June 2013 LSAT Section 1 Question 14

Investigators have not proved that the forest fire was started by campers. Nor have they proved that lightning trigge...

ikarus on August 6, 2020

What's the flaw?

I read through the other threads for an explanation but came up lacking. I was able to get the correct answer but do not understand what the flaw is exactly. What I am reading is: P1: have not proven the fire was started by campers P2: have not proven the fire was started by lightning C: therefore, have not proven the fire was started by campers or lightning But this leaves open the possibility that it could still be proven that it was campers or lightning that caused the fire. It almost feels like there is a typo in the question stimulus, as if the conclusion should read: "So the investigators CAN not prove that the blaze was caused by campers or lightning." This replaces the the "have" with a "can" and rules out possibility that campers or lightning could still be proven to be the cause of the fire. This, then is a recognizable logical fallacy.... just because we haven't done something yet, doesn't mean we won't be able to do it in the future. What am I missing?

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shunhe on August 6, 2020

Hi @ikarus,

Thanks for the question! The flaw is basically that the argument went from concluding that “Can’t say for sure it’s A. And can’t say for sure it’s B. So can’t say for sure it’s A or B.”

In other words, let’s say we have a university that only has two majors: philosophy, and mathematics. John enrolls in this university. Now, we don’t know if John is a philosophy major. We also don’t know if John is a math major. Does that mean we also don’t know that John is either a philosophy major or a math major? No, it doesn’t, John has to be one of the two! We’ve narrowed it down to these two options, even if we don’t know which exactly it is. But the argument makes the flaw of saying that if we don’t know it’s either option, we can’t know that those are the only two options. The investigators haven’t proven the campers started the fire. They also haven’t proven that lightning started the fire. Thus, the argument concludes, the investigators can’t have narrowed it down to either the campers or lightning. That’s the flaw with this argument that is then replicated in the correct answer choice.

Hope this helps! Feel free to ask any other questions that you might have.

ikarus on August 6, 2020

@Shunhe, I understand what you're saying IF it is limited to those two options. However, I don't see anything in the stimulus (or in the correct answer choice) that actually limits it to only those two options. Couldn't it have been arsonists or plane crash that started the fire? Couldn't there be a third individual running for election in the correct answer choice?

shunhe on August 7, 2020

Maybe. But it also could've been just those two things. Going from the first two premises to say "It's definitely NOT just those two options" is a logical flaw.

Mazen on February 22, 2022

Does the flaw explained above fall under the umbrella of "using the absence of evidence as evidence for the absence of" flaw, or is it a distinct classification in its own right?

Mazen on February 22, 2022

Please disregard the post above. I figured it out!
Thank you