Recall the rules of simple conversion:
Universal affirmative → particular affirmative, with existence.
Universal negative → universal negative.
Particular affirmative → particular affirmative.
Particular negative → no generally valid simple converse.
Thus “converting the premises” is not permission to reverse every sentence at will. Directly converting a particular negative can turn an apparently convincing reduction into an invalid proof.
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The Fourth Figure: Productive Conditions, Moods and Proofs
Recall the rules of simple conversion:
Universal affirmative → particular affirmative, with existence.
Universal negative → universal negative.
Particular affirmative → particular affirmative.
Particular negative → no generally valid simple converse.
Thus “converting the premises” is not permission to reverse every sentence at will. Directly converting a particular negative can turn an apparently convincing reduction into an invalid proof.