Logical Reasoning

PT142 · S2 · Q23 Every brick house on River Street

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Every brick house on River Street has a front yard.

Conclusion (so)

Most of the BH's on R St. have two stories.

Evidence

Every BH on R St. has a front yard. Most of the H's on R St. w/ front yards also have two stories.

Evaluation

Since this argument has symbol matches ("brick house" / "river street" / "front yard" / "two stories" all appear twice) and quantifiers ("every / most / most"), this argument is ripe for being turned into an algebraic recipe.

In doing so, we don't really need to worry about the flaw. As long as we re-create this recipe, it'll re-create the flaw.

Every [BH, R] has FY. A and B → Y Most [H, R, FY] have 2S. Most *A, B, Y = Z ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Most [BH, R] have 2S. Most A, B = Z

The *A is because the first sentence is about "brick houses" and the second sentence is about "houses".

That switch from a narrower category "brick houses" to a broader one "houses" is why we can't infer the conclusion.

Goal

We have our recipe, but we can hopefully make quick eliminations by just scanning for things such as - 1 All Premise, 1 Most Premise, 1 Most Conclusion - The All Premise should be a subset of the group mentioned in the Most Premise

23.

Which one of the following is most appropriate as an analogy demonstrating that the reasoning in the argument above is flawed?

  1. By that line of reasoning,

    Bad Premise Match

    Both premises (the All and the Most) are about "legislators". To mimic the original argument, you'd need one of those to be a subset of the other. f.e. "since all female legislators are politicians and most legislators have run for office".

    24% picked this

  2. By that line of reasoning,

    Bad Premise Match

    We don't really have to read this one, since it has three Most ideas. One of our premises should be an All/No statement.

    15% picked this

  3. By that line of reasoning,

    Bad Conclusion Match

    Our conclusion should be a Most claim. This one is a "Not all / Some" claim, so we could bail as soon as we see the conclusion. Were you to keep reading, you'd also see that we have an All premise and a Some premise (but that 2nd premise should be a Most claim).

    3% picked this

  4. Correct

    By that line of reasoning,

    Why this is right

    This is crazy hard to see, but a good one to guess: All + Most → Most This and (A) are the only two answers that had an All Premise, a Most Premise, and a Most Conclusion. And the All and Most premises do switch from narrower to broader. ALL = legislators Most = public servants (a legislator is just one type of public servant. Police officers, mayors, governors, etc. are other types) Picture the original argument this way: Every [narrower type] has a front yard. Most [broader type] has two stories. Thus, most [narrower type] has two stories. This answer is saying something similar: Every [narrower type] is a public servant. Most [broader type] has not run for office. Thus, most [narrower type] hasn't run for office.

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

    56% picked this

  5. By that line of reasoning,

    Bad Premise Match

    Both Premises are "most" claims, so we don't need to read any further on this one.

    3% picked this

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