Consider the source example:
Original: Some animals are not human.
Attempted converse: Some humans are not animals.
The attempted converse does not follow: nonhuman animals exist, but humans are animals. Thus a particular negative has no generally valid simple converse.
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A special rule: particular negatives
Simple Conversion of Propositions: Definition, Method and Rules
Consider the source example:
Original: Some animals are not human.
Attempted converse: Some humans are not animals.
The attempted converse does not follow: nonhuman animals exist, but humans are animals. Thus a particular negative has no generally valid simple converse.