Which rule requires that if both premises are affirmative, the conclusion must also be affirmative?

• A. No conclusion can follow from two negative premises
• B. If both premises are affirmative, the conclusion must be affirmative
• C. If either premise is negative, the conclusion must be negative
• D. The middle term must be distributed at least once

Answer: (B)

In categorical syllogisms, the quality of the conclusion follows the quality of the premises. When both premises are affirmative, the conclusion must also be affirmative. This is seen in the standard Barbara form: All S are M and All M are P, therefore All S are P. The chain of inclusions from subject through the middle term to the predicate preserves affirmation, so the conclusion remains affirmative.

This rule is about the propagation of affirmation through a syllogism. The other statements describe different, separate constraints: two negative premises lead to no valid conclusion in standard syllogistic, the middle term must be distributed at least once is a structural requirement, and if a premise is negative, the conclusion must be negative. These are distinct rules and do not address the scenario described.