In “Some animals are not humans,” animals form a wider class while humans form a narrower subclass. Horses and birds make the original true.
The reversal “Some humans are not animals” demands a human outside the wider animal class. But every human belongs to that class.
Thus switching terms in a particular negative does not preserve exclusion in the way it does for two completely disjoint classes.
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Simple Conversion: Definition, Categorical Rules and Reductio
In “Some animals are not humans,” animals form a wider class while humans form a narrower subclass. Horses and birds make the original true.
The reversal “Some humans are not animals” demands a human outside the wider animal class. But every human belongs to that class.
Thus switching terms in a particular negative does not preserve exclusion in the way it does for two completely disjoint classes.