#20 - Astronomer: Earth was bombarded repeatedly by comets

[Administrator]

Complete Question Explanation

Method of Reasoning—AP. The correct answer choice is (C)

Here, the astronomer discusses a theory attempting to explain the origin of life on Earth. This is a structurally complicated argument, containing four premises, a subsidiary conclusion, and a main conclusion. The argument can be deconstructed as follows:

• Premise: Earth was bombarded repeatedly by comets and asteroids early in its history
  
  Premise: this bombardment probably sterilized the surface and prevented life from originating during this early period in Earth’s geological history
  
  Premise: Mars escaped severe bombardment
  
  Subconclusion: There could have been microbial life on Mars prior to there being such life on Earth
  Premise
  
  Premise: many meteorites originating from Mars have landed on Earth
  
  Conclusion: thus, life on Earth may have started when living microbes were carried here from Mars on a meteorite

The question stem identifies this as a Method of Reasoning—Argument Part question. More specifically, it asks you to identify the logical role played by the statement identified in the question stem and italicized above. This statement is a subconclusion, which is supported by evidence and then used to support the main conclusion.

Notice that the argument’s subconclusion has a conclusion indicator, “so,” but the main conclusion does not. This is an often used tactic in Method of Reasoning—Argument Part questions, by which your attention is focused on the subconclusion as “the” conclusion, increasing the chances that you will fail to identify the main conclusion.

Answer choice (A): This choice is incorrect because the statement “Mars escaped severe bombardment” provides support for the subconclusion.

Answer choice (B): This choice is wrong not only for the reason stated above regarding choice (A), but also because the claim that there may have been microbial life on Mars before such life appeared on the Earth, even if true, would not justify the conclusion, that life on Earth may have started when living microbes were carried to Earth from Mars on meteorites.

Answer choice (C): This is the correct answer choice. This choice is correct because it properly describes the statement’s role as a subconclusion. Further, the statement is required for the conclusion to be valid, because if the logical opposite of the statement were the case, i.e., “there could not have been microbial life on Mars prior to there being such life on Earth,” then the conclusion that such life was carried from Mars to Earth would be invalid. The addition of this language that the statement is required for the conclusion is an interesting complication that makes this question more difficult than the typical Argument Part question. You were required not only to properly identify the structure of the argument, but also the logical weight of the language.

Answer choice (D): This choice is incorrect because it overstates both the strength of the premises supporting the subconclusion and the strength of the subconclusion supporting the main conclusion. In neither case is justification present. Instead, there is simply support for each.

Answer choice (E): This choice is a classic “Half Right, Half Wrong” option that can make for an attractive incorrect answer choice. The first part of this choice is correct. The statement, in its role as a subconclusion, provides some support for the argument’s conclusion. However, the second half of the choice, which denies the statement is required to establish the conclusion, is incorrect for the reasons stated above regarding choice (C).

[mhsk]

Hi,

I am having a difficulty understanding the answer choices, especially with regard to specific wordings.

1. I am supposing that there is a difference between "some justification is provided" (in answer choice C) and "justification is provided" (in answer choice D). How does this degree of justification affect the answer choice? I am thinking that the statement stipulated in the question stem has "justification" - more than merely "some justification" -, but "some justification" also can be true because "some" can be extended to include "all." Therefore, can I correctly say that the first part of both answer choice C and D are correct? In the same vein, can I also correctly say that the first part of answer choice E - "it is a claim that provides some support for the argument's conclusion" - is also correct?

2. I am struggling with the phrases "required in order to establish the argument's main conclusion" (in answer choice A, C, E) and "ensure(s)/establish(es) the truth of the argument's main conclusion" (in answer choice B, D, E). Are both phrases same in meaning, so that one phrase replacing another phrase does not affect the meaning at all? For example, if second part of answer choice C is replaced with "and that, if true, ensures the truth of the argument's main conclusion," would answer choice C still be the correct answer? Or is "the truth" not relevant for this specific question only because the conclusion is qualified with "may (have started)" ?

I would very much appreciate if you could help me clarify these points.

Thanks!

-Rachel

[Jonathan Evans]

Hi, Rachel,

Good questions. The Administrator explanation does a good job of addressing some of your concerns, so I encourage you to review this explanation again. However, I'd also like to respond to some of your specific observations.

"Some" does not contradict "all"—as you noted—but "some" does not imply "all." In other words, "all" is strong; it's very restrictive. "All swans are white" is a stronger statement than "some swans are white." Answer choice (D) is more in the "all swans are white" category. It's powerful; it's strong (as the Administrator explanation notes). "Some" here operates as a qualifying statement. Sure, "some" justification is provided. Is justification sufficient to establish the validity of the claim provided? Who knows? "Some" qualifies and weakens answer choice (C). "Some" leaves things more open. Because (D) is so strong, it ends up overplaying its hand. We really don't have sufficient evidence to know that there "could have been microbial life on Mars." We have "some" evidence, but it's not exactly case-closed.

So, long story short, (D) is incorrect even from the outset. The initial clause of (D) is prima facie worse than that of (C).

In re your second question, no, these statements differ markedly. In fact, this distinction is very much akin to the difference between Assumption questions and Justify the Conclusion questions. Basically:

1. "required in order to establish the argument's main conclusion" this refers to a necessary condition, a prerequisite for the argument to be valid.
2. "ensure(s)/establish(es) the truth of the argument's main conclusion" this refers to a sufficient condition, a statement the truth of which would guarantee the validity of the argument.

In other words, these statements are not interchangeable. Point (1) above is not nearly as strong as point (2). This distinction is rather significant, and for further explication on this point, I recommend you review the lessons/chapters on conditional reasoning, assumption questions, and justify the conclusion questions.

I hope this helps!

[mhsk]

Hi Jonathan,

Thank you very much for your clarifications. Your explanations are very helpful! I will also make sure to review parts that you have mentioned

-Rachel

[PresidentLSAT]

Hello LSAT freinds,

This question was unreasonably difficult; though I still need some help on a few clarifications. I ruled out C because the conclusion in the stimulus wasn't a certainty- "life may have started " I thought that if the conclusion is a probability, the role of the premise we are asked to identify isn't necessary for the argument's conclusion to hold, since the argument is a probability, anyway.

I do get the if there were no life on mars, our conclusion is bs anyway, tho.

Is D wrong because if microbial life existed on mars before earth, it doesn't automatically mean that life on earth may have started when living microbes were carried from mars. Mother nature could have had some tricks up her sleeves that probably halted such evolvement.

[Rachael Wilkenfeld]

Exactly, President. Answer choice (D) is saying that the claim we are analyzing is enough to prove the conclusion true. We don't know that. Just because life could have developed on Mars before it could have developed on Earth, we can't conclude life on Earth must have been brought from microbial life on Mars. There are way too many other possibilities that aren't eliminated here for the statement to prove the conclusion true.

Hope that helps!