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How does (D) weaken the argument? Isn't the stimulus basically: "When someone lies--> Lie-detector tests can determine"?
(D) says that sometimes people who aren't lying could produce physiological reactions identical to the ones that accompany the act of lying, but that doesn't make lie-detectors any less likely to determine when people DO lie (at least that's what I thought).. Am I missing something here? Can anyone help?

The argument here is:

Lying accompanied by physiological reactions -> Lie detector tests that detect these reactions are sure way to determine when someone is lying

(D) Weakens the answer by explaining that physiological reactions are also caused by the act of taking a lie detector test. The key here is the absolute "sure." Had the stimulus been less absolute in its conclusion, (D) would not be a credited answer choice.

Which answer choice did you choose as the credited choice? B, C, & E jump out at me as being completely irrelevant. Did you select A?

Yeah, I chose (A), but now I get how it doesn't really weaken the argument because the argument doesn't rely on the assumption that lie-detector tests measure every physiological reaction that occurs when someone is lying.

But I'm still not clear on how (D) weakens the argument because (correct me if I'm wrong in my thought process here) the stimulus doesn't say that the lie-detector tests are a "sure way" of determining when someone is NOT lying - it only says that they are a sure way of detecting when someone IS lying. So if the detector can determine both a liar AND someone who didn't lie, isn't it still effectively determining when someone is lying?

armedwithamind wrote:The argument here is:

Lying accompanied by physiological reactions -> Lie detector tests that detect these reactions are sure way to determine when someone is lying

(D) Weakens the answer by explaining that physiological reactions are also caused by the act of taking a lie detector test. The key here is the absolute "sure." Had the stimulus been less absolute in its conclusion, (D) would not be a credited answer choice.

Which answer choice did you choose as the credited choice? B, C, & E jump out at me as being completely irrelevant. Did you select A?

Ask yourself, is a lie detector that says everything anyone says is a lie, really a lie detector?
The lsat isn't always a formal logic test. Sometimes you just have to think about it. Introducing information that shows the lie detector is effected by things that aren't lying, weakens the argument that lie detectors are a "sure fire" way of detecting lying.

Clearlynotstefan wrote:Ask yourself, is a lie detector that says everything anyone says is a lie, really a lie detector?
The lsat isn't always a formal logic test. Sometimes you just have to think about it. Introducing information that shows the lie detector is effected by things that aren't lying, weakens the argument that lie detectors are a "sure fire" way of detecting lying.

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