manishcmu wrote:
GMATNinja wrote:
surbhi1991 wrote:
Hi
How is B Not correct?
Also If we negate E, It doesn't negate the conclusion.
To understand the problem with answer choice (B), first take another look at the conclusion of the passage:
Quote:
it is impossible to know any mathematical proposition to be true.
Note that the conclusion deals exclusively with
mathematical propositions. Keep this in mind while reading answer choice (B):
Quote:
(B) Observation alone cannot be used to prove the truth of any proposition.
The key word in this answer choice is "
any." Answer choice (B) would apply to a broader range of propositions than just the mathematical propositions mentioned in the conclusion (artistic propositions, philosophical propositions, culinary propositions... or whatever). These other types of propositions are not relevant to the conclusion of the passage. Answer (B) does not provide any additional links between the evidence and the specific conclusion of the passage, so it is not an assumption upon which the author relies.
For (E), you don't really need to use the negation technique. (More on the limitations of the negation technique
here.) Instead, you could think of it this way: if (E) is assumed, will the conclusion logically follow from the facts given in the passage? Here's one of those key facts again:
Quote:
No mathematical proposition can be proven true by observation.
And here is answer choice (E), our potential assumption:
Quote:
(E) Knowing a proposition to be true requires proving it true by observation.
Now we know from the passage that mathematical propositions
cannot be proven true by observation. We also know that knowing a proposition to be true
requires proving it true by observation. It follows that "it is impossible to know any mathematical proposition to be true."
Answer choice (E) has provided the missing link between the evidence and the conclusion, and so it is the correct answer.
I hope this helps!
Hi
GMATNinja, even though I picked E, I believe that we need
mathematical proposition instead of
a proposition. The argument is about
mathematical proposition. By saying a proposition, we are including all kinds of proposition which is not required by the argument. We need something bare minimum. What is your view on this?
Think about it this way: the question asks "The conclusion follows logically if which one of the following is assumed?"
So, we need an answer choice that, if stuck into the passage as evidence, makes it so the conclusion MUST be true. Broken down, it should look something like this:
- Fact #1 from the passage
- Correct answer choice
- Given the two things above, conclusion that MUST be true
Let's try that out with answer choice (E):
- Fact from passage: "No mathematical proposition can be proven true by observation."
- Answer choice (E): Knowing a proposition to be true requires proving it true by observation.
- Conclusion: It MUST follow that "it is impossible to know any mathematical proposition to be true."
You are correct that (E) applies to all propositions and not just mathematical propositions. However, that is not a problem for this exact question -- the conclusion "follows logically" if (E) is assumed, so that is the correct answer. A slightly different question (for example, "on which assumption does the argument depend?") would require a bare minimum answer choice.
Given all of that, I'll amend my explanation of answer choice (B) using the same technique:
- Fact from passage: "No mathematical proposition can be proven true by observation."
- Answer choice (B): Observation alone cannot be used to prove the truth of any proposition.
- Conclusion: MUST it follow that "it is impossible to know any mathematical proposition to be true"?
Here we learn that observation alone cannot prove the truth of a proposition. But what about other methods of proof? For example, what if mathematical propositions could be proven true just with theoretical calculations? In that case, it
would not follow that "it is impossible to know any mathematical proposition to be true." Because the conclusion does not necessarily hold true if (B) is assumed, (B) is not the correct answer choice. (E), on the other hand, offers no wiggle room at all -- the conclusion MUST follow if (E) is assumed.
Rashed12 wrote:
Would anybody explain why D is not correct?
Eliminating (D) is tricky, and comes down to how the phrase "only if" affects the meaning of the sentence. For a good explanation of "if" vs. "only if," please check out
this article.
Take another look at (D):
Quote:
(D) Knowing a proposition to be true is impossible only if it cannot be proven true by observation.
Based on the logic explained in the article linked above, we can reconstruct this answer choice to read as follows: "IF knowing a proposition to be true is impossible, THEN it cannot be proven true by observation."
We
cannot infer the inverse, that "IF a proposition cannot be proven true by observation, THEN knowing that proposition to be true is impossible." This would be the more relevant construction, because it would prove that the conclusion follows logically from the answer choice and the evidence in the passage. For this reason, (D) is out.
I hope that helps!
close
24 Dec 2018, 07:17
53%
(02:05)
wrong 
