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Introduction
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Arguments about groups often use words such as “all,” “some,” and “none.” A small change in scope can make a conclusion much broader than its premises.
This lesson explains the undistributed middle, illicit major, illicit minor, and the four-term fallacy. The names come from traditional logic. You do not need to memorize that vocabulary to understand the central check: which groups do the premises actually connect?
A categorical syllogism is a deduction with two premises and a conclusion about membership in groups. It uses three terms, each naming a group, to connect the statements. The examples below show what happens when that connection is incomplete or changes meaning.