The claim (muddaʿā) is the proposition to be established; its contradictory has the opposite truth value in every case. They cannot both be true or both be false.
The contradictory of “Every human is an animal” is “Some human is not an animal,” not “No human is an animal.” The former correctly denies the universal assertion.
Reductio must begin with the genuine contradictory. Refuting a different, stronger or weaker statement may leave the original claim unproved.
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Qiyās al-Khalf: Compound Reductio, Impossibility and Proof of the Claim
The claim (muddaʿā) is the proposition to be established; its contradictory has the opposite truth value in every case. They cannot both be true or both be false.
The contradictory of “Every human is an animal” is “Some human is not an animal,” not “No human is an animal.” The former correctly denies the universal assertion.
Reductio must begin with the genuine contradictory. Refuting a different, stronger or weaker statement may leave the original claim unproved.