Two checks are central to reductio:
1. Entailment: does the contradictory really lead to the alleged consequence?
2. Impossibility: is that consequence genuinely impossible, rather than merely unexpected?
“If the road is blocked we arrive late; we arrived late, so the road was blocked” is an invalid inference. Many other causes could explain lateness, and arriving late is not impossible.
Both failure of entailment and failure to establish an impossibility would undermine a purported reductio.
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Qiyās al-Khalf: Compound Reductio, Impossibility and Proof of the Claim
Two checks are central to reductio:
1. Entailment: does the contradictory really lead to the alleged consequence?
2. Impossibility: is that consequence genuinely impossible, rather than merely unexpected?
“If the road is blocked we arrive late; we arrived late, so the road was blocked” is an invalid inference. Many other causes could explain lateness, and arriving late is not impossible.
Both failure of entailment and failure to establish an impossibility would undermine a purported reductio.