Categorical proposition

In logic, a categorical proposition, or categorical statement, is a proposition that asserts or denies that all or some of the members of one category (the subject term) are included in another (the predicate term). The study of arguments using categorical statements (i.e., syllogisms) forms an important branch of deductive reasoning that began with the Ancient Greeks.

The Ancient Greeks such as Aristotle identified four primary distinct types of categorical proposition and gave them standard forms (now often called A, E, I, and O). If, abstractly, the subject category is named S and the predicate category is named P, the four standard forms are:

• All S are P. (A form)
• No S are P. (E form)
• Some S are P. (I form)
• Some S are not P. (O form)

A surprisingly large number of sentences may be translated into one of these canonical forms while retaining all or most of the original meaning of the sentence. Greek investigations resulted in the so-called square of opposition, which codifies the logical relations among the different forms; for example, that an A-statement is contradictory to an O-statement; that is to say, for example, if one believes "All apples are red fruits," one cannot simultaneously believe that "Some apples are not red fruits." Thus the relationships of the square of opposition may allow immediate inference, whereby the truth or falsity of one of the forms may follow directly from the truth or falsity of a statement in another form.

Modern understanding of categorical propositions (originating with the mid-19th century work of George Boole) requires one to consider if the subject category may be empty. If so, this is called the hypothetical viewpoint, in opposition to the existential viewpoint which requires the subject category to have at least one member. The existential viewpoint is a stronger stance than the hypothetical and, when it is appropriate to take, it allows one to deduce more results than otherwise could be made. The hypothetical viewpoint, being the weaker view, has the effect of removing some of the relations present in the traditional square of opposition.

Arguments consisting of three categorical propositions — two as premises and one as conclusion — are known as categorical syllogisms and were of paramount importance from the times of ancient Greek logicians through the Middle Ages. Although formal arguments using categorical syllogisms have largely given way to the increased expressive power of modern logic systems like the first-order predicate calculus, they still retain practical value in addition to their historic and pedagogical significance.

Translating statements into standard form

not all members can use the lift

Properties of categorical propositions

Categorical propositions can be categorized into four types on the basis of their "quality" and "quantity", or their "distribution of terms". These four types have long been named A, E, I and O. This is based on the Latin affirmo (I affirm), referring to the affirmative propositions A and I, and nego (I deny), referring to the negative propositions E and O.

Quantity and quality

Quantity refers to the amount of members of the subject class that are used in the proposition. If the proposition refers to all members of the subject class, it is universal. If the proposition does not employ all members of the subject class, it is particular. For instance, an I-proposition ("Some S are P") is particular since it only refers to some of the members of the subject class.

Quality refers to whether the proposition affirms or denies the inclusion of a subject within the class of the predicate. The two possible qualities are called affirmative and negative. For instance, an A-proposition ("All S are P") is affirmative since it states that the subject is contained within the predicate. On the other hand, an O-proposition ("Some S are not P") is negative since it excludes the subject from the predicate.

Name	Statement	Quantity	Quality
A	All S are P.	universal	affirmative
E	No S are P.	universal	negative
I	Some S are P.	particular	affirmative
O	Some S are not P.	particular	negative

An important consideration is the definition of the word some. In logic, some refers to "one or more", which could mean "all". Therefore, the statement "Some S are P" does not guarantee that the statement "Some S are not P" is also true.

Distributivity

The two terms (subject and predicate) in a categorical proposition may each be classified as distributed or undistributed. If all members of the term's class are affected by the proposition, that class is distributed; otherwise it is undistributed. Every proposition therefore has one of four possible distribution of terms.

Each of the four canonical forms will be examined in turn regarding its distribution of terms. Although not developed here, Venn diagrams are sometimes helpful when trying to understand the distribution of terms for the four forms.

A form

An A-proposition distributes the subject to the predicate, but not the reverse. Consider the following categorical proposition: "All dogs are mammals". All dogs are indeed mammals but it would be false to say all mammals are dogs. Since all dogs are included in the class of mammals, "dogs" is said to be distributed to "mammals". Since all mammals are not necessarily dogs, "mammals" is undistributed to "dogs".

E form

An E-proposition distributes bidirectionally between the subject and predicate. From the categorical proposition "No beetles are mammals", we can infer that no mammals are beetles. Since all beetles are defined not to be mammals, and all mammals are defined not to be beetles, both classes are distributed.

I form

Both terms in an I-proposition are undistributed. For example, "Some Americans are conservatives". Neither term can be entirely distributed to the other. From this proposition it is not possible to say that all Americans are conservatives or that all conservatives are Americans.

O form

In an O-proposition only the predicate is distributed. Consider the following: "Some politicians are not corrupt". Since not all politicians are defined by this rule, the subject is undistributed. The predicate, though, is distributed because all the members of "corrupt people" will not match the group of people defined as "some politicians". Since the rule applies to every member of the corrupt people group, namely, "all corrupt people are not some politicians", the predicate is distributed.

The distribution of the predicate in an O-proposition is often confusing due to its ambiguity. When a statement like "Some politicians are not corrupt" is said to distribute the "corrupt people" group to "some politicians", the information seems of little value since the group "some politicians" is not defined. But if, as an example, this group of "some politicians" were defined to contain a single person, Albert, the relationship becomes more clear. The statement would then mean, of every entry listed in the corrupt people group, not one of them will be Albert: "all corrupt people are not Albert". This is a definition that applies to every member of the "corrupt people" group, and is therefore distributed.

Summary

In short, for the subject to be distributed, the statement must be universal (e.g., "all", "no"). For the predicate to be distributed, the statement must be negative (e.g., "no", "not").

Name	Statement	Distribution
		Subject	Predicate
A	All S are P.	distributed	undistributed
E	No S are P.	distributed	distributed
I	Some S are P.	undistributed	undistributed
O	Some S are not P.	undistributed	distributed

Criticism

Peter Geach and others have criticized the use of distribution to determine the validity of an argument. It has been suggested that statements of the form "Some A are not B" would be less problematic if stated as "Not every A is B," which is perhaps a closer translation to Aristotle's original form for this type of statement.

Operations on categorical statements

There are several operations (e.g., conversion, obversion, and contraposition) that can be performed on a categorical statement to change it into another. The new statement may or may not be equivalent to the original. [In the following tables that illustrate such operations, rows with equivalent statement shall be marked in green, while those with inequivalent statements shall be marked in red.]

Some operations require the notion of the class complement. This refers to every element under consideration which is not an element of the class. Class complements are very similar to set complements. The class complement of a set P will be called "non-P".

Conversion

Main article: Logical conversion

The simplest operation is conversion where the subject and predicate terms are interchanged.

Name	Statement	Converse	Obverted Converse	Subaltern	Converse per accidens	Obverted Converse per accidens
A	All S are P.	All P are S.	No P are non-S.	Some S are P (if S or P exists).	Some P are S (if S or P exists).	Some P are not non-S (if S or P exists).
E	No S are P.	No P are S.	All P are non-S.	Some S are not P (if S exists).	Some P are not S (if P exists).	Some P are non-S (if P exists).
I	Some S are P.	Some P are S.	Some P are not non-S.	N/A
O	Some S are not P.	Some P are not S.	Some P are non-S.

From a statement in E or I form, it is valid to conclude its converse. This is not the case for the A and O forms.

Obversion

Main article: Obversion

Obversion changes the quality (that is the affirmativity or negativity) of the statement and the predicate term. For example, a universal affirmative statement would become a universal negative statement.

Name	Statement	Obverse
A	All S are P.	No S are non-P.
E	No S are P.	All S are non-P.
I	Some S are P.	Some S are not non-P.
O	Some S are not P.	Some S are non-P.

Categorical statements are logically equivalent to their obverse. As such, a Venn diagram illustrating any one of the forms would be identical to the Venn diagram illustrating its obverse.

Contraposition

Main article: Contraposition

Name	Statement	Contrapositive	Obverted Contrapositive	Contrapositive per accidens	Obverted Contrapositive per accidens
A	All S are P.	All non-P are non-S.	No non-P are S.	N/A
E	No S are P.	No non-P are non-S.	All non-P are S.	Some non-P are not non-S (if S exists).	Some non-P are S (if S exists).
I	Some S are P.	Some non-P are non-S.	Some non-P are not S.	N/A
O	Some S are not P.	Some non-P are not non-S.	Some non-P are S.

See also

• Square of opposition
• Term logic

External links

• ChangingMinds.org: Categorical propositions
• Catlogic: An open source computer script written in Ruby to construct, investigate, and compute categorical propositions and syllogisms