Logical Reasoning

PT134 · S3 · Q6 Local resident: An overabundance of algae

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Local resident: An overabundance of algae must be harmful to the smaller fish in this pond.

Conclusion

An overabundance of algae must be harmful to the smaller fish in this pond.

Evidence

During the 15 or so years I've lived here, the few times that I've seen large numbers of dead small fish wash ashore in late summer coincide exactly with the times that I have noticed abnormally large amounts of algae in the water.

Evaluate

There aren't obvious keywords here indicating Conclusion vs. Evidence, but the 2nd sentence is clearly evidence, because it's a historical fact, not an opinion or inference the author is deriving.

The first sentence's keyword is "must". That word often shows up in conditional statements (such as ).

But it's also used to convey that tone when we're trying to solve a causal mystery (my wife said goodbye, but then I heard the door open and close again a second later: she must have forgotten something and come back inside to grab it.)

Whenever we see an argument where the author's conclusion is saying or assuming a causal explanation for some curious fact in the evidence, we use our Explain Curious Fact mindset:

Curious Fact ... how come the times I've seen large numbers of dead small fish wash ashore, it's coincided with lots of algae in the water?

Explanation ... the algae is killing the small fish.

The two pressure points for such an argument are 1. Alternate Explanations for the curious fact 2. the plausibility of the Author's Explanation

However, when we're doing Flaw and see Explain Curious Fact, the correct answer almost always deals with #1.

One of the 10 Famous Flaws is this idea of Causal Overconfidence. Authors are too sure of one possible explanation, while failing to consider other possible explanations.

Goal

Let's look for an answer that simply calls out the illegal move from Correlation to Causation (i.e. just because high algae and dead small fish are correlated doesn't allow the author to be this sure that there's a causal relationship between those two things).

Or look for an answer that alludes to an Alternate Explanation for the correlation. Maybe it's Reverse Causality (when small fish die, their bodies emit chemicals that promote the growth of algae). Maybe it's some Third Factor (a local factory is polluting the pond with a chemical that both kills small fish and promotes algae).

6.

The local resident's argument is most vulnerable to criticism on the grounds that it

  1. presumes, without providing justification, that

    Not Assumed: more susceptible

    Since this answer starts with presumes / takes for granted, we ask ourselves whether the author was assuming the idea that follows. The author never discusses larger fish, nor does she say anything comparative in relation to small vs. large fish. The fact that she only mentions small fish being harmed doesn't mean that she thinks only small fish are harmed (or that they're harmed more). If we negate this supposed assumption and say, "Actually, author, larger fish are just as susceptible to harm from algae as are smaller fish", that doesn't weaken the argument. The author could still be totally correct that algae is harming the smaller fish.

    4% picked this

  2. fails to consider that the

    Not an Objection

    Not an Objection: other bodies of water Since this answer starts with fails to consider / overlooks the possibility, we ask ourselves whether the idea that follows would weaken. Would it hurt the argument if we said, "Hey, author - smaller fish wouldn't be as severely affected by high algae if this were a larger body of water"? Nope, that's totally out of scope. The author is only trying to convince us that in this body of water, the pond, an abundance of algae is harming smaller fish.

    1% picked this

  3. ignores the possibility that the

    Not an Objection: other size fish

    Since this answer starts with fails to consider / ignores the possibility, we ask ourselves whether the idea that follows would weaken. Would it hurt the argument if we said, "Hey, author - that high amount of algae would have different effects on medium or larger size fish"? Nope, that's totally out of scope. The author is only trying to convince us that a high amount of algae is harming the smaller fish in this pond. What it does to other fish is not relevant.

    2% picked this

  4. Correct

    ignores the possibility that the

    Why this is right

    Since this answer starts with fails to consider / ignores the possibility, we ask ourselves whether the idea that follows would weaken. Would it hurt the argument if we said, "Hey, author - it's possible that the high algae and dead small fish are both effects of some common cause"? Yes, that would be an Alternate Explanation, specifically the one we nickname "Third Factor". Whenever an author looks at a correlation between X and Y and then concludes that X causes Y, we always have two alternate explanations to reflexively consider: 1. could it be reverse cause? It is possible that Y is causing X, and that's why they're correlated? 2. could it be a third factor? It is possible that Z is causing both X and Y, and that's why X and Y are correlated? Maybe when the weather is too warm, it both creates a lot of algae and kills smaller fish. That would be a common cause that would account for why this author sees lots of algae and dead small fish occur at the same time.

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

    93% picked this

  5. ignores the possibility that below-normal

    Not an Objection: scarcity of algae

    Since this answer starts with fails to consider / ignores the possibility, we ask ourselves whether the idea that follows would weaken. Would it hurt the argument if we said, "Hey, author - a very small amount of algae can be bad for smaller fish"? Not really. We might be tempted to think that we're undermining the author's claim that "big amounts of algae are harmful to smaller fish" by saying that "small amounts of algae are harmful to smaller fish", but it's very compatible for both things to be true. Very low amounts of solar radiation (sunlight) are bad for humans and so are very high amounts of solar radiation. It's coherent to think that both too much and too little of a given thing can be harmful.

    1% picked this

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