Q17 - People who have never been

[jiyoonsim]

So I understand why A is correct answer, and why E (my initial answer) is wrong. But as I go through all the answers, I was confused by D. Is D incorrect answer because it does not have flaw?

And if there is any way to cut down some time for questions like this, please let me know. I usually go for conclusion checks, but I figured sometimes it works fine and sometimes it backfires badly.

[giladedelman]

Thanks for your question!

Looking at the conclusions is one tool for application/matching questions, but it shouldn't be your main tool. It can be useful in that if the original argument concludes, "Therefore, any X is Y" (conditional statement), and answer (B) concludes, "Therefore, my uncle is a doctor" (not conditional) then we can probably get rid of (B) because the conclusion isn't the same kind of statement. This works better on normal application questions than on "match the flaw" questions, which can be looser in terms of structure, as long as the flaw is there.

Which brings us to our main tool for "match the flaw" questions, that is, we've got to identify the flaw first and foremost. Here, the flaw is reversed logic. The premise tells us

never been asked more than what's easy ----> never do all they can

then, from the second premise that Alex hasn't done all he can, it concludes that he hasn't been asked to do more than what's easy:

hasn't done all he can ----> never been asked more than what's easy

So we're looking for the answer that contains the same illegal reversal.

(A) is correct, like you said, because it reverses a conditional relationship. We're told that anyone with a dog knows the value of companionship. Then, from the fact that Alicia knows the value of companionship, the argument concludes that she must have a dog. This is clearly reversed logic.

(B) starts with a conditional statement, so at first it looks okay, but we can rule it out once we see "Fran has surely never discovered something new" -- this answer negates the conditions in a way the original doesn't. It's flawed, but it's an illegal negation, not a reversal.

(C) also negates the conditions, so it's not a match. It turns out that this answer actually reverses and negates, so it's logically valid.

(D) is, as you suspect, logically valid, so it can't be a match. If any closed plane figure bounded by straight lines is a polygon, and that thing on the board is a closed figure bounded by straight lines, then yes, it's a polygon. Case closed.

(E) is incorrect because the second premise introduces a new condition that's not part of the original conditional statement.

Does that clear this one up for you at all? Remember, on "match the flaw" questions, it's all about identifying the flaw and staying focused on that.

[zainrizvi]

I'm a bit perplexed by choice (D). I understand that closed plane, bounded -> polygon, but wouldn't the opposite be held true as well? Polygon -> close plane, bounded.... so is the condition of having closed plane, bounded both necessary and sufficient in this case?

[irini101]

I have the same issue of zainrizvi: why diagram the sentence "a polygon is...closed plane" as: cp --> plg, instead of plg--> cp or
plg <--> cp???

I am so confused, could any one please help explain?

Thanks in advance!

[giladedelman]

Yep, I think you guys are right: because it says this is the "definition" of a polygon, we can treat this as a biconditional: if it's a polygon, it must be a closed plane figure bounded by straight lines, and if it's a closed plane figure bounded by straight lines, it must be a polygon. So answer (D) is logically valid.

[goriano]

It can be useful in that if the original argument concludes, "Therefore, any X is Y" (conditional statement), and answer (B) concludes, "Therefore, my uncle is a doctor" (not conditional) then we can probably get rid of (B) because the conclusion isn't the same kind of statement.

I've always wondered whether it is safe to do this, because "my uncle is a doctor" can be considered a conditional statement. My uncle --> doctor. So where do we draw the line?

[timmydoeslsat]

We have to be very wary of diagramming definitions on the LSAT.

A true definition is biconditional <--->

However, such is the case with the dad/doctor example, you cannot be deceived into believing that any sentence with "is" is a definition.

On the LSAT, very rarely is this concept tested. When you see something like (D), it tells us "by definition...." so we know that it is in fact a biconditional statement.

I would be hesitant to equate something as a definition unless I see something like D where it says "Object A is any plane that has 23 lines..." When you see distinct wording such as "any," that is an indicator of a likely definition. I would ask myself, "Could I really have the necessary condition bring about the sufficient?"

In the case of dad/doctor, no. In choice D, yes.

[giladedelman]

Hey, Goriano:

For our purposes, a conditional statement gives us some kind of rule, something that is true in all cases. "My uncle is a doctor" isn't a conditional statement because it's only about one case. Being "my uncle" isn't a condition. You couldn't really translate this into, "If you are my uncle, you are a doctor." It's just a description of a person.

[redcobra21]

Hey

Thanks for the great discussion. Just a quick follow-up question

For (A), the flaw is: A--> B; therefore, B --> A

For (B), the flaw is: A-->B; therefore, ~A-->~B

Is it the case that these two should be considered differently even though their flaws produce the same conditional relationship via the contrapositive?

Thanks