All intelligent people are nearsighted. I am very nearsighted. So I must be a genius. Which one of the following exhibits both of the logical flaws exhibited in the argument above?
(A) I must be stupid because all intelligent people are nearsighted and I have perfect eyesight.
(B) All chickens have beaks. This bird has a beak. So this bird must be a chicken.
(C) All pigs have four legs, but this spider has eight legs. So this spider must be twice as big as any pig.
(D) John is extremely happy, so he must be extremely tall because all tall people are happy.
(E) All geniuses are very nearsighted. I must be very nearsighted since I am a genius.
my take on this is: B
please explain the answer.
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- RonPurewal
- Students
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- JonathanSchneider
- ManhattanGMAT Staff
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Correct answer: D.
We are asked to mimic the 2 logical flaws in the argument. These are:
wrong direction: we are told that all intelligent people are nearsighted. we are NOT told that all nearsighted people are intelligent. we cannot assume that this is a two-way street.
wrong amount: we are told that intelligent people are nearsighted. we are NOT told that nearsightedness and intelligence increase in direct proportion. we cannot assume that the more nearsighted, the more intelligent.
Only D mimics both flaws. We are told that tall people are happy. We are NOT told that all happy people are tall; and we are NOT told that the taller you are, the happier you are.
We are asked to mimic the 2 logical flaws in the argument. These are:
wrong direction: we are told that all intelligent people are nearsighted. we are NOT told that all nearsighted people are intelligent. we cannot assume that this is a two-way street.
wrong amount: we are told that intelligent people are nearsighted. we are NOT told that nearsightedness and intelligence increase in direct proportion. we cannot assume that the more nearsighted, the more intelligent.
Only D mimics both flaws. We are told that tall people are happy. We are NOT told that all happy people are tall; and we are NOT told that the taller you are, the happier you are.
- san
Note carefully that this question refers to two logical
flaws that are in the argument. It requires the test taker to
select the option that exhibits both of those flaws. It is
worth a bit of time to make clear what the two flaws are.
The first sentence identifies a group, intelligent people,
and says all its members have a particular characteristic,
nearsightedness. In the second sentence, the speaker admits
to being nearsighted then concludes, in the third
sentence, that he or she must be a member of the group
of intelligent people (a genius). The justification for this
inference, presumably, is that since the speaker has one
characteristic that belongs to all members of a certain
group, then the speaker also belongs to that group. This
is one flaw. The error can be seen by noting that the first
sentence speaks of all intelligent people but not of all
nearsighted people. So we cannot legitimately infer from
this statement that any particular nearsighted person,
here the speaker, is (or is not) intelligent.
The second flaw also arises from attributing something
to the first sentence that is not legitimate. The
member of that group. (B), however, does not commit
the intensification flaw seen in the original. So, responses
(A) and (B) each exhibit one of the flaws in the original
argument, but neither of them exhibits both.
Response (C) illicitly presumes that size is directly
proportional to the number of legs a creature has, and
so concludes that the spider with eight legs is twice as
big as a pig which has four legs. This is an interesting
mistake, but not the two errors committed by the
original argument.
(D) is the correct response. Even though the order of
presentation is different than the original argument, the
structure of the reasoning is the same, and exhibits both
of the flaws. (D) makes an inference from "all tall people
are happy" and "John is extremely happy" to "he
must be extremely tall." This is to take a characteristic
belonging to all members of a class as sufficient indication
that an individual having that characteristic is also
a member of that class. It also infers from the intensification
of that characteristic to the possession of an
intense degree of the defining characteristic of the class.
(D) thus exhibits both flaws seen in the original argument;
it is the credited response.
Response (E) exhibits neither of the flaws. Indeed, it is
a valid argument. Note that the intensification appears
in the main premise: all geniuses, i.e., very intelligent
people, are said to be very nearsighted. And the conclusion
states of one particular genius, the speaker, that he
or she is very nearsighted. This of course must be true if
the first premise is true. So (E) is incorrect.
flaws that are in the argument. It requires the test taker to
select the option that exhibits both of those flaws. It is
worth a bit of time to make clear what the two flaws are.
The first sentence identifies a group, intelligent people,
and says all its members have a particular characteristic,
nearsightedness. In the second sentence, the speaker admits
to being nearsighted then concludes, in the third
sentence, that he or she must be a member of the group
of intelligent people (a genius). The justification for this
inference, presumably, is that since the speaker has one
characteristic that belongs to all members of a certain
group, then the speaker also belongs to that group. This
is one flaw. The error can be seen by noting that the first
sentence speaks of all intelligent people but not of all
nearsighted people. So we cannot legitimately infer from
this statement that any particular nearsighted person,
here the speaker, is (or is not) intelligent.
The second flaw also arises from attributing something
to the first sentence that is not legitimate. The
member of that group. (B), however, does not commit
the intensification flaw seen in the original. So, responses
(A) and (B) each exhibit one of the flaws in the original
argument, but neither of them exhibits both.
Response (C) illicitly presumes that size is directly
proportional to the number of legs a creature has, and
so concludes that the spider with eight legs is twice as
big as a pig which has four legs. This is an interesting
mistake, but not the two errors committed by the
original argument.
(D) is the correct response. Even though the order of
presentation is different than the original argument, the
structure of the reasoning is the same, and exhibits both
of the flaws. (D) makes an inference from "all tall people
are happy" and "John is extremely happy" to "he
must be extremely tall." This is to take a characteristic
belonging to all members of a class as sufficient indication
that an individual having that characteristic is also
a member of that class. It also infers from the intensification
of that characteristic to the possession of an
intense degree of the defining characteristic of the class.
(D) thus exhibits both flaws seen in the original argument;
it is the credited response.
Response (E) exhibits neither of the flaws. Indeed, it is
a valid argument. Note that the intensification appears
in the main premise: all geniuses, i.e., very intelligent
people, are said to be very nearsighted. And the conclusion
states of one particular genius, the speaker, that he
or she is very nearsighted. This of course must be true if
the first premise is true. So (E) is incorrect.
- JonathanSchneider
- ManhattanGMAT Staff
- Posts: 370
- Joined: Sun Oct 26, 2008 3:40 pm
If B were to have both of the flaws, it would read something like this:
All chickens have beaks. This bird has a very large beak. Therefore, this bird must be an extremely chicken-y chicken.
The first flaw is one of direction. Just because all A's are B's does not mean that all B's are also A's.
The second flaw is one of severity. Just because all A's are B's does not mean that a very A A will be a very B B.
All chickens have beaks. This bird has a very large beak. Therefore, this bird must be an extremely chicken-y chicken.
The first flaw is one of direction. Just because all A's are B's does not mean that all B's are also A's.
The second flaw is one of severity. Just because all A's are B's does not mean that a very A A will be a very B B.
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