predicable

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predicable

logic

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Key People:: Aristotle

Related Topics:: predicate

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predicable, in logic, something that may be predicated, especially, as listed in Boethius’ Latin version of Porphyry’s Isagoge, one of the five most general kinds of attribution: genus, species, differentia, property, and accident. It is based upon a similar classification set forth by Aristotle in the Topics (a, iv–viii), which has “definition,” however, in place of “species.”

Aristotle treated only statements of the form “A is B,” in which subject and predicate are both universal. He noted that in every true statement of this type the predicate either is convertible with the subject (i.e., “B is A” follows from “A is B”) or else it is not. If the predicate is convertible and states its essence, then it is the definition of the subject; whereas if it is convertible but does not state the essence, it is a property of the subject. On the other hand, if the predicate is not convertible with the subject but is part of the definition, it is the genus or differentia of the subject, for a definition always consists of genus and differentia. Finally, if the predicate is not convertible and is not part of the definition, it is an accident of the subject.

Some Aristotelian examples may be briefly mentioned. In the true statement “Man is a rational animal,” the predicate is convertible with the subject and states its essence; therefore, “rational animal” is the definition of man. The statements “Man is an animal” and “Man is rational,” while true, are not convertible; their predicate terms, however, are parts of the definition and hence are the genus and differentia of man. On the other hand, the statement “Man is capable of learning grammar” is true and convertible; but “capable of learning grammar” does not state the essence of man and is therefore a property of man. The true statement “Man is featherless” offers an example of an accident. Its predicate is not convertible with its subject, nor is it part of the definition; accordingly, it expresses only an accidental characteristic of man.

Porphyry gave the following examples of predicable relationships in which the subject is “man”: of genus, animal; of differentia, rational; of property, risible; and of accident, white.

deduction

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Aristotle, Greek Philosopher, by Joos Ghent (Justus van Ghent) and Pedro Berruguete; in the Louvre, Paris.

deduction

reason

Also known as: deductive inference, deductive reasoning

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Last Updated: May 31, 2025 • Article History

Key People:: John Neville Keynes

Related Topics:: logic; reason; transcendental argument; inference

On the Web:: College of DuPage Digital Press - Exploring Communication in the Real World - Deductive Reasoning (May 31, 2025)

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deduction, in logic, a rigorous proof, or derivation, of one statement (the conclusion) from one or more statements (the premises)—i.e., a chain of statements, each of which is either a premise or a consequence of a statement occurring earlier in the proof. This usage is a generalization of what the Greek philosopher Aristotle called the syllogism, but a syllogism is now recognized as merely a special case of a deduction. Also, the traditional view that deduction proceeds “from the general to the specific” or “from the universal to the particular” has been abandoned as incorrect by most logicians. Some experts regard all valid inference as deductive in form and, for this and other reasons, reject the supposed contrast between deduction and induction. See also axiomatic method; formal system; inference.

This article was most recently revised and updated by Brian Duignan.