Consider:
All humans are living beings.
All birds are living beings.
“No human is a bird” may be factually true, but it does not logically follow from these two premises, since both are affirmative. Their shared predicate does not establish exclusion between the subjects.
A valid inference differs from merely arriving at a true statement. Validity asks whether the conclusion could be false while all premises remain true.
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The Second Figure: Conditions of Validity and Four Productive Moods
Consider:
All humans are living beings.
All birds are living beings.
“No human is a bird” may be factually true, but it does not logically follow from these two premises, since both are affirmative. Their shared predicate does not establish exclusion between the subjects.
A valid inference differs from merely arriving at a true statement. Validity asks whether the conclusion could be false while all premises remain true.