Discussion
Fact 1: Humans are mammals.
Fact 2: Some humans are engineers.
Fact 3: Engineers build houses.
If the above statements are facts, which of the following can be logically inferred?
I. All mammals build houses.
II. Engineers are mammals.
III. Some humans are not engineers.
Answer: B
The answer is III only.
Statement I does not follow: the facts say humans are mammals and engineers build houses, but nothing makes every mammal an engineer or a builder. Statement II does not follow either: some humans are engineers, but the facts never say that every engineer is human, so engineers as a class need not be mammals.
Statement III is taken to follow from Fact 2 read in its everyday sense: saying that some humans are engineers implies that not all of them are, so some humans are not engineers. (In strict syllogistic logic 'some' does not exclude 'all', which is why this question is sometimes argued; the key adopts the everyday reading.) The options containing I or II are therefore wrong.
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