In philosophical logic, the square of opposition is a diagram representing the relations between the four basic categorical propositions. The origin of the square can be traced back to Aristotle making the distinction between two oppositions: contradiction and contrariety. But Aristotle did not draw any diagram. This was done several centuries later by Apuleius and Boethius.
In traditional logic, a proposition (Latin: propositio) is a spoken assertion (oratio enunciativa), not the meaning of an assertion, as in modern philosophy of language and logic. A categorical proposition is a simple proposition containing two terms, subject (S) and predicate (P), in which the predicate is either asserted or denied of the subject.
Every categorical proposition can be reduced to one of four logical forms, named A, E, I, and O based on the Latin affirmo (I affirm), for the affirmative propositions A and I, and nego (I deny), for the negative propositions E and O. These are:
In tabular form:
|Universal affirmative||A||Omne S est P.||Every S is P. (S is always P.)||affirmo (I affirm)|
|Universal negative||E||Nullum S est P.||No S is P. (S is never P.)||nego (I deny)|
|Particular affirmative||I||Quoddam S est P.||Some S is P. (S is sometimes P.)||affirmo (I affirm)|
|Particular negative||O||Quoddam S nōn est P.||Some S is not P. (S is not always P.)||nego (I deny)|
*Proposition 'A' may be stated as "All S is P." However, Proposition 'E' when stated correspondingly as "All S is not P." is ambiguous because it can be either an E or O proposition, thus requiring a context to determine the form; the standard form "No S is P" is unambiguous, so it is preferred. Proposition 'O' also takes the forms "Sometimes S is not P." and "A certain S is not P." (literally the Latin 'Quoddam S nōn est P.')
Aristotle states (in chapters six and seven of the Peri hermēneias (Περὶ Ἑρμηνείας, Latin De Interpretatione, English 'On Interpretation')), that there are certain logical relationships between these four kinds of proposition. He says that to every affirmation there corresponds exactly one negation, and that every affirmation and its negation are 'opposed' such that always one of them must be true, and the other false. A pair of affirmative and negative statements he calls a 'contradiction' (in medieval Latin, contradictio). Examples of contradictories are 'every man is white' and 'not every man is white' (also read as 'some men are not white'), 'no man is white' and 'some man is white'.
'Contrary' (medieval: contrariae) statements, are such that both cannot at the same time be true. Examples of these are the universal affirmative 'every man is white', and the universal negative 'no man is white'. These cannot be true at the same time. However, these are not contradictories because both of them may be false. For example, it is false that every man is white, since some men are not white. Yet it is also false that no man is white, since there are some white men.
Since every statement has a contradictory opposite, and since a contradictory is true when its opposite is false, it follows that the opposites of contraries (which the medievals called subcontraries, subcontrariae) can both be true, but they cannot both be false. Since subcontraries are negations of universal statements, they were called 'particular' statements by the medieval logicians.
Another logical opposition implied by this, though not mentioned explicitly by Aristotle, is 'alternation' (alternatio), consisting of 'subalternation' and 'superalternation'. Alternation is a relation between a particular statement and a universal statement of the same quality such that the particular is implied by the other. The particular is the subaltern of the universal, which is the particular's superaltern. For example, if 'every man is white' is true, its contrary 'no man is white' is false. Therefore, the contradictory 'some man is white' is true. Similarly the universal 'no man is white' implies the particular 'not every man is white'.
These relationships became the basis of a diagram originating with Boethius and used by medieval logicians to classify the logical relationships. The propositions are placed in the four corners of a square, and the relations represented as lines drawn between them, whence the name 'The Square of Opposition'.
Subcontraries, which medieval logicians represented in the form 'quoddam A est B' (some particular A is B) and 'quoddam A non est B' (some particular A is not B) cannot both be false, since their universal contradictory statements (every A is B / no A is B) cannot both be true. This leads to a difficulty that was first identified by Peter Abelard. 'Some A is B' seems to imply 'something is A'. For example, 'Some man is white' seems to imply that at least one thing is a man, namely the man who has to be white, if 'some man is white' is true. But, 'some man is not white' also implies that something is a man, namely the man who is not white, if the statement 'some man is not white' is true. But Aristotelian logic requires that necessarily one of these statements is true. Both cannot be false. Therefore, (since both imply that something is a man) it follows that necessarily something is a man, i.e. men exist. But (as Abelard points out, in the Dialectica) surely men might not exist?
Abelard also points out that subcontraries containing subject terms denoting nothing, such as 'a man who is a stone', are both false.
Terence Parsons argues that ancient philosophers did not experience the problem of existential import as only the A and I forms had existential import.
He goes on to cite medieval philosopher William of Moerbeke
And points to Boethius' translation of Aristotle's work as giving rise to the mistaken notion that the O form has existential import.
In the 19th century, George Boole argued for requiring existential import on both terms in particular claims (I and O), but allowing all terms of universal claims (A and E) to lack existential import. This decision made Venn diagrams particularly easy to use for term logic. The square of opposition, under this Boolean set of assumptions, is often called the modern Square of opposition. In the modern square of opposition, A and O claims are contradictories, as are E and I, but all other forms of opposition cease to hold; there are no contraries, subcontraries, or subalterns. Thus, from a modern point of view, it often makes sense to talk about 'the' opposition of a claim, rather than insisting as older logicians did that a claim has several different opposites, which are in different kinds of opposition with the claim.
Gottlob Frege's Begriffsschrift also presents a square of oppositions, organised in an almost identical manner to the classical square, showing the contradictories, subalternates and contraries between four formulae constructed from universal quantification, negation and implication.
The traditional square of opposition is now often compared with squares based on inner- and outer-negation.
The square of opposition has been extended to a logical hexagon which includes the relationships of six statements. It was discovered independently by both Augustin Sesmat and Robert Blanché. It has been proven that both the square and the hexagon, followed by a "logical cube", belong to a regular series of n-dimensional objects called "logical bi-simplexes of dimension n." The pattern also goes even beyond this.
The logical square, also called square of opposition or square of Apuleius has its origin in the four marked sentences to be employed in syllogistic reasoning: Every man is bad, the universal affirmative and its negation Not every man is bad (or Some men are not bad), the particular negative on the one hand, Some men are bad, the particular affirmative and its negation No man is bad, the universal negative on the other. Robert Blanché published with Vrin his Structures intellectuelles in 1966 and since then many scholars think that the logical square or square of opposition representing four values should be replaced by the logical hexagon which by representing six values is a more potent figure because it has the power to explain more things about logic and natural language.
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