LSAT 90 - Section 2 - Question 14

This is a Flaw/Descriptive Weakening question.

The stimulus starts with distinguishing categories of celestial objects. Stars versus planets. Stars generate light. Planets do not generate light, they only reflect it. Next, the argument defines a domain: this galaxy. It says that under this domain (in this galaxy), there are celestial objects that are neither stars nor planets, but it doesn’t give them names. (Fortunately, we already know their names: moons, asteroids, black holes, etc.) The argument then concludes that these nameless objects (i.e., these celestial objects that are neither planets nor stars) generate light.

Wait, what? The premises don’t support the conclusion at all! The premises establish that planets don’t generate light. It also establishes that there are other non-star, non-planet, nameless objects. But it says nothing about whether those nameless objects generate light. Where did that even come from?

As it turns out, Correct Answer Choice (E) reveals that the argument made the oldest mistake in the book: sufficiency-necessity. But this was such an unforced and bizarre error that it very successfully obscured itself. I certainly didn’t see this coming because I never would have expected the argument to confuse “planets don’t generate light (p→/gl)” with “only planets don’t generate light (/gl→p).” Had the premises established that “only planets don’t generate light” and there are non-star and non-planet objects, then those nameless objects would generate light. But the argument in fact only established that “planets don’t generate light.” It failed to consider the possibility that planets are not the only celestial objects that don’t generate light. For example, moons and asteroids also don’t generate light.

Answer Choice (A) says the argument failed to consider the possibility that this galaxy contains no celestial objects that reflect light other than planets. This is such an irrelevant consideration that I guess the argument failed to consider it. But that’s not why the argument’s reasoning is flawed. The conclusion is claiming that the nameless objects generate light. (A) is concerned with whether these nameless objects reflect light. Who cares if they reflect light or not?

Answer Choice (B) says the argument failed to consider the possibility that celestial objects in this galaxy comprise only a minute percentage of all the celestial objects in the universe. Similar to (A), this is such an irrelevant consideration that I guess the argument failed to consider it. But that’s not why the argument’s reasoning is flawed. The argument already established its domain: this galaxy. The conclusion does not exceed the bounds of that domain.

Answer Choice (C) says the argument failed to consider the possibility that celestial objects in this galaxy are not the only celestial objects that generate light. Similar to (A) and (B), this is such an irrelevant consideration that I guess the argument failed to consider it. But that’s not why the argument’s reasoning is flawed. (B) claims that there may be celestial objects in other galaxies that generate light. Okay, but who cares? The argument already established its domain: this galaxy. The conclusion does not exceed the bounds of that domain.

Answer Choice (D) says the argument failed to consider the possibility that there are numerous features that distinguish stars from planets besides the ability to generate light. Similar to (A), (B), and (C), this is such an irrelevant consideration that I guess the argument failed to consider it. But that’s not why the argument’s reasoning is flawed. It doesn’t matter what other features distinguish planets from stars. Stars tend to be bigger. Stars tend to live longer. Great. Who cares?