LSAT 90 – Section 2 – Question 19

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Type Tags Answer
Curve Question
PT90 S2 Q19
Necessary assumption +NA
+Hardest 146.031 +SubsectionMedium

This is an NA question.

This is a very difficult argument to understand substantively. Let’s first approach this question by breaking it up into its parts. Conveniently, it progresses in order from minor premise to major premise to main conclusion. The two conclusion indicator words “thus” and “therefore” help us recognize this.

There may be a way to get to the right answer without fully understanding the argument substantively, but I wouldn’t bank on it. You’ll have to pick up in the main conclusion the new reference to “other characteristics.” What other characteristics? We’ve only talked about a star’s brightness. So the “other characteristics,” whatever they may be, had better be determinable.

If you picked up on that, then you’re probably down to Answer Choice (B) and Correct Answer Choice (D). The problem with (B) is that it’s too specific and hence unnecessary. We don’t need “differences in the elements each is burning” to be detectable from Earth. We just need some difference to be detectable from Earth, like how (D) has it.

Okay, but let’s back up and try to actually understand this argument.

The minor premise tells us that the distance between Earth and a distant galaxy overwhelms the distance between Earth and any object in that distant galaxy. Imagine you’re on a tiny island and we’re trying to measure the distance between you and two people on a different faraway tiny island. They’re not equidistant from you. One of them is actually closer, because, say, one is standing at the shore and the other is standing on the other side of the island. But the first sentence is saying that the distance between the two islands is so vast that that’s the only thing that really matters. The islands themselves are so small and the vast space between the islands so large that it hardly matters where anyone is standing on their islands. That difference is so small that it’s negligible.

So it follows that if two stars are in the same distant galaxy (two people on the same distant island), then the distance between those two stars to Earth will in effect be the same (because whatever difference is negligible). But if we still observe a difference in their brightness, that difference can’t be due to their (negligibly different) distance. It must be due to their actual brightness as in how bright they’re actually burning and not just how bright they appear to be.

Now the argument reaches for its main conclusion. It concludes that we should be able to figure out the correlation between two stars' relative actual brightness and the two stars’ other characteristics. We see “other characteristics” appear out of nowhere. We did talk about brightness and how if two stars are in the same distant galaxy, we can in effect treat them as being equidistant from Earth in terms of their brightness. But in order to correlate their brightness with “other characteristics,” we first have to be able to detect and measure those “other characteristics.”

Again, this is what Correct Answer Choice (D) picks up on. It says that there are stars in distant galaxies that have characteristics, other than brightness, discernible from Earth. This must be true. If this were false, that would mean that we could discern only a distant star’s brightness and nothing else. If that’s true, then we would be unable to correlate brightness with anything else.

Note that (D) doesn’t care to specify what the “characteristics” are. That’s good because the conclusion didn’t care to specify either. This is what makes Answer Choice (B) unnecessary. While (B) would certainly strengthen the argument, we don’t need the differences in the elements each is burning to be detectable from Earth. Who knows what “other characteristics” the conclusion wanted to correlate with brightness? Maybe it’s the elements that each is burning. Maybe it’s the color of the stars, their size or mass, or their temperature. It could be any of those characteristics that need to be detectable from Earth.

Answer Choice (A) says that if two stars are in two different galaxies... Eh, we can stop. We don’t care. The argument cares about two stars in the same distant galaxy. (A) talks about two stars in two different (near or far?) galaxies. Whatever else (A) is about to say for these two stars will be irrelevant so you should move on to the next answer.

But for review, we can negate this and see that it has no impact on the argument. So what if it is possible to determine their distances from Earth? That doesn’t hurt the argument.

Answer Choice (C) is similarly irrelevant for it talks about stars in our own galaxy. Again, for the same reasons as in (A), we don’t care. We should move on.

If we negate (C), that’s just fine for the argument. It would be absolutely bizarre if all the stars in the Milky Way were all approximately the same distance from Earth but it wouldn’t affect the argument.

Answer Choice (E) can also be similarly eliminated as soon as you see that it’s talking about stars that are significantly different in distance from Earth. The argument contemplated two stars that are not significantly different in distance from Earth. That’s what the sub-argument established by placing the two stars in distant galaxies to begin with. We should move on.

(E) goes on to say that if there are significant differences in how far away two stars are from Earth, then those stars will differ significantly in apparent brightness. This isn’t required. It’s fine for the argument if two stars of significant difference in distance from Earth are about the same in brightness. The argument contemplated two stars of insignificant difference in distance but significant difference in brightness. After all, a major assumption of the argument is that distance is only one factor in determining a star’s brightness.

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