Logical Reasoning

PT108 · S2 · Q2 John: For 40 years, fluoride

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John: For 40 years, fluoride has been added to public drinking water.

Conclusion (should)

I should stop drinking fluoridated water.

Intermediate Conclusion

Only if I stop drinking fluoridated water will I be sure not to develop bone cancer.

Evidence

In a study, fluoridated water given to lab rats caused bone cancer in 90% of the males (0% of the females).

Evaluate

There seem to be a couple potential issues here -- 1. are we really convinced that fluoridated water poses a cancer risk to humans?

2. are we really convinced that if John avoided fluoridated water, then he will be sure not to develop bone cancer?

When it comes to #1, we might object that fluoridated water could be carcinogenic to rats but not to humans. Or we could just point to the weirdness that 90% of males and 0% of females were affected as evidence that something screwy is going on with this experiment or with the physiology of male rats.

When it comes to #2, obviously eliminating one risk of bone cancer would not guarantee that John will not develop bone cancer.

The use of "only then" with "I will be sure" seems to convey a bi-conditional idea.

if he doesn't stop → won't be sure to avoid drinking FL water bone cancer

if he does stop → will be sure to avoid drinking FL water bone cancer

Goal

Look for an answer resisting the move from , or an answer resisting the move from .

2.

Which one of the following is the strongest criticism of John's reasoning?

  1. John does not consider how

    Out of Scope: other people

    We can't complain about anything being left out of an argument, unless the thing being left out is relevant to the conclusion being drawn. But since the conclusion is only about John, there's no reason his premises need to consider how other people are affected by fluoridated water.

    2% picked this

  2. John does not consider whether

    Out of Scope: other diseases

    We can't complain about anything being left out of an argument, unless the thing being left out is relevant to the conclusion being drawn. But since the conclusion is only about John developing bone cancer, there's no reason his argument needs to consider other diseases.

    1% picked this

  3. John does not consider whether

    Not an Objection

    This answer is zooming in on a background claim that wasn't even part of the logic. It wouldn't hurt this argument at all if there have been brief periods during the last 40 years in which fluoride wasn't added.

    2% picked this

  4. John does not focus on

    Out of Scope: positive effects

    If John's conclusion were some broad statement like, "Thus, the city should not fluoridate the public's drinking water", then adjudicating that claim would involve assessing all the pro's and con's of fluoridating water. We'd have to weigh the potentially increased risk of bone cancer against all the positive things that fluoridated water bring us. But the conclusion is narrowly about John and his risk of bone cancer. Nothing else is relevant to adjudicating that claim other than "would he or wouldn't he be sure not to develop bone cancer."

    5% picked this

  5. Correct

    John does not consider the

    Why this is right

    John seems to think that if he stops drinking fluoridated water, then and only then will he be sure not to develop bone cancer. This answer objects by saying, "there are other possible causes of bone cancer, dude. Cutting fluoridated water out of your diet might lower your risk of bone cancer, but you can't be sure you're not going to develop bone cancer because there are other potential causes of bone cancer."

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

    90% picked this

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