Logical Reasoning

PT11 · S4 · Q15 A certain experimental fungicide

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A certain experimental fungicide causes no harm to garden plants, though only if it is diluted at least to ten parts water to one part fungicide.

Conclusion (thus)

This fungicide, as long as it is sufficiently diluted, provides a means of eliminating powdery mildew from rose plants that involves no risk of harming the plants.

Evidence

This fungicide causes no harm to garden plants, only if it's diluted to at least ten parts water to one part fungicide.

This fungicide is known to be so effective against powdery mildew that it has the capacity to eliminate it completely from rose plants.

Evaluate

It's hard to argue with this conclusion. We'd either be arguing that the fungicide doesn't provide a means of eliminating powdery mildew or that it would involve some risk of harming the rose plants.

But we were told that if the fungicide is diluted to a 1:10 ratio with water then it causes no harm to garden plants. Is a "rose plant" a garden plant? Presumably, but maybe they're testing us on that assumption.

We were also told that the fungicide has the capacity to eliminate powdery mildew completely from rose plants. So how could we argue that this fungicide doesn't provide a means of eliminating powdery mildew?

Goal

We might figure out the tiny possibility for objection, or we might feel stumped (as I'm modeling here).

The correct answer, if negated, should weaken the argument, meaning it should allow us to argue that the fungicide, sufficiently diluted, would either not provide a means of eliminating powdery mildew or would involve some risk of harming the plants.

15.

Which one of the following is an assumption on which the argument depends?

There is not an alternative
Irrelevant: other methods
This author didn't act like this fungicide is the only method that can do what the conclusion promises. So the author doesn't need to assume that there aren't other methods that could achieve the same outcomes. If we negate this and say, "there is an alternative method", it doesn't weaken at all. The author will just be like, "Cool -- I said this fungicide provides a means, not the only means".
8% picked this
When the fungicide is sufficiently
Out of Scope: risk to non-plants
The conclusion is only promising that this method will involve no risk of harming the plants. She didn't promise no harm to people, animals, or insects. If we negate this and say "it does present some risk for people / animals / insects" we wouldn't be hurting anything she was arguing.
13% picked this
Powdery mildew is the only
Too Strong
Too Strong: the only Only Thing Mentioned ≠ Only Thing The only fungal infection afflicting rose plants that was mentioned in the paragraph was powdery mildew, but that doesn't mean that the author is assuming that powdery mildew is the only fungal affliction that afflicts rose plants. If I make an argument and San Francisco is the only Californian city I mention, that doesn't mean I'm assuming that "San Francisco is the only city in California".
3% picked this
If a fungicide is to
Too Strong
Too Strong: must Internal to a Premise There's no reason that the author has to define "effective" as "eliminating completely". The author just said, "this fungicide is so effective that it eliminates the powdery mildew completely." If we say, "He's so tall that he can dunk a basketball without jumping", does that mean we're assuming "If a person is to be considered tall, they must be able to dunk a basketball without jumping"? Of course not. We should be deeply suspicious of any Necessary Assumption answer that is trying to connect language from within one premise claim. This answer is just taking language from the 2nd sentence and trying to create a "missing link" out of it.
13% picked this
Correct
The effectiveness of the fungicide
Why this is right
This has the lovable "ruling-out" not that is found in so many correct, Defender-style answers on Necessary Assumption. If we negate it, does it weaken? Can we object to the argument by saying that "the effectiveness of the fungicide does depend on its being more concentrated than one part in ten parts of water"? Yes! This gives us a way to argue that the fungicide, sufficiently diluted, won't provide a means of eliminating powdery mildew. Sure it's known to be effective against powdery mildew ... in its potent, concentrated form. But this negation is saying that once you water it down so that it's too diluted to harm the rose plant, then you no longer get its effectiveness at getting rid of the powdery mildew fungus. In essence, you can either choose to have this diluted enough to involve no risk of harming the rose plants, or you can choose to have this potent enough for it to eliminate powdery mildew. But you can't have both at the same time.
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.
62% picked this

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