Which rule requires that the middle term be distributed at least once?

• A. Rule I: There must be three terms
• B. Rule II: The middle term must not occur in the conclusion
• C. Rule III: If a term is distributed in the conclusion, then it must be distributed in the premises
• D. Rule IV: The middle term must be distributed at least once

Answer: (D)

Distribution in categorical syllogisms means a term is distributed when the proposition about it claims something of all of its members. The middle term is the link between the two outer terms and appears in both premises. It must be distributed at least once in the premises so that it truly represents all of its class and can properly connect the outer terms. If the middle term isn’t distributed anywhere, you can’t guarantee a real connection between the outer terms, which leads to the common undistributed middle fallacy. By requiring distribution of the middle term in at least one premise, the argument’s link between the outer terms is solid, allowing a valid conclusion to follow.