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Complete Question Explanation

Must Be True—SN. The correct answer choice is (E)

The stimulus contains a fact set without a conclusion, but the combination of the first two premises yields a conclusion that is then stated in the correct answer. Let us review the stimulus sentence by sentence.

The stimulus opens with a conditional statement that can be diagrammed as:
  • MS ..... :arrow: ..... MKT
Note that the necessary indicator “only” modifies “marketplaces” and thus “marketplaces is the necessary condition. The next sentence states that Mesopotamian cities of the fourth century B.C. did not have marketplaces, which is the same as indicating that these particular cities did not meet the necessary condition stated in the first sentence:
  • Premise 2: ..... MKTMesopotamian
The combination of these two statements immediately yields the inference that Mesopotamian cities of the fourth century B.C. did not have monetary systems. As this inference is restated in answer choice (E), (E) is correct.

Structurally, the two premises in the first two sentences add together to create a conclusion:

  • Premise 1: ..... MS ..... :arrow: ..... MKT

    Premise 2: ..... MKTMesopotamian

    Conclusion/Answer (E): ..... MSMesopotamian
The last sentence introduces an issue designed to distract you from the inference discussed above. According to the last sentence, Greek cities of that period did have marketplaces, and according to further information provided, since money was traded there some type of monetary system existed.

The question stem requires you to find an answer choice that must be true, and based on the combination of a fact set with conditional reasoning, you should expect some type of contrapositive.

Answer choice (A): This answer choice can be eliminated because of the word “only.” Although the last sentence of the stimulus indicates that money (and thus a monetary system) was present in Greek cities of the fourth century B.C. there is no indication that Greek cities were the “only” such cities to have a monetary system.

Answer choice (B): The stimulus discusses a conditional relationship wherein the presence of a monetary system means that a marketplace must be present. This answer makes a claim about the origins of marketplaces, specifically that monetary systems cause marketplaces to arise. The stimulus contains no information that would support this statement.

Answer choice (C): The stimulus discusses both the Greeks and Mesopotamians, and also states that the Mesopotamians engaged in trade, but the stimulus does not state that the Greeks and Mesopotamians traded with each other.

Answer choice (D): This answer choice discusses an occurrence “after the fourth century B.C.,” but the stimulus was confined to discussing events “through the fourth century B.C.” Hence this answer choice—like the prior three answers—fails the Fact Test and is incorrect.

Answer choice (E): This is the correct answer choice. As discussed above, this answer choice completes the contrapositive begun in the first two sentences.
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Please help me better understand how to do Conditional Reasoning Questions with two premises. I am stil confused despite the lesson 2 explanation.


How do I do this with 2 premises.
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 Dave Killoran
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Hey B,

Thanks for the question! This is a great question that shows one classic way the LSAT uses basic conditional reasoning in a Must Be True question. So, let's break it down.

  • The first sentence is a premise, and it contains a conditional statement:

    ..... Premise: ..... Monetary System :arrow: Marketplace.

    The second sentence indicates that one of the conditions, a marketplace, never happened in the fourth century B.C., Mesopotamian cities. This is another premise:

    ..... Premise: ..... Marketplace 4thMeso

    So, when you reach this point, you should immediately try to connect those two elements. why? Because the necessary condition in the first premise involves marketplaces, and the second sentence/premise states that this necessary condition did not occur in fourth century B.C. Mesopotamian cities. Thus, the combination of the two enacts a contrapositive, and allows us to draw a conclusion, namely that fourth century B.C. Mesopotamian cities did not have monetary systems:

    ..... Conclusion: ..... Monetary System 4thMeso
The question stem is a Must Be True, and since we know that the conclusion we drew above follows, we should immediately seek it in the answer choices. and a review of those answers shows that answer choice (E) is the same as our conclusion.

If we were to look at what happened here in an abstract sense, it looks like this argument form:

  • Premise: A :arrow: B
    Premise: B

    Conclusion: A
In this question, they removed the conclusion form the stimulus, and used it as the correct answer. But, they could manipulate this in other ways. for example, they could drop one of the premises, and then ask an assumption question. then the answer would be the missing premise. Or they could take the whole argument, ask a Parallel Reasoning question, and then supply an answer with an identical form.

Not every LSAT question has such a standard structure, but in Lesson 2 we want to look at ideas that will help us explore other, more advanced ideas on the test. This question is an awesome example of a question that can really help you see how the test makers think, and how formulaic they can be at times.

Please let me know if that helps. Thanks!
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Im a little confused about the distinction between the conditional logic used in the stimulus and the causal reasoning used in answer choice and why its incorrect. The stimulus says:

Historically, monetary systems have developed only in population centers with market places.
Obviously conditional... MS :arrow: MKP

B.) The development of monetary systems has historically led to the development of marketplaces.

These seem very similar although answer choice B doesn't seem to use language strong enough to draw a conditional relationship. We have: "monetary systems have developed" vs "The development of monetary systems has".

Is the difference that in the stimulus we first have marketplaces which lead to the development of monetary systems and in B we have monetary systems first which lead to marketplaces? Is the order of events what is distinguishing between correctness vs incorrectness? Or would I still be wrong if B said "The development of marketplaces has historically led to the development of monetary systems." This is a tricky question.
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 Beatrice Brown
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Hi Andwer! This is a great question that gets at the difference between causal and conditional language.

What signifies the conditional reasoning in the part of the stimulus you refer to is the word "only," which indicates the necessary condition (so if there are monetary systems, then this is a population center with a marketplace).

Answer choice (B), however, only contains causal language. "Has historically led" is another way of saying that the development of monetary systems causes the development of marketplaces.

But just because there is a conditional relationship between two entities doesn't mean that they are also causally related! For a conditional relationship, we don't necessarily know anything about the ordering of the two variables. The sufficient condition may come first, but the necessary condition can also occur first. Take the following example:

Student does well on exam :arrow: student studies
This conditional statement tells us that if a student does well on an exam, then the student studied. Notice that the order of the sufficient and necessary conditions doesn't actually tell us the proper order in which they occurred. Presumably, the necessary condition (a student studies) actually happens before the sufficient condition (a student does well on the exam).

Similarly, the conditional statement in the stimulus doesn't tell us the order in which the two events occurred. It could be the case that the monetary systems arose first, or it could be the case that there was a population center with a marketplace first.

So to summarize, the ordering of the events in answer choice (B) does not matter, as each ordering would be incorrect. From conditional statements, we cannot infer that there is a causal relationship between the sufficient and necessary conditions. Answer choice (B) requires us to infer a causal relationship, so it cannot be correct.


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