Q19 - If understanding a word...

[noah]

This is a tricky question! I wouldn't go for diagramming this, as I think it falls apart more quickly staying within the content of the argument and applying the statements.

The stimulus establishes that if some skill - understanding a word - requires specific knowledge - knowing it's definition - then that skill requires some other specific knowledge - knowing the meaning of all the words in that definition. It must be, we can assume, that it's because knowing a definition requires knowing the meaning of all of the words in that definition. The stimulus goes on to give us one more tid bit: babies don't know the definition of some of the words they utter. So, those babies don't have that special knowledge- knowing the dictionary definition - that is supposedly required. (E) capitalizes on this fact by noting that if babies are able to understand words, then it turns out the special knowledge actually is not necessary, since babies manage to understand words without it.

If I were to diagram, I would use this:

[U = understanding a word; KD = know a definition; KW = know all words in a def; B= babies!]

(U --> KD) --> (U --> KW)
B (all) --> ~ KW (for some words)

The most important thing here is to notice that the first statement is NOT stating that (U --> KD), it simply is playing out the hypothetical effect of that being true.

We can infer something based on that, since that hypothetical effect is stated as a fact: If we turn it into this: (A --> B) --> (A --> C), what is missing? It requires (assumes) B --> C. So, we know that (KD --> KW), knowing a definition requires knowing the words in that definition.

Answer choice (E) states that if there are some babies who understand all the words they utter, then understanding a word doesn't require knowing all the words in the definition. In short, this must be true because it establishes that U --> KD would NOT be true if Babies U, since we already know that KD is not true (from the stimulus). It'd be as if I suggested that it might be possible that loving someone requires knowing his or her name, but, if we find out that someone loves someone without knowing her name, then that suggested rule is not true.

From a test-smarts point of view, (E) "triggers" the "all babies" rule given in the stimulus, so in a pinch, this would be a smarter guess than some other answer choices.

More intuitively and working from wrong to right:

(A) is extremely tempting! But we don't know this; we only know that babies don't know the definitions of some words they utter. IF we knew that knowing the definition was necessary for understanding a word (if we assumed that the hypothetical conditional statement were true), this would be a valid inference. But it may be the case that it's not true that U --> KW.

(B) is very tempting and similar to (E). However, it jumps to saying that it's not true that U --> KD, but we haven't learned that the necessary part (U --> KW) is not true. It's particularly tempting also because it seems to be where the argument "is heading", but that's sloppy, non-lawyerly reading.

(C) this might be true -- babies are uttering some words for which they don't know the definitions, however we don't know that they understand a thing they are saying. This answer establishes that U --> KD is not true, but then infers that ~ KD --> U.

(D) from sheer mental exhaustion this answer choice might be tempting! However, it's negating the sufficient side of the initial statement, which does not mean we can negate the necessary side. (D) establishes that ~ (U --> KD) --> ~ (U --> KW), which is a negation of both sides. In common sense talk: if the rule about understanding a word requiring knowing it's definition is not true, does that mean we don't need to know the words in the definition -- maybe, but maybe not! But notice, the answer choice expands this to state that you don't need to know ANY other word. That's quite a leap (and it's a leap within an illegal negation of logic).

#officialexplanation

[cyruswhittaker]

I'm still a little confused on this:

So basically we're forming the contrapositive and showing that the necessary condition of the first sentence does not have to be true: "Understanding a word does NOT require understanding the words that occur in that definition."

And for choice E, we're told that some babies do understand the words they utter without knowing the dictionary definitions (from stimulus) of some of the words they utter. But I am not clear about how this actually denies the necessary condition (which is the conditional in sentence 2).

Afterall, it is based on understanding the words that occur in the definition.

Maybe the babies don't know the definitions, but the definitions are formed on words that they DO understand. In other words, how can we claim that the babies don't actually understand the words in the definition?

[noah]

You actually don't need to use the second part of the greater conditional to infer (E). All you need is this:

Understanding word --> knowing it's definition

If we know that babies understand all words they utter, as (E) proposes hypothetically, then according to the proposed rule, babies would know the definitions of all the words they utter. However, we know from the stimulus that babies in fact do not know the dictionary definitions of all the words they utter. So, the bold rule above is not true, as (E) states.

If you want to think of this with the contrapositive of the above:

NOT know definitions --> NOT understand words

And we know that babies trigger the sufficient for at least some of the words they utter, so they should end up NOT understanding some of the words they utter. So, as (E) suggests - contrapositively? - the rule must not be true.

I hope that clear up this rather dense problem with an answer that doesn't even use all of it's density!

[clarafok]

hello,

i'm also still kind of confused with why E is right...

i understand
(U -> KD) -> (U->KW)
and KD -> KW

but i don't really understand how the babies part was diagrammed. shouldn't it be B(all) -> ~KD (for some words) instead of ~KW?

and i don't really understand anything after that, specifically

In short, this must be true because it establishes that U --> KD would NOT be true if Babies U, since we already know that KD is not true (from the stimulus). It'd be as if I suggested that it might be possible that loving someone requires knowing his or her name, but, if we find out that someone loves someone without knowing her name, then that suggested rule is not true.

is it the contrapositive of B(all) -> ~KD/KW (which ever is the correct one because as stated earlier, i don't understand why it's ~KD)

any help would be appreciated!

[ManhattanPrepLSAT1]

Let me try and explain this from the almost mathematical perspective of conditional logic...

We're given two statements in the stimulus.

1. (UW ---> KD) ---> (UW ---> UWD)
2. B ---> ~KD

Notation Key: UW = understand a word, KD = know the definition, UWD = understand all the word in the definition, B = babies

The first statement is actually bait. It's not used in arriving at the correct answer.

Using the second statement and the "if" part of answer choice (E):

B some UW
B ---> ~KD
--------------

we can infer:

UW some ~KD

If some people (namely babies) can understand words without knowing the dictionary definition of a word, then understanding a word does not always involve knowing its dictionary definition. Keep in mind that to refute the idea that A requires B, you do so by showing that some A's are not B's. So given that

UW some ~KD

we know that

~(UW ---> KD)

in English: it is not true that understanding a word requires knowing its dictionary definition.

Here's an similar example.

Stimulus:
Suppose I know that all lawyers went to law school.

Answer choice:
Then it must be true that if some lawyers are not rich, then being rich is not a requirement of going to law school.

It's a complicated problem and in it's most generic outline it would look like:

Stimulus: A ---> B
Answer choice: (A some ~C) ---> ~(B ---> C)

Does that help you see where answer choice (E) is coming from?

[daniel.g.winter]

Wow, this is a very dense and convoluted problem at first glance (especially on a timed PT toward the end of your third straight section). However, after spending some time with it, it is NOT that bad. Here's my thoughts.

The stimulus tells us that if understanding a word involves knowing its dictionary definition, then understanding a word requires understanding the words in that definition. It also tells us that all babies do not know the dictionary definitions of some words they utter. Think about how the stimulus phrases this: "BUT CLEARLY there are people - for example, all babies - who do not know the dictionary definitions of some of the words they utter." The question stem is asking us what follows logically. Well, obviously the statement following is going to refute that hypothetical conditional in the first sentence or weaken it in some way. The way the argument is going just leads me to think that way. Why would it say "But clearly" then?

E says that if there are some babies who understand the words they utter (and from the stimulus we know these same babies DO NOT know dictionary definitions of ALL of these words) then it must not be true that understanding a word requires knowing its dictionary definition.

I think that the "But Clearly" is a huge tipoff here that you're looking for an answer choice that will attack that hypothetical conditional in some way.

And I had chosen A for this thinking it was a very simple problem. I diagrammed the first sentence as UW --> KD. The second sentence seemed to say ~KD, thefore A, ~UW. Thought I nailed this one on my test haha. Oh well, live and learn.

[ManhattanPrepLSAT1]

The question stem is asking us what follows logically. Well, obviously the statement following is going to refute that hypothetical conditional in the first sentence or weaken it in some way. The way the argument is going just leads me to think that way. Why would it say "But clearly" then?

I love your instincts here! The word "clearly" indicates that we're moving into a conclusion, and the word "but" indicates that the conclusion will be a rebuttal of something stated earlier. The evidence in this one doesn't refute the conditional though, instead it allows for a clever application of the contrapositive of the first statement. But since we're reversing and negating the conditional in the application, the "but" is the implication that the necessary condition of the first statement has not been met - though as it's stated the claim following "but clearly" does not actually challenge the condition.

Nice work daniel.g.winter! Process and a technical understanding of what's happening can help you with future questions, but your instincts will be more valuable in guiding timely decisions.

[shaynfernandez]

Like many posters this question completely threw me off and even after spending a while looking at it I am still not sure I understand exactly what we are doing. When I initially read this I saw it was a inference question with formal logic. So I adjusted and pre phrased in a since that the two conditional statements in part 1 would yield a "some" inference, because this is frequent in conditional reasoning on late inference questions.

My initial thought was to look for: some who know dictionary definition<---> understand words in the definition

For this is a logical inference (some B <--> some C) from a
A-->B
A-->C
Conditional.

I was extremely surprised that the answers looked like sufficient assumption answers where they offered a trigger and result.

[syousif3]

I chose A but I'm still not convinced with E

Here's what I have:

Underst. a word---> knowing its Dictionary Def.

Underst. a word---->underst. words occur in that def.

And then connecting knowing its Dictionary Def---> underst. words occur in that def.

All babies dont know Dictionary Def of some of the words they utter

But then E says if underst. all the words they utter, then this is not true underst. a word--->knowing its Dictionary Def. How can you infer that.. I really want to understand what I'm missing :/

By uttering a word they are assuming they understand the word correct? So obviously then if understand a word and not know the dictionary definition the necessary condition no longer follows and the 1st conditional statement is no longer valid.. is that right?

[ManhattanPrepLSAT1]

This is a tough one... let me work this one through again.

Lets take the "if" part of answer choice (E) for granted. If we add it to the second statement in the stimulus, the "then" part of answer choice (E) follows logically. The first statement in the stimulus ends up being irrelevant on this one.

"If" part of answer choice (E):
Some babies understand all the words they utter.
B <-s-> UW

Second statement in the stimulus:
All babies do not know the dictionary definition of all the words they utter.
B ---> ~KDD

if we add these two statements together we can infer:
UW <-s-> ~KDD

which says understanding a word does not always involve knowing its dictionary definition.

[robinzhang7]

Hey LSAT Geeks,

I just wanted to confirm my understanding on this problem. The first sentence of the stimulus is an "IF" statement, thereby isn't absolutely true. That is why we can basically disregard it in its entirety? Only the second statement about babies must be taken to be a fact. However, this brings me to my question: I thought that all premises were supposed to be taken as a fact and completely true. Why, then, can we effectively ignore the application of the first sentence?
Because if ALL babies don't know the definition of words (~KD) then wouldn't we have to trigger the following link:

Understand word --> KD into ~KD --> ~Understand word

Then using that ^ , we could make no other inferences so it's time to look at the answer choices - the most attractive being (A) and (E). Wouldn't (A) be correct according to the conditionals we just proved?

I understand your solution, Matt: by using second sentence of stimulus and "if" part of (E). However, I want to know how to use the 1st sentence to also arrive at this answer because to me, (A) seems valid according to the conditionals in the 1st sentence. Thanks!

Understand words <-------some-------> Babies ----> DO NOT understand dictionary definition

[maryadkins]

I just wanted to confirm my understanding on this problem. The first sentence of the stimulus is an "IF" statement, thereby isn't absolutely true. That is why we can basically disregard it in its entirety? Only the second statement about babies must be taken to be a fact. However, this brings me to my question: I thought that all premises were supposed to be taken as a fact and completely true. Why, then, can we effectively ignore the application of the first sentence?

You don't ignore it. Just because it says "If" doesn't mean you can ignore it. It IS true. It's just that what is true may or may not happen.

In this case we're told if understanding a word means knowing it's dictionary definition, then it means understanding the words that make up that definition. Let's symbolize this as:

If UW means Know Dict Def --> Know Words in Def

So the contrapositive would be:

~Know Words in Def--> ~UW means Know Dict Def [Note: This doesn't mean you don't understand the word. It means understanding a word DOESN'T MEAN you have to know its dictionary definition]

We don't know if any of this is true or not. Because it's an "if." It's like being told, "If the world ends tomorrow, you'll do Y." I can take that as 100% true, but we have to wait and see if the world ends tomorrow.

This is what is happening here. The first part isn't triggered until we know that "UW means Know Dict Def." So in (A), we have no clue if they understand the words they utter or not. We don't know if the first sentence in the stimulus even applies. It's not that it DOESN'T apply. It's that we have no clue whether it does or not, and so (A) is not a valid inference.

[ganbayou]

Hello,
Thank you for the explanation, but actually I do not understand the bold part.
Do we have to do this? I thought by combining the second sentence and if part of E, we can infer the rest part of E and our job is done there.
What is this bold part for?
Why do we need to rebut?
Thank you

[ohthatpatrick]

You're correct. Matt was just demonstrating via conditional logic how the 2nd sentence + the if-half of (E) would prove the latter half.

Since you understood that fine without conditional logic, no need to prove it.

We're all just saying that if you have these two ideas:
babies utter words for which they don't know the dictionary definition
+
some babies understand all the words they utter

then you can prove
understanding a word doesn't ALWAYS involve knowing its dictionary definition