Logical Reasoning

PT144 · S4 · Q22 On Wednesdays, Kate usually buys

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On Wednesdays, Kate usually buys some guava juice.

Conclusion

Kate must sometimes shop at the local health food store on Wednesdays.

Evidence

Kate usually buys guava juice on Wednesdays. The only place you can buy guava juice is the local health food store.

Evaluate

This is definitely an airtight argument. It does have some quantified and conditional words (usually / the only / sometimes), so we could try to put it into some algebra.

On Wednesdays, K Most Wednesdays, usually buys guava Kate buys guava

Only place to buy buys → local health guava is local health guava food stores food stores.

Thus, on some Wednesdays, Some Wednesdays, Kate goes to local health Kate shops local health food store. food store

It's weird that the conclusion is milder in strength than it could have been. We could have concluded that on .

What makes the argument work is that we know Kate does something (buy guava on Wed) and we know there's only one way to do that something (shop at local health food store), so that guarantees us that Kate uses that "only one way".

Goal

Look for an argument with two premises. One of them should be conditional. The other one should provide a fact that triggers the conditional. The conclusion should involve a (possibly watered-down) inference about the outcome of that conditional.

Prem 1: Most/Some A's are B. Prem 2: The only way to be B is to be C. Conc: Thus, Most/Some A's are C.

22.

The argument above is most similar in its pattern of reasoning to which one of the following arguments?

  1. Correct

    Only teachers at the Culinary

    Why this is right

    We've got a conditional premise: use Culinary Institute's → teacher at main kitchen Culinary Institute We've got a premise that triggers that rule. Since most dinners at Cafe Delice are prepared in Culinary Institute's main kitchen, we know that Culinary Institute's main kitchen got used, so we know that teachers were doing the preparing. Thus, we can accept the conclusion that some dinners at Cafe Delice were prepared by Culinary Institute teachers. This argument mimicked the feature of the original that the conclusion is weaker than it could have been. It would have been fair to conclude that "most dinners at Cafe Delice are prepared by Culinary Institute teachers", but this answer uses at least just as the original argument used sometimes.

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

    49% picked this

  2. All dinners at Cafe Delice

    Bad Premise Match Bad Conclusion Match

    A quick glance at the conclusion would show us that it has a mismatching strength. The original conclusion was "sometimes she must shop at store". This conclusion is "all dinners must be prepared by teachers". Both of these premises are conditional (all / only). We wanted one conditional premise and one factual premise that would trigger it (preferably in most format, like the original). These two premises combine to form a conditional chain: dinner prepared in prepared by at → kitchen of → Culinary Institute Cafe Delice Culinary Institute teachers So the conclusion is a valid one; it's just the argument has fewer structural similarities to the original than (A) does, because (A) matched the Most premise and the Sometimes conclusion.

    10% picked this

  3. Most dinners at Cafe Delice

    Invalid Logic

    There is a conditional premise, you teach at Culinary Institute → can use Culinary Institute's kitchen But nothing triggers that rule. The premise tells us about using Culinary Institute's kitchen, so it would be a flaw to read backwards and think that it tells us anything about teachers. In other words, just because all Culinary Institute teachers are allowed to use the main kitchen, that doesn't mean that if we know the main kitchen was used that we know it was a teacher who did the preparing.

    6% picked this

  4. Most teachers at the Culinary

    Invalid Logic

    There is a conditional premise, dinner at Cafe Delice → prepared using Culinary Institute's kitchen But nothing triggers that rule. The premise tells us about using Culinary Institute's kitchen, so it would be a flaw to read backwards and think that it tells us anything about dinner at Cafe Delice. In other words, just because most Culinary Institute teachers are allowed to use the main kitchen, that doesn't mean that any of the Cafe Delice dinners that were prepared in that kitchen were prepared by Culinary Institute teachers.

    18% picked this

  5. Only teachers at the Culinary

    Invalid Logic

    There is a conditional premise, allowed to use main kitchen → teach at Culinary Institute But nothing triggers that rule. The factual premise tells us about Culinary Institute teachers, so it would be a flaw to read backwards and think that it tells us anything about main kitchen. In other words, just because most Cafe Delice dinners are prepared by Culinary Institute teachers, that doesn't mean that the Culinary Institute teachers used the main kitchen of Culinary Institute to cook the meal. We're told that "only Culinary Institute teachers can use the main kitchen", but this argument is assuming "the main kitchen is the only one that Culinary Institute teachers ever use". It's possible that Culinary Institute teachers prepare most dinners at Cafe Delice in the Cafe's own kitchen. Nothing in the evidence rules out that possibility.

    18% picked this

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