Logical Reasoning

PT154 · S1 · Q9 Gecko lizards are found

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Gecko lizards are found in any environment where there is an abundant population of gnats.

Conclusion (consequently)

The climate here must not be wet.

Intermediate Conclusion (must)

There aren't lots of gnats here.

Evidence (because)

There are no geckos here.

Anywhere you find lots of gnats, you'll find Geckos. Lots of gnats → Geckos

Gnats can survive only in wet climates. Gnats → wet climate

Evaluate

We're doing a Flaw question, and we see conditional logic indicators (any, only). What should we be thinking? We should deeply suspect that this is going to commit the #1 Famous Flaw, Necessary vs. Sufficient, which just means that the author presents a conditional logic premise and then botches using it by making an illegal reversal or negation.

The author draws two conclusions, so we should look at each one.

The first (intermediate) conclusion is that there must not be abundant gnats. Why? Because there are no geckos, and geckos are found in every environment with abundant gnats.

~Geckos → ~Lots of Gnats Here = ~Geckos Thus, Here = ~Lots of Gnats

This mini-argument seems airtight. It just applies a fact (no geckos) that triggers the contrapositive of the conditional in the first sentence (lots of gnats → geckos).

Okay the main conclusion was that the climate is not wet.

We were told that gnats survive only in a wet climate. Only / only if are necessary indicators (the only is sufficient), so the idea of "wet climate" goes on the right of the arrow.

Gnats → Wet climate ~Wet climate → ~Gnats

The author's move from the 2nd to last claim to the final claim is, But that move looks like this:

~Gnats → ~Wet climate

That's not what the rule said. It said "if yes gnats, then yes wet climate", but the author is trying to argue "if no gnats, then no wet climate". That's an illegal negation.

Goal

Look for an answer describing the author's Necessary vs. Sufficient error when it comes to the 2nd conditional (the gnats / wet climate one).

Or look for an answer making the conversational objection of, .

9.

The reasoning in the argument is flawed in that it

  1. presumes, without providing justification, that

    Not Assumed Too Strong: only

    This answer is itself making a Necessary vs. Sufficient error. We were told that abundant gnats are sufficient to indicate geckos. lots of gnats ? geckos This answer is saying that the author was assuming that abundant gnats are necessary for geckos. geckos ? lots of gnats This answer would work if the author had messed up the conditional in the first sentence, because it's saying, "The author incorrectly saw the conditional in the first sentence in reverse". The author didn't misuse the conditional in the first sentence; he botched the one in the second sentence.

    24% picked this

  2. Correct

    presumes, without providing justification, that

    Why this is right

    Did the author assume that Wet climate ? abundant gnats ~abundant gnats ? ~Wet climate ? Yes! After all, the author's final move was thinking, "There is no abundant population of gnats here. Consequently, the climate must not be wet". That's making this move: ~abundant gnats ? ~Wet climate When we're reading a Necessary Assumption answer choice, on Necessary Assumption or on Flaw (takes for granted / presumes / fails to establish), and that answer is conditional, we generally want to just diagram how that conditional looks, and then ask ourselves whether the author made that move. Did the author go from establishing the idea on the left to thinking/concluding the idea on the right? Crucially, we need to consider the contrapositive of the answer we're looking at, because correct answers are usually written in a contraposed form of how we would recognize the argument. For example, here the answer gives us Wet climate ? abundant gnats but the contrapositive form matches the argument. ~abundant gnats ? ~Wet climate

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

    68% picked this

  3. does not consider whether small

    Not an Objection

    Since this begins with fails to consider / overlooks the possibility, we can ask ourselves whether the following idea would be a weakening Objection. Can we hurt the author's argument by saying, "small populations of gnats can survive in non wet climates"? No that would be easily tolerated by the author. He thinks there is no abundant population of gnats, but he can still allow for there being a small population of gnats. And he thinks this climate is not wet, which is what the answer is also saying. So this "objection" is perfectly compatible with what the author was saying. A correct answer would be does not consider whether small populations or no populations of gnats are sometimes found in climates that are wet. This is just an awkward way of saying, "just because there's not an abundant population of gnats doesn't mean the climate isn't wet. You could have a wet climate, that doesn't have an abundant population of gnats."

    4% picked this

  4. does not mention whether gecko

    Irrelevant

    It's true, the author didn't mention whether geckos eat things other than gnats. In fact, the author never mentioned that geckos eat gnats. Why are we talking about what geckos eat? That has nothing to do with the logical flaw of this argument, which involves assuming that "since gnats require a wet climate, any place without gnats must not be a wet climate".

    2% picked this

  5. fails to establish that some

    Not Assumed

    When an answer begins presumes / takes for granted / fails to establish, we treat it like Necessary Assumption. Was this author needing to assume that "some geckos couldn't survive in a dry climate containing only a small population of gnats". It doesn't seem like the author was every specifically addressing any scenario like that. Let's negate it and see if it would weaken: "no gecko lizards could not survive in a dry climate with few gnats". Let's remove the double negative: "all geckos could survive in a dry climate with few gnats" Would it hurt the argument to say, "All geckos could survive in a dry climate with few gnats"? Nope. We can really only hurt the argument by saying something like, "Isn't it possible that there are wet climates that lack abundant populations of gnats"?

    2% picked this

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