Logical ReasoningDifficulty: Hard

PT140 S2 Q25 Explanation

At one time, many astronomers

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsNecessary Assumption

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Stimulus

At one time, many astronomers assumed that Earth remains motionless while the stars revolve around it. They concluded from this that the stars were not more than a few million miles from Earth. They reasoned that if the stars were farther away, they would have to move at tremendously day and reappear in roughly the same positions each night.

What this question is testing

Necessary Assumption

Your task

Find the assumption the argument requires in order for its conclusion to hold.

Common trap

Answers that would help the argument but aren't strictly required (sufficient, not necessary).

Winning move

Negate each choice — the right one breaks the argument when negated.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
25.

Which one of the following is an assumption required by the reasoning

Answer choices

  1. Out of Scope: don't revolve8% picked this

    If the stars do not revolve around Earth, it is possible for at least some stars to be more than a

    These astronomers were assuming that the stars revolve, and their whole argument is predicated on that idea. So they weren't assuming anything about a hypothetical world in which stars don't revolve around the Earth. This answer choice is doing some sort of Fake Opposite ju-jitsu. Since we spoke about a world in which stars do revolve around Earth and it was not possible for the stars to be more than a few million miles away, they're just writing an answer choice that flips those ideas.

  2. Too Strong: all / exactly same15% picked this

    All stars move at exactly the same speed when they are

    On Necessary Assumption, we're extra-wary of extreme language, and it doesn't get much more extreme than this. The astronomers' argument and worldview wouldn't be undermined if some stars move at different speeds than others.

  3. Opposite7% picked this

    Earth does not remain motionless while the stars revolve

    This is the opposite of what the astronomers believe. They are assuming that the Earth is motionless while the stars revolve around it.

  4. Correct57% picked this

    Stars do not move at tremendously

    Why this is right

    This is the fact we need to establish to kill off the possibility of living in a world where the stars are farther than a few million miles away. If stars were farther than few million miles, stars would have to move at crazy fast speeds. [ stars do not move at crazy fast speeds ] Thus, stars not farther than few million miles

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

  5. Tricky Elimination14% picked this

    A star that is more than a million miles from Earth could reappear in roughly the

    This definitely feels like an idea we can infer. It's so weakly worded. The astronomers think it's unfeasible for stars to be 3+ million miles away because they'd have to go unrealistically quickly to make it back to roughly the same spot in the sky the next night. And the astronomers conclude that the stars are not more than a few million miles from Earth. So presumably they think within that range, it's feasible for a star to circle around back to roughly the same position. If we negate this, it'll say: if a star is more than a million miles away, a star could not reappear in the same spot Doesn't that jack up the astronomers' story? No, because the conclusion is "not more than a few million". If this negation is telling us "even at 1.1 million miles, you're already too far out to be able to feasibly zip around the Earth and arrive in the same spot, then the implication of that is that these stars must be even closer than 1 million miles! The thing is, that doesn't disagree with the author's conclusion. If we say, "I bet you Joey show up here with no more than twenty bucks", and then Joey shows up with $4, was I right or wrong? I was right. Anything $20 or less and I am right. It has to be more than $20 for me to be wrong. Similarly, concluding that the stars are "no more than a few million miles from Earth", means that you're right if stars are between 0 and 3,000,000 miles from Earth. You're only wrong if there are stars farther than 3,000,000 miles from Earth. Negating this answer isn't allowing us to argue that this author is wrong. It makes her conclusion actually seem truer than ever, since the stars seem closer than ever.

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