Claim: Every square has four sides.
Contradictory: Some square does not have four sides.
By definition, a square is a four-sided figure with equal sides and right angles. Assuming a square that does not have four sides conflicts with the very definition used to identify it as a square.
The assumed contradictory is therefore rejected. Here having four sides belongs to the definition of the square, not to a merely observed regularity.
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Qiyās al-Khalf: Compound Reductio, Impossibility and Proof of the Claim
Claim: Every square has four sides.
Contradictory: Some square does not have four sides.
By definition, a square is a four-sided figure with equal sides and right angles. Assuming a square that does not have four sides conflicts with the very definition used to identify it as a square.
The assumed contradictory is therefore rejected. Here having four sides belongs to the definition of the square, not to a merely observed regularity.