subcontrary

logic

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July 14, 2026

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In traditional logic, subcontrary propositions are a pair of statements that share the same subject and predicate. These propositions have a specific relationship: while both can be true, they cannot both be false. This concept is often illustrated using the square of opposition, a diagram that shows the relationships between different types of categorical propositions. For example, "Some S is P" and "Some S is not P" are subcontrary propositions. This logical relationship was studied by Aristotle and is a fundamental concept in understanding logical inference.12345

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history of logic
Categorical forms
…although they were later called subcontraries. Subcontraries cannot be false together, although, as Aristotle remarked, they may be true together. The same holds for indefinite affirmatives and negatives, construed as equivalent to the corresponding particular forms. Note that if a universal proposition (affirmative or negative) is true, its contradictory is…
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…although they were later called subcontraries. Subcontraries cannot be false together, although, as Aristotle remarked, they may be true together. The same holds for indefinite affirmatives and negatives, construed as equivalent to the corresponding particular forms. Note that if a universal proposition (affirmative or negative) is true, its contradictory is…
Read More