Logical reasoning PrepTest 158 · Section 3 · Question 21

This answer choice might strengthen the argument if true — it refutes the premise supporting the light-absorbing explanation-favoring scientists' hypothesis — but it isn't a necessary premise. To draw the conclusion that we have no reason to believe in the light-absorbing explanation, we don't need to establish that the light-absorbing explanation can't even explain star dimness. We just need to show that the alternative explanation is enough, by itself, to explain star dimness.

The contrapositive of this answer choice would read, "If the hypothesis of an invisible light-absorbing medium in outer space does not adequately explain the low visibility of other star systems from Earth, then it is not correct." That answer choice would strengthen this professor's conclusion if the professor established that the light-absorbing explanation didn't explain star dimness. However, the professor never said that. The professor just claimed that there's already the theory-of-relativity explanation that can explain star dimness. As on (A), we don't need to establish that the light-absorbing explanation can't explain star dimness. We just need to show that the theory-of-relativity explanation is enough, by itself, to explain star dimness.

The inverse of this answer choice ("A hypothesis is likely to be incorrect if there's an existing theory that already adequately accounts for the phenomenon") would strengthen this argument. It would show that there's no reason to believe the light-absorbing explanation since the existing theory of relativity already accounts for star dimness. But as written, (C) doesn't help strengthen the argument. We're looking for premises that would help support the conclusion that there's no evidence for the light-absorbing explanation. So we need rules for what makes a hypothesis incorrect, not correct.

This answer choice is irrelevant. Correctness does not depend upon public or even scientific acceptance. The Earth revolved around the sun long before most scientists accepted that theory. So whether or not scientists accept the theory of relativity does not affect whether the theory of relativity fully explains star dimness.

Argument or Facts:
Argument

Question Type:
Strengthen with a Necessary Premise

Stimulus Summary:
Some people theorize space contains an invisible, "light-absorbing medium" which would explain why stars look dim when viewed from Earth. But the general theory of relativity has already explained the dim stars, so there's no reason to believe there's an invisible light-absorbing medium in space.

Strategy Overview:

1. Argument or facts? Always argument, so identify premises and conclusions of argument
2. Anticipate why the premises are not enough to show that the conclusion is true
3. Use anticipations to select the answer choice that:
   1. Strengthens the conclusion of the argument, and
   2. Is necessary for the conclusion of the argument
      1. Can use "negation test" to double-check: if this answer choice isn't true, does the conclusion become untrue as well? If so, it's necessary for the conclusion of the argument, and it is the correct answer

Answer Anticipation:
On Strengthen with a Necessary Premise questions, it's our job to find the answer choice that: (a) strengthens the argument's conclusion and (b) must be true to draw the conclusion. Finding an answer choice that matches both criteria is, at first, very challenging for many test-takers.

But with much practice, experienced LSATers develop a reliable strategy. All Strengthen with a Necessary Premise questions feature flawed arguments, and it's much easier to find the correct answer after identifying why the argument is flawed. After all, fixing that flaw will strengthen the argument, and showing that the flaw is fixable is necessary to draw the conclusion. (For most Strengthen with a Necessary Premise questions, the flaw involves introducing a new, important idea in the conclusion, creating a gap between the premises and conclusion. That is not the case here, however.)

So what's the flaw here? The first thing that may come to mind is that the professor seems to suggest that the only reason to believe in this theoretical light-absorbing medium is its ability to explain other stars' low visibility from Earth. Perhaps there are other reasons to believe in the light-absorbing medium. As it happens, the answer choices don't include that easy anticipation (probably because it's too late in the section to make things that simple).

So, what other flaws does this argument possess? The only support for the conclusion is that the general theory of relativity has already explained star dimness. (If these astrophysical terms are throwing you off, don't worry. In the magical world of the LSAT, the theory of relativity can be anything you want it to be.) To put this more plainly, the professor assumes that just because one theory explains star dimness, we have no reason to believe in the other theory.

Arguments on the LSAT do this sort of thing a lot. They assume that there can only be one explanation. They forget that phenomena can sometimes have multiple explanations or that multiple explanations can together account for a phenomenon. This, by the way, can be considered an exclusivity flaw — it assumes that the theory-of-relativity explanation and light-absorbing-medium explanation are mutually exclusive. (Strong language in the conclusion, such as "no reason to believe" here, is a strong indication that the argument commits an exclusivity flaw.) The correct answer will likely address this exclusivity issue, perhaps by showing that the theory-of-relativity explanation and the light-absorbing-medium explanation are mutually exclusive.

Finally, we can double-check answer choices with the negation test if we need to. To apply the negation test, we can simply ask, "If this answer choice weren't true, does the conclusion become untrue as well?" If so, that answer choice is necessary for the argument's conclusion and it is correct. If the conclusion could still be true, then that answer choice is wrong.

Answer Choice Explanation:
This precisely matches our anticipation that the correct answer would show that the theory-of-relativity explanation and the light-absorbing-medium explanation are mutually exclusive. This answer choice states that the theory-of-relativity explanation doesn't rely on the light-absorbing-medium explanation. This strengthens the argument, as it shows that the theory-of-relativity explanation can stand on its own — it doesn't need a light-absorbing medium to explain star dimness.

Moreover, the negation test will confirm that this answer choice is necessary to the argument. Suppose that the general theory of relativity depended on the existence of an invisible light-absorbing medium in outer space. In that case, is the conclusion now untrue? Yep. The professor's belief in the theory of relativity's ability to explain star dimness would be evidence in favor of the existence of a light-absorbing medium, not evidence against its existence. Therefore, this answer choice is necessary to the argument and is correct.

Key Takeaways:
The LSAT likes to intimidate you by using advanced scientific concepts in fairly simple logical arguments. Paying attention to things like the strength of language in the argument's conclusion ("no reason to believe") can help us elide the tricky subject matter and see the repetitive logical flaws that underly the arguments.