The commentary on printed page 71 also discusses modal converses. A proposition qualified by necessity, duration, possibility, or a particular time cannot always be transformed like a bare categorical proposition.
“Necessarily” is not equivalent to “sometimes.” Mishandling the modality may destroy truth preservation.
The text points to larger treatises for the full rules. The key lesson here is that a valid contraposition must preserve truth with its modal qualifications.
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Contraposition: Classical Rules, Validity and the Later Convention
The commentary on printed page 71 also discusses modal converses. A proposition qualified by necessity, duration, possibility, or a particular time cannot always be transformed like a bare categorical proposition.
“Necessarily” is not equivalent to “sometimes.” Mishandling the modality may destroy truth preservation.
The text points to larger treatises for the full rules. The key lesson here is that a valid contraposition must preserve truth with its modal qualifications.