Logical Reasoning

PT106 · S1 · Q12 The author’s conclusion in the

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People who say that Dooney County is flat are clearly wrong.

Conclusion (clearly)

People who say Dooney County is flat are clearly wrong.

rephrase: DC is not flat.

Evidence

Farmers whose land is flat don't build terraces to prevent erosion.

But I hear DC's farms have terraces.

Evaluate

This argument tries to work by contrapositive logic. We were given this rule:

Land flat → don't build terraces to prevent erosion

The contrapositive would be a rule that allows to prove that land isn't flat:

do build terraces to prevent erosion → land not flat

The author is trying to conclude that DC's land isn't flat, so she needs to establish this trigger:

DC builds terraces to prevent erosion.

However, the premise just says that terraces exist there. It doesn't say why they were built.

Goal

We have a classic case of an Untriggered Conditional. The argument never established that DC farms are building terraces to prevent erosion, only that DC farms have a bunch of terraces.

Let's look for a Missing Idea assumption like . (Technically, the argument is also assuming that "what I heard it true". We don't know factually that DC has terraces, only through hearsay. So an answer could establish that the rumor is true.)

12.

The author's conclusion in the passage depends on the assumption that

  1. The only cause of soil

    Too Strong (the only)

    The only cause of soil erosion mentioned was water, but that doesn't mean this author ever committed to the idea that water is the only thing in the universe that causes soil erosion.

    7% picked this

  2. Correct

    There are terraces on farmland

    Why this is right

    This is what we were predicting. The argument is trying to use this rule to prove that DC isn't flat: build terraces to prevent erosion → land not flat She told us that there are terraces on DC farms, but never that they were terraces built to prevent erosion. This answer is supplying that missing trigger idea. If we negated it, it would surely weaken to say, "Hey author, sure there are lots of terraces in DC, but no terraces in DC were built to prevent soil erosion."

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

    73% picked this

  3. Terraces of the kind found

    Term Shift (shown to prevent)

    We need to know that the terraces in DC were built in order to prevent erosion, not that they have been shown to prevent erosion. The language of the conditional rule is about the intent of building the terraces, not proven results. If we negated this and said "the sort of terraces the farmers in DC built have not been shown to prevent soil erosion" it doesn't weaken the argument. The author can say, "I never said these terraces have been shown to prevent soil erosion. I'm just saying the farmers built them to prevent erosion, which indicates that their land isn't flat."

    11% picked this

  4. On flat land there is

    Too Strong (no soil erosion)

    The argument says that on flat land, soil erosion by water isn't a problem. But there still might be some soil erosion. It just might be unproblematic. Or maybe there is soil erosion of a different type (maybe there's soil erosion by rodents or by wind). Saying "soil erosion by water isn't a problem" does not mean the same thing as "there is no soil erosion".

    7% picked this

  5. The only terraces in Dooney

    Too Strong (the only)

    Just like (A), the only terraces in DC that were mentioned were on farms, but that doesn't mean the author is saying that only farms have terraces.

    2% picked this

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