Which of the following propositions can be directly inferred from the statement - "All mammals are four-legged animals?
Some mammals are four-legged animals.
Concept: The statement "All mammals are four-legged animals" is a universal affirmative (A-type) proposition: All S are P, where S = mammals and P = four-legged animals. An immediate inference is a conclusion drawn directly from a single premise.
Working: By subalternation in the classical square of opposition, the truth of a universal affirmative (All S are P) guarantees the truth of the corresponding particular affirmative (Some S are P). Hence from "All mammals are four-legged animals" we may directly infer "Some mammals are four-legged animals." Both keep the same subject (mammals) and predicate (four-legged animals).
Why the other choices are wrong:
- "All animals are four-legged mammals" swaps and enlarges the terms (subject becomes all animals, predicate becomes four-legged mammals); this is not a valid conversion of an A proposition.
- "No four-legged mammals are animals" is a universal negative that contradicts the original meaning and cannot follow from it.
- "All four-legged animals are mammals" is the simple converse of the A proposition; an A proposition does not convert to another A (converse of All S are P is only Some P are S), so this is an illicit conversion.
Takeaway: From "All S are P" the only safe immediate inference of this list is the subaltern "Some S are P."
Correct answer: Some mammals are four-legged animals.
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