Logical Reasoning

PT142 · S1 · Q25 Student: If a person has an immunity

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Student: If a person has an immunity to infection by a microorganism, then that microorganism does not cause them to develop harmful symptoms.

Conclusion (it follows that)

Many people have an immunity to infection by staphylococcus.

Evidence (since)

If you have immunity → then don't develop to infection by X harmful symptoms

Many people don't develop harmful symptoms from staphylococcus.

Evaluate

Whenever we're doing Flaw or Parallel Flaw and we see Conditional Logic, we suspect that we're about to see the #1 famous flaw of them all -- Necessary vs. Sufficient, which just means that the author presents a conditional rule in the evidence and then applies that rule in an illegal backwards or opposite fashion.

EXAMPLE If you go to Harvard Law, you'll be a good lawyer. Tanya is a good lawyer. Thus, Tanya attended Harvard Law.

This argument gave us a conditional in the first sentence, then gave us a premise saying that many people are the Right Side of that conditional rule. Then it concluded that many people are the Left Side of that rule.

Goal

So we need two premises, one a conditional logic rule and one a factual claim about the outcome of the rule. The conclusion then should be a factual claim about the trigger of the rule.

Premise 1: A → B Premise 2: Thing X is B. Conclusion: Thing X is A.

25.

The student's argument is most similar in its flawed pattern of reasoning to which one of the following?

  1. Everything morally right is just,

    Valid Logic

    This provides a conditional Morally right → Just To match the original argument, we should now hear Something is Just. Thus, Something is morally right. Instead we hear Some X's are not just. Thus, some X's are not morally right. That isn't flawed logic. That's correctly reasoning via the contrapositive.

    8% picked this

  2. Advertisers try to persuade people

    Weaker Match

    Weaker Match: Negation vs. Reversal Weak Conclusion Match This is somewhat tempting, because we could construe the first claim as a conditional, and then the argument does botch that rule by applying it in an illegal way. But this answer basically inverts the conditional, whereas the original argument reversed the conditional. ORIGINAL THIS ANSWER Premise 1: A → B A → B Premise 2: Thing X is B. Thing X is ~A. Conclusion: Thing X is A. Thing X is ~B. Technically, an illegal reversal and an illegal negation are the same thing. But if you're trying to find the most similar argument, it's still more similar to match reversal with reversal, or negation with negation. Additionally, the conclusion here is not certain of itself, which is not a good match for the original conclusion, which was certain of itself.

    10% picked this

  3. Isabel said that she would

    Valid Logic

    This provides a conditional take medication → cures or reduces symptoms To match the original argument, we should now hear Isabel's symptoms are gone. Thus, she must have taken medication. Instead we hear Isabel's symptoms are not gone. Thus, she must not have taken medication. That isn't flawed logic. That's correctly reasoning via the contrapositive.

    9% picked this

  4. Correct

    When business owners are subjected

    Why this is right

    This provides a conditional Excessively taxed → less willing to expand To match the original argument, we should now hear Something is less willing to expand. Thus, something is excessively taxed. Indeed we do, although it is weird how the final premise and conclusion get grammatically squeezed into the final sentence. The recent decline in business expansions = business owners are less willing to expand their business. Thus, this shows their taxes are too high = they are excessively taxed It's a bit of a stinker as a correct answer because "a recent decline in expansions" is not a perfect match for "business owners are less willing to expand". In the original argument "without developing any harmful symptoms" was a strong match for "does not cause them to develop harmful symptoms". But this is the only answer choice that offered a conditional and then tried to reason backwards through it.

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

    70% picked this

  5. Studies show that doctors tend

    Trap

    Bad Evidence Match Bad Conclusion Match Topic Trap There is only one premise, so that's already a problem for this argument to match the original. The premise isn't quite conditional; it's just tends to. It's also a big red flag when we see a similar topic (medical / disease / germs) on a Parallel or Analogy answer choice. In order to match the original argument, after the first sentence we would want to hear something like, "Betty tends to wash her hands more often than health care professionals. Thus she must be a doctor."

    3% picked this

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