If we converted “All humans are animals” into “All animals are humans,” the statement would remain universal but become false. The predicate animal is broader than the subject human.
Original: every member of the human class is an animal.
Invalid reversal: every animal is human.
Valid converse: some animals are human.
The broader class has members outside the narrower one. Hence universal affirmation must be restricted to particular affirmation during conversion.
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Simple Conversion: Definition, Categorical Rules and Reductio
If we converted “All humans are animals” into “All animals are humans,” the statement would remain universal but become false. The predicate animal is broader than the subject human.
Original: every member of the human class is an animal.
Invalid reversal: every animal is human.
Valid converse: some animals are human.
The broader class has members outside the narrower one. Hence universal affirmation must be restricted to particular affirmation during conversion.