How we change what others think, feel, believe and do

Four Terms

Disciplines > Argument > Fallacies > Four Terms

Description | Discussion | Example | See also

Description

All A are B. All C are D. So All A are D.

Make two statements and make an unspoken leap that connects these statements to allow a third, conclusive statement to be made.

Example

All dogs are mammals. All fish are animals. So all dogs are animals. [true, but not proven by the first two statements]

Man is an intelligent animal. No woman is a man. Therefore no women are intelligent animals. ['man' has two different meanings]

Discussion

Syllogisms should have only three terms, with one term being the bridge between the major and minor premise that forms the conclusion. So where four terms appear in the major and minor premises, these two statements are logically disconnected and no logical conclusion may be drawn.

Sometimes it may appear that there are three terms, as in the second example above. This can still cause problems where one term actually has different meanings (equivocation) in either term. For example, 'man' can mean 'humanity' or 'male'.

Classification

Syllogistic, Assumptive