Originally posted by Stoic
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Two propositions are contradictory iff they cannot both be true and they cannot both be false.
Two propositions are contraries iff they cannot both be true but can both be false.
Two propositions are subcontraries iff they cannot both be false but can both be true.
A proposition is a subaltern of another iff it must be true if its superaltern is true, and the superaltern must be false if the subaltern is false.
Two propositions are contraries iff they cannot both be true but can both be false.
Two propositions are subcontraries iff they cannot both be false but can both be true.
A proposition is a subaltern of another iff it must be true if its superaltern is true, and the superaltern must be false if the subaltern is false.
PNC applies to the first. PEM applies to 1 always and to 2 and 3 when the disjunction is exclusive.
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