Minor: No fish are birds.
Major: All birds are living things.
Invalid conclusion: No fish are living things.
The first premise separates fish from birds. The second says birds are living things. Being outside birds does not imply being outside all living things.
Fish can be living things while both premises remain true. Therefore the stated universal negative conclusion does not follow. This illustrates the affirmative-minor requirement.
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The First Figure: Conditions of Validity and Four Productive Moods
Minor: No fish are birds.
Major: All birds are living things.
Invalid conclusion: No fish are living things.
The first premise separates fish from birds. The second says birds are living things. Being outside birds does not imply being outside all living things.
Fish can be living things while both premises remain true. Therefore the stated universal negative conclusion does not follow. This illustrates the affirmative-minor requirement.