Valid argument:
All sparrows are birds.
No fish is a bird.
Therefore no sparrow is a fish.
Invalid arrangement:
All sparrows are birds.
Some pets are not birds.
Therefore no sparrow is a pet — this does not follow.
The second major premise excludes only some pets from birds. Other pets might themselves be birds. It lacks the universality required to rule out the entire major class.
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The Second Figure: Conditions of Validity and Four Productive Moods
Valid argument:
All sparrows are birds.
No fish is a bird.
Therefore no sparrow is a fish.
Invalid arrangement:
All sparrows are birds.
Some pets are not birds.
Therefore no sparrow is a pet — this does not follow.
The second major premise excludes only some pets from birds. Other pets might themselves be birds. It lacks the universality required to rule out the entire major class.