The subject is what the proposition is about. Both propositions must concern the same subject.
Example 1: Zayd is learned.
Example 2: Amr is not learned.
Both may be true because they concern different people. Replace Amr with Zayd and, provided the other respects are identical, direct contradiction becomes possible.
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Identity of subject
Contradiction and the Eight Conditions
The subject is what the proposition is about. Both propositions must concern the same subject.
Example 1: Zayd is learned.
Example 2: Amr is not learned.
Both may be true because they concern different people. Replace Amr with Zayd and, provided the other respects are identical, direct contradiction becomes possible.