Logical Reasoning

PT138 · S4 · Q1 Jim's teacher asked him to

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Jim's teacher asked him to determine whether a sample of a substance contained iron.

Conclusion

The substance has iron.

Evidence

A magnet was attracted to the substance, and magnets attract iron.

Evaluate

Consider this analogy --

We would object by saying

This is the famous Necessary vs. Sufficient flaw. We were given a conditional rule: Iron in it → Magnet is attracted.

From that rule Jim thought, He's reasoning backwards through that conditional statement, a.k.a. confusing the sufficient condition with the necessary condition.

Goal

Similarly, Jim is failing to consider that magnets might be attracted to other things besides iron. Maybe they're also attracted to nickel and copper.

The fact that the magnet was attracted to the substance doesn't prove it's iron, because there might be other things besides iron that magnets are attracted to.

1.

Jim's reasoning is questionable in that it fails to consider the possibility that

  1. iron sometimes fails to be

    Not an Objection

    This gets at the notion that we might get a False Negative when we test for iron. Just because a magnet isn't attracted to something wouldn't prove there's no iron in it, because sometimes iron isn't attracted to magnets.

    3% picked this

  2. iron is attracted to other

    Not an Objection

    This would fight a move from "Since iron was attracted to this thing, this thing must be a magnet". But our author never made that move so pointing out that iron is attracted to other things has no impact on the author.

    5% picked this

  3. the magnet needed to be

    Not an Objection

    Like (A), this sounds like an objection we would make if the author were saying, "Thus, there is NOT iron in the substance". We would sound the alarm about False Negatives: nuh-uh ... sometimes iron just fails to attract to a magnet ... sometimes you just need to orient the magnet the correct way. We need an objection that would allow us to argue that this substance is not iron. This answer isn't helping us to argue that Anti-Conclusion.

    1% picked this

  4. Correct

    magnets attract substances other than

    Why this is right

    This gives us the False Positive objection we're looking for ... "Just because the magnet was attracted doesn't prove that it is iron. Magnets are attracted to copper and nickel, too, so maybe the substance is made of copper or nickel, not iron."

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

    90% picked this

  5. some magnets attract iron more

    Out of Scope Comparison: more strongly

    We aren't concerned with the relative level of attraction. If we were fighting a move from "Since the magnet wasn't attracted, it isn't iron", we could respond with this False Negative objection of ... there's iron in there: the magnet test just failed because this type of iron doesn't attract strongly enough for Jim to have noticed any attraction.

    2% picked this

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