Sometimes a premise is false and the inference is invalid as well. The argument is then defective both materially and formally.
Example:
Every bird is a stone.
Every fish lives in water.
Therefore every fish is a bird.
The first premise is false, and the two premises do not even supply an appropriate common middle term. Finding one defect does not remove the other.
A complete evaluation must therefore check the truth of the premises and the validity of the inference separately.
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Sophistical Syllogism: Illusory Premises, Deceptive Resemblances and Fallacies
Sometimes a premise is false and the inference is invalid as well. The argument is then defective both materially and formally.
Example:
Every bird is a stone.
Every fish lives in water.
Therefore every fish is a bird.
The first premise is false, and the two premises do not even supply an appropriate common middle term. Finding one defect does not remove the other.
A complete evaluation must therefore check the truth of the premises and the validity of the inference separately.