Simple conversion is valid for which forms?

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

Answer: (A)

Simple conversion in categorical logic is the idea of interchanging the subject and predicate terms in a proposition, and it preserves truth only for universal statements. When a proposition says all S are P, its simple conversion yields all P are S; when a proposition says no S are P, its simple conversion yields no P are S. These conversions are valid for universal forms (the ones that say “all” or “no” about the classes).

For the particular forms—some S are P or some S are not P—the same swap does not reliably preserve truth, so they do not have a valid simple conversion.

So the forms that support simple conversion are the universal ones: universal affirmative and universal negative.