“No humans are horses” does not entail “No nonhorses are humans.”
A human is both a nonhorse and a human, so the proposed universal denial is false.
This shows why the quantity cannot simply remain universal. In the book's traditional scheme, the valid result is particular negative.
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Contraposition: Classical Rules, Validity and the Later Convention
“No humans are horses” does not entail “No nonhorses are humans.”
A human is both a nonhorse and a human, so the proposed universal denial is false.
This shows why the quantity cannot simply remain universal. In the book's traditional scheme, the valid result is particular negative.