Subcontraries: Which pair are subcontraries?

• A. A and E statements
• B. A and I statements
• C. I and O statements
• D. E and O statements

Answer: (C)

Subcontraries are the two propositions that share the same subject and predicate form and can both be true, but cannot both be false. The pair that fits this is the particular affirmative and the particular negative: Some S are P and Some S are not P. It’s possible for some individuals to be P and for some to not be P, so both statements can be true. But they cannot both be false, because that would force No S are P and All S are P at the same time, which is impossible for the same subject and predicate. So the subcontrary pair is the two particular propositions. (The universal forms All S are P and No S are P are contraries, not subcontraries.)