Logical Reasoning

PT154 · S4 · Q26 Current evidence indicates that there

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Current evidence indicates that there is no methane on Planet 253.

Conclusion

If there's no methane on Planet 253, then there's no life on Planet 253.

Evidence (since)

Microbes always produce methane. (if there is no methane, no microbes)

Evaluate

It's very annoying to find the Conclusion here because none of the claims sound obviously like an opinion, and there aren't argument indicator words otherwise helping to guide us.

The one indicator word we get is the Since. Any time we see, Since _____ , _____ . it means that the Since-claim is support for the second claim, which is a conclusion.

But there's no way the conclusion we're supposed to be proving with the correct answer is the second half of that last sentence.

Why? Because that's already a logically guaranteed conclusion. All they're doing in that last sentence is saying,

Since microbes always produce methane Micr → Meth We know if there's no methane, no microbes ~Meth → ~Micr

Okay, so the final sentence is just a statement and its contrapositive. That can't be the conclusion we need to prove.

The first sentence is just a factual claim about current evidence. So by process of elimination, we need to be considering the 2nd sentence the main conclusion.

The author is trying to prove that "if there's no methane, there's no life", which would look like this, if we elongated the arrow to show the intended logic path:

CONCLUSION ~methane ------------------------> ~life

EVIDENCE ~methane ----> ~microbes

MISSING PIECE OF THE PATH

~microbes ----> ~life

Goal

Let's find an answer saying, , or its contrapositive, "Wherever there is life, there are microbes".

Since we need to prove a claim about "no life" but the evidence never spoke about that, the correct answer absolute must be a claim that allows us to prove "no life". We can summarily reject any answer that doesn't mention Life or any answer that would only let you prove "there is life".

26.

The conclusion drawn above follows logically if which one of the following is assumed?

  1. There is, in fact, no

    Unrelated to Goal

    This doesn't mention "life" so it's immediately wrong. The conclusion of this argument was a conditional statement: if there's no methane, then there's no life Whenever we encounter conditional conclusions, we will never be trying to strengthen or weaken the idea that the trigger is true. Suppose I made this conditional conclusion: if humans colonized Mars, we would build roller coasters there. You can't strengthen this by saying that we will colonize Mars or weaken this by saying we won't. I'm not saying we're actually going to colonize Mars. I'm only making a definitive claim "we would build roller coasters" about a hypothetical world in which we have colonized Mars. So when we see a conditional conclusion, we just accept the Trigger as an implicit premise. You can only debate my roller coaster conclusion if you first just accept the supposition that this is a world where we've colonized Mars.

    4% picked this

  2. If methane exists on Planet

    Unrelated to Goal

    Since this doesn't mention "life", we can immediately reject it.

    8% picked this

  3. If there is no methane

    Unrelated to Goal

    Since this doesn't mention "life", we can immediately reject it.

    3% picked this

  4. Correct

    If there are no microbes

    Why this is right

    This is what we predicted. The evidence told us this: if no methane, no microbes This answer adds on this: if no microbes, then no life which allows us to derive the conclusion: If no methane on P253, then no life on P253

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

    80% picked this

  5. If there is methane on

    Unrelated to Goal

    This mentions "life", but this is a rule that is set up to prove "there is life". We need a rule that proves there isn't life.

    5% picked this

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