Aristotelian logic

Aristotle recognised 4 kinds of quantified sentences, each of which contain a subject and a predicate:

• Universal affirmative: Every S is a P.
• Universal negative: No S is a P.
• Particular affirmative: Some S is a P.
• Particular negative: Not every S is a P.

There are various ways to combine such sentences into syllogisms, both valid and invalid. In Mediaeval times, students of Aristotelian logic classified every possibility and gave them a name. For example, the Barbara syllogism is as follows:

• Every X is a Y.
• Every Y is a Z.
• Therefore, every X is a Z.

Aristotle also recognised the various immediate entailments that each type of sentence has. For example, the truth of a universal affirmative entails the truth of the corresponding particular affirmative, and the falsity of the corresponding universal negative and particular negative. The square of opposition lists all these logical entailments.

Famously, Aristotelian logic runs into trouble when one or more of the terms involved is empty (has no members). For example, under Aristotelian logic, "all trespassers will be prosecuted" implies the existence of at least one trespasser.

See also: logic