The textbook does not rely only on an example; it also gives an indirect proof (burhān bil-khalf). We temporarily suppose the intended converse false, then combine that supposition with the known true original and derive an impossibility.
Original: “No human is a stone.”
Proposed converse: “No stone is a human.”
If the proposed converse were false, its contradictory would be true: “Some stones are humans.” We then combine this claim with the original universal negative.
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Simple Conversion: Definition, Categorical Rules and Reductio
The textbook does not rely only on an example; it also gives an indirect proof (burhān bil-khalf). We temporarily suppose the intended converse false, then combine that supposition with the known true original and derive an impossibility.
Original: “No human is a stone.”
Proposed converse: “No stone is a human.”
If the proposed converse were false, its contradictory would be true: “Some stones are humans.” We then combine this claim with the original universal negative.