Original: Some birds are not birds of prey.
Take a bird that is not predatory. It is a non-predatory thing but is not a nonbird, because it is a bird.
Result: Some non-predatory things are not nonbirds.
The same existing instance witnesses the new proposition, explaining why a particular negative contraposes to a particular negative.
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Contraposition: Classical Rules, Validity and the Later Convention
Original: Some birds are not birds of prey.
Take a bird that is not predatory. It is a non-predatory thing but is not a nonbird, because it is a bird.
Result: Some non-predatory things are not nonbirds.
The same existing instance witnesses the new proposition, explaining why a particular negative contraposes to a particular negative.