Reading Excerpted by Christopher Lay

Los Angeles Pierce College

Department of History, Philosophy, and Sociology

Determining Validity for Categorical Syllogisms

(The following comes from Chapter Eight: "Patterns of Deductive Thinking" in Burton F. Porter's The Voice of Reason: Fundamentals of Critical Thinking, Oxford University Press, 2002.) 

A subject or a predicate is called a "middle term" "because it appears twice in the premises."

A subject or a predicate is distributed if it covers every member of the class (subjects and predicates "are distributed, [if] they cover every member of the class." 

1) "At least one of the premises must be affirmative."

2) "If a premise is negative then the conclusion must also be negative, and if the conclusion is negative then a premise must be negative."

3) "The middle term must be distributed at least once."

4) "Any [subject or predicate] distributed in the conclusion must also be distributed [somewhere in] a premise."