@Indy5 The biggest difference between A & B is that B is too broad. We don’t know from the prompt if there are any other ways to prove something definitively, just that it can’t be proven with a photograph.
What we do know from the prompt is that if a photograph cannot show the whole truth, it is false - it cannot furnish definitive proof. So we know that something that does not show the whole truth does not show definitive proof.
MehranJuly 3, 2018
Hi @Indy5, thanks for your post.
Let's start with a carefully assessment of the stimulus, as we always do in Logical Reasoning.
This stimulus presents an argument. The conclusion is: Nothing can ever be definitively proved with a photograph. (The word "therefore" is a structural indicator that often denotes a conclusion.)
What premises are given in support of this conclusion? 1. Every photo must in some obvious sense be true, because it involves the light rays that something emits hitting film. 2. But no photo can express the whole truth, so in a sense, every photo is false.
Notice that the premises are about what is true or false. The conclusion, however, is about what can be proved. These are different concepts. In many people's minds, these concepts are similar - that's how the LSAT writers trap students. But these are different concepts, and the conclusion here (which is about proof) is *not* supported by the premises (which are about what is true vs. what is false).
The question stem asks us to strengthen the weak argument presented in the stimulus by identifying the missing (necessary) assumption. Remember you can negate the answer choices on a Strengthen with Necessary Premise question - when you negate the correct answer, the argument in the stimulus falls apart.
Answer choice (B) can be eliminated, because it doesn't help bridge the gap between true/false and proof. Negate (B): The whole true may be known. That does not address the gap between true/false and proof.
Answer choice (A), however, sets forth a missing (necessary) premise: whatever is false (in the sense that it cannot express the whole truth) cannot furnish definitive proof. See how this answer choice combines the concept of true/false and the issue of what can be proved? It bridges the gap in the stimulus. If you negate answer choice (A) - "whatever is false CAN nonetheless furnish definitive proof" - the argument in the stimulus falls apart.
Hope this helps! Please let us know if you have any additional questions.
Indy5July 3, 2018
This really helped, thanks!
jskaggsMarch 6, 2019
I am kind of confused why we are using Strengthen with Necessary premises and negating answer choices, when this says, and is indicated by the question stem that it is a strengthen with sufficient premise question...?
Jacob-RMarch 7, 2019
Hi @jskaggs
I certainly agree that the question stem’s phrasing (i.e., “permitâ€) generally indicates that we are looking for an answer that is sufficient. And it is excellent that you are on the lookout for precise language to distinguish between sufficient and necessary questions.
But in this case, no other answer other than what is expressed in answer A can properly close the gap in the logic of the passage in order to get to its stated conclusion. In other words, the “necessariness†of the answer is perhaps indicated by putting the emphasis on “properly drawn†in the stem rather than focusing solely on “permit.â€
But even if you view this as a sufficient question, hopefully you still got to correct answer A, because surely that answer is still sufficient to permit the conclusion! This is one question where that distinction probably doesn’t matter as much, but again, kudos for being on the lookout for that distinction!
megmcdermottSeptember 5, 2019
@mehran I thought that you can only use the tactic that when negate the answer choice the argument falls apart in a Strengthen with Necessary Premise question... isn't this a strengthen with Sufficient Premise question? Can the negation trick be used for both types of questions?
EatenJanuary 11, 2022
I am still confused on this for the same reasons listen above
EatenJanuary 11, 2022
listed*
AbigailJanuary 24, 2022
Hello @eaten and @megmcdermott,
Yes, you are correct that this is a Strengthen with Sufficient Premise question as indicated by the wording in the question stem "permit the conclusion to be properly drawn." You should only use the Assumption Negation Test on Strengthen with Necessary Assumption. The Negation Test only works 100% of the time on Necessary Assumption. It sometimes (like in the example above) will work for Sufficient Assumption questions. But, to be safe, it is generally not a good idea to use it on Sufficient Assumption questions.
