LSAT 158 - Section 3 - Question 16

This is a Must Be True question.

This question tests your ability to manipulate grammar to reveal underlying logical relationships between sets. In particular, we are dealing with sets of intersections and supersets and subsets.

The stimulus states that if a belief is based on information from a reliable source, then it is reasonable to maintain that belief. This is a standard conditional claim using the “if... then...” formulation. Let's kick the idea of “a belief” up into the domain. This will simplify the analysis. You wouldn't know at this moment to do this. You have to finish reading the stimulus.

Kicking the idea of “a belief” as the subject up into the domain, we get to talk about the properties of beliefs. And it's those properties that have a sufficiency necessity relationship. The sufficient property is “based on reliable information.” If that's true, then the necessary property is “reasonableness.”

The next claim is an intersection claim. We know this from the presence of the word “some.” Some beliefs are based on information from a reliable source and yet are neither self-evident nor grounded in observable evidence. This is where you might notice that the subject once again is belief. The sentence here is talking about a triple intersection between three different sets of properties of beliefs.

1. based on reliable information (same set as the sufficient condition in the previous sentence)
2. not self-evident
3. not grounded in observable evidence

These three sets have an intersection. But we also know from the first sentence that if based on reliable information, then reasonable. That implies “reasonable” gets to join this intersection with “not self-evident” and “not grounded in observable evidence” as well.

Using logic, first, represent the intersection of 2 and 3 as simply A. B will represent based on reliable information. C will represent reasonable. The triple intersection can now be represented as A ←s→ B. The conditional is B → C. The valid inference is A ←s→ C. Again, that’s just the triple intersection between “reasonable” and 2 and 3.

If that was confusing, consider this. If a cat is mild-mannered, then it’s domestic. Some mild-mannered cats are large and fluffy. Therefore, some domestic cats are large and fluffy. That’s analogous to the question here. Kick the subject “cats” up into the domain. The “some” premise describes a triple intersection between mild-mannered, large, and fluffy. Because mild-mannered implies domestic, we know that domestic also intersects with large and fluffy.

Hopefully that clears up the logic. Translating it back into English reveals that we have many options. “Some domestic cats are large and fluffy” is probably the most straightforward translation. But we could also say, “Among the large cats, some domestic ones are fluffy.” Or we could say, “Among the fluffy cats, some large ones are domestic.” The order of the modifiers in “some” intersections doesn’t matter. “Some” can be reversibly read.

So, there are many ways to translate the valid inference from the actual stimulus back into English. We could say, “Some beliefs that are neither self-evident nor grounded in observable evidence are nonetheless reasonable.” Or we could state this as Correct Answer Choice (E) does. Among reasonable beliefs that are not self-evident, there are some beliefs that are not grounded in observable evidence. Or even differently still. Grammar is the reason for this flexibility. As long as you know that you're just trying to express a triple intersection, you should be all set.

Answer Choice (A) says beliefs for which a person does not have observable evidence are unreasonable. This is a mishmash of the concepts in the stimulus. What must be true is that some beliefs for which a person does not have observable evidence are reasonable. Or alternatively, we could say that unreasonable beliefs must not be based on unreliable information.

Answer Choice (B) says beliefs based on information from a reliable source are self-evident. This is also a mishmash of the concepts above. We have no information to make claims about self-evident beliefs.

Answer Choice (C) says all reasonable beliefs for which a person has no observable evidence are based on information from a reliable source. This does not validly follow from the premises. But if we changed the quantifier “all” into the quantifier “some,” then it would follow validly.

Answer Choice (D) says if the belief is not grounded in observable evidence then it is not self-evident either. This also does not follow. The relationship between these two concepts is only one of intersection. We don't know if the two sets have a superset-subset relationship.