Logical Reasoning

PT154 · S4 · Q7 Late blight is a disease

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Late blight is a disease affecting tomato plants that causes blue-grey patches to appear on the leaves.

Evidence

Applying a copper solution to blighted plants is an effective treatment for the disease [late blight].

Conclusion (Therefore)

Late blight must be caused by a deficiency of copper in the plant’s mineral intake.

Evaluate

LR questions often contain arguments that contain evidence about a correlation—two things that tend to happen together. Based on just that correlation, the argument claims that one of the things causes another. This is called a Causal Flaw, which is often explained by saying "a correlation does not prove causation."

This argument's conclusion claims that late blight is caused by a copper deficiency. But the evidence is a bit stronger than a mere correlation. The copper solution is described as an "effective treatment." This indicates that the copper solution causes the plants to be cured of the disease.

So, what's the exact flaw here? The conclusion implies that tomato plants need copper. The assumption is that the copper solution provides the plants with this copper that they need. But none of that is stated in the evidence. We're never told what the copper solution actually does, only that it works. It certainly might cure a copper deficiency, but there are other, equally plausible alternative explanations.

For example, late blight could be caused by some type of organism that is killed by the copper solution. If this is true, the conclusion doesn't really make sense. It would be the same as saying,

Goal

Look for an answer with evidence that describes some type of causal connection without explaining exactly how that connection works. The conclusion will be based on an assumption that there is only one explanation, when other reasonable possibilities exist. Watch out for answers containing arguments that aren't flawed, or ones that contain a completely different flaw.

7.

The flawed nature of the argument can be most effectively demonstrated by noting that, by parallel reasoning, we could argue that

  1. since radiation therapy can be

    Bad Conclusion Match

    The conclusion of the original argument doesn't claim that higher doses of copper solution would be more effective in treating late blight. The conclusion in this answer choice doesn't involve the same flaw of favoring one explanation when others are just as plausible.

    1% picked this

  2. since scurvy is caused by

    Bad Evidence Match Bad Conclusion Match

    This wouldn't be a terrible answer, except the premise and conclusion are completely reversed. It would be similar to the original argument if it read, "since vitamin C is an effective treatment for scurvy, it follows that scurvy is caused by a deficiency of vitamin C."

    3% picked this

  3. since malnutrition can cause health

    Bad Conclusion Match

    This is similar to answer choice A. The original argument does not claim that, because something is effective, even more of that thing is more effective.

    1% picked this

  4. Correct

    since paracetamol is effective in

    Why this is right

    This describes the same type of situation as the original argument. If we replace "paracetamol" with "copper," replace "headaches" with "late blight," and replace "bloodstream" with "mineral intake," we basically have the same argument with the same flaw.

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

    93% picked this

  5. since calcium deficiency in apple

    Bad Evidence Match Bad Conclusion Match

    This is similar to answer choice B in that this reverses the premise and conclusion. The conclusion of the original argument claims that X causes Y, while that is stated as the evidence in this answer choice.

    2% picked this

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