Syllogistic Fallacy: Undistributed Middle

This is a fallacy involving three sets: A, B and C. It is the assumption that because all A are members of C, and all B are members of C, that all A are B.

Examples

Consider:

• A. All cherry tomatoes are red things.
• B. All cherries are red things.
• C. Therefore all cherry tomatoes are cherries.

Again:

• A. Fred is an atheist who lives in California.
• B. Ted is also an atheist living in California.
• C. Therefore Fed and Ted are actually the same person.

Weaknessess

There is usually no reason to believe that two members of a group are identical or related, nor two subgroups of a group.