Let the original subject be S and predicate P. The earlier form is:
Original subject S → new predicate non-S.
Original predicate P → new subject non-P.
Example:
Original: Every human is an animal.
Contrapositive: Every nonanimal is nonhuman.
The pattern alone does not guarantee a valid inference for every quantity. The four categorical cases must be checked separately.
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Contraposition: Classical Rules, Validity and the Later Convention
Let the original subject be S and predicate P. The earlier form is:
Original subject S → new predicate non-S.
Original predicate P → new subject non-P.
Example:
Original: Every human is an animal.
Contrapositive: Every nonanimal is nonhuman.
The pattern alone does not guarantee a valid inference for every quantity. The four categorical cases must be checked separately.