Logical Reasoning

PT146 · S1 · Q21 Most kinds of soil contain clay

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Most kinds of soil contain clay, and virtually every kind of soil contains either sand or organic material, or both.

Evidence

Most soil has clay Most A are B Most soil has sand Most A are C or D or organic material

Conclusion

Some soil is clay and sand Some A are C and and Some soil is clay and org Some A are D

Evaluate

Because we see “most” in one premise and “some” in the conclusion, we want to think of “virtually every” (i.e. “almost all”) as providing us another “most” claim.

Why? Well, this argument comes close to making the famous and legal “most + most” inference, which looks like this:

Most A’s are B + Most A’s are C -------------------- Some B are C

With the ingredients we were provided here, we could have made this inference:

Most S’s have C + Most S’s have SD or OM ------------------------------- Some C are (SD or OM)

What the author is doing illegally is thinking he can break that up into two different takeaways: Some C are SD, and Some C are OM.

Conversationally, the problem with the conclusion is that it’s possible that sand never overlaps with clay, or that organic material never overlaps with clay. It’s not possible that both of them never overlap. At least one of them does need to overlap with clay, but both of them don’t have to.

If you crave numbers for seeing this flaw, let’s say there are 11 types of soil. 6 clay, 5 no clay (most soil has clay) 8 sand, 2 org material, 1 neither (almost all has sand or org material)

Does there have to be soil that has clay and sand? Yes. 6 + 8 add up to 14, but there are only 11 types of soil, so we know that 3 of them overlap.

Does there have to be soil that has clay and organic material? No. It’s very possible that the 2 types of soil that have organic material are 2 of the 5 types of soil that don’t have clay.

Goal

The cool thing about Parallel Flaw is that we don’t have to thoroughly understand the original argument’s flaw in order to match the argument, when the original argument has conditional or quantified logic.

We need two Most premises, and one of them needs to have an “or” outcome. The conclusion needs to make two Some claims, one for each member of that “or” outcome.

Premise 1: Most A’s are B Premise 2: Most A’s are C or D Conclusion: Some A’s are C, and Some A’s are D

21.

The pattern of flawed reasoning in which one of the following arguments is most parallel to that in the argument above?

  1. Most pharmacies sell cosmetics. Virtually

    Bad Conclusion Match

    This conclusion is conditional. We want a conclusion that makes two Some claims. The premises did match: Most pharmacies sell cosmetics. Most pharmacies sell shampoo or toothpaste. But the conclusion would have needed to be “Some places that sell cosmetics also sell shampoo, and some places that sell cosmetics also sell toothpaste”.

    14% picked this

  2. Undoubtedly, most pharmacies sell cosmetics,

    Bad Conclusion Match

    This conclusion is a Most claim. We want a conclusion that makes two Some claims. The words “for, after all, since, because” (F.A.B.S.) directly indicate premises and indirectly indicate conclusions. Ideally, we don’t read any of this answer once we’ve already seen that the first claim was the conclusion and that the conclusion was a mismatch.

    8% picked this

  3. Most pharmacies sell cosmetics. Nearly

    Bad Conclusion Match

    This conclusion is conditional. We want a conclusion that makes two Some claims. The premises did match: Most pharmacies sell cosmetics. Most pharmacies sell shampoo or toothpaste. But the conclusion would have needed to be “Some places that sell cosmetics also sell shampoo, and some places that sell cosmetics also sell toothpaste”.

    4% picked this

  4. Virtually every pharmacy that sells

    Weak Premise Match

    This conclusion makes two Some claims, so it’s worth reading. The problem is that the first premise isn’t saying “Most P’s are S or T”, it’s saying “Most P’s that are S are also T”. In the original argument, both Most claims were about the same group: “types of soil”. This answer would only match had the original argument said, Most pharmacies that sell shampoo also sells toothpaste. Most sandy soil has organic material. Consider the difference in meaning between these two claims: Most people have an iPhone or an Android phone. vs. Most people that have an iPhone also have an Android phone.

    17% picked this

  5. Correct

    Nearly all pharmacies sell either

    Why this is right

    This has two Some claims in the conclusion, so it's worth reading. Premise 1: Most pharmacies sell cosmetics. Premise 2: Most pharmacies sell shampoo or toothpaste. Conclusion: Some cosmetics sell shampoo, and some cosmetics and toothpaste.

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

    57% picked this

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