men, so our noncategorical syllogism can be justified by use of the equivalence above and then citing BARBARA.
If a statement includes a term so that the statement is false if the term has no instances (is not instantiated) then the statement is said to entail existential import with respect to that term. In particular, a universal statement of the form All A is B has existential import with respect to A if All A is B is false if there are no As.
The following problems arise:
For example, if it is accepted that AiB is false if there are no As and AaB entails AiB, then AiB has existential import with respect to A, and so does AaB. Further, if it is accepted that AiB entails BiA, then AiB and AaB have existential import with respect to B as well. Similarly, if AoB is false if there are no As, and AeB entails AoB, and AeB entails BeA (which in turn entails BoA) then both AeB and AoB have existential import with respect to both A and B. It follows immediately that all universal categorical statements have existential import with respect to both terms. If AaB and AeB is a fair representation of the use of statements in normal natural language of All A is B and No A is B respectively, then the following example consequences arise:
and so on.
If it is ruled that no universal statement has existential import then the square of opposition fails in several respects (e.g. AaB does not entail AiB) and a number of syllogisms are no longer valid (e.g. BaC,AaB>AiC).
These problems and paradoxes arise in both natural language statements and statements in syllogism form because of ambiguity, in particular ambiguity with respect to All. If "Fred claims all his books were Pulitzer Prize winners", is Fred claiming that he wrote any books? If not, then is what he claims true? Suppose Jane says none of her friends are poor; is that true if she has no friends? The firstorder predicate calculus avoids the problems of such ambiguity by using formulae that carry no existential import with respect to universal statements; existential claims have to be explicitly stated. Thus natural language statements of the forms All A is B, No A is B, Some A is B and Some A is not B can be exactly represented in first order predicate calculus in which any existential import with respect to terms A and/or B is made explicitly or not made at all. Consequently the four forms AaB, AeB, AiB and AoB can be represented in first order predicate in every combination of existential import, so that it can establish which construal, if any, preserves the square of opposition and the validly of the traditionally valid syllogism. Strawson claims that such a construal is possible, but the results are such that, in his view, the answer to question (e) above is no.
The Aristotelian syllogism dominated Western philosophical thought from the 3rd Century to the 17th Century. At that time, Sir Francis Bacon rejected the idea of syllogism and deductive reasoning by asserting that it was fallible and illogical.^{[6]} Bacon offered a more inductive approach to logic in which experiments were conducted and axioms were drawn from the observations discovered in them.
In the 19th Century, modifications to syllogism were incorporated to deal with disjunctive ("A or B") and conditional ("if A then B") statements. Kant famously claimed, in Logic (1800), that logic was the one completed science, and that Aristotelian logic more or less included everything about logic there was to know. (This work is not necessarily representative of Kant's mature philosophy, which is often regarded as an innovation to logic itself.) Though there were alternative systems of logic such as Avicennian logic or Indian logic elsewhere, Kant's opinion stood unchallenged in the West until 1879 when Frege published his Begriffsschrift (Concept Script). This introduced a calculus, a method of representing categorical statements — and statements that are not provided for in syllogism as well — by the use of quantifiers and variables.
This led to the rapid development of sentential logic and firstorder predicate logic, subsuming syllogistic reasoning, which was, therefore, after 2000 years, suddenly considered obsolete by many. The Aristotelian system is explicated in modern fora of academia primarily in introductory material and historical study.
One notable exception to this modern relegation is the continued application of Aristotelian logic by officials of the Congregation for the Doctrine of the Faith, and the Apostolic Tribunal of the Roman Rota, which still requires that arguments crafted by Advocates be presented in syllogistic format.
People often make mistakes when reasoning syllogistically.^{[7]}
For instance, from the premises some A are B, some B are C, people tend to come to a definitive conclusion that therefore some A are C.^{[8]} However, this does not follow according to the rules of classical logic. For instance, while some cats (A) are black things (B), and some black things (B) are televisions (C), it does not follow from the parameters that some cats (A) are televisions (C). This is because first, the mood of the syllogism invoked is illicit (III), and second, the supposition of the middle term is variable between that of the middle term in the major premise, and that of the middle term in the minor premise (not all "some" cats are by necessity of logic the same "some black things").
Determining the validity of a syllogism involves determining the distribution of each term in each statement, meaning whether all members of that term are accounted for.
In simple syllogistic patterns, the fallacies of invalid patterns are:
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