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June 2007, Logical Reasoning 2, Question 9


Question nine. When a question ask you for an assumption on which the argument depends, you're dealing with a necessary assumption question. In a necessary assumption question, your job is to find something that the argument requires in order for it to be true, something that if it weren't true, the argument would fall apart, the evidence would no longer be able to prove its conclusion.

There can be a lot of necessary assumptions, we just have to find one that appears in the answers. But in order to find them, we need to understand the argument as a whole. So break it down into its conclusion and its evidence. The conclusion of this argument is actually stated twice, once in the first sentence and once in the last.

Basically, the Tasmanian tiger doesn't exist anymore. It's gone extinct. We get two pieces of evidence to back this up. The first is that sheep farmers drove the Tasmanian tiger off of its natural habitat. The second is that since then, naturalists have been working hard to find evidence and haven't found any hard evidence that there's any tigers left.

Now that little sentence in black, in spite of the alleged sightings, that's a little piece of counter evidence that doesn't play a direct role in proving this conclusion. There are two major assumptions in this argument. First, that if the evidence were there, the naturalists would have found it. So naturalists are good enough at looking for traces of Tasmanian tigers that they would have found any evidence that was there.

And second, the tigers aren't hiding somewhere else in the area or somewhere else in the world. Like if there's a colony of Tasmanian tigers who are living in Cincinnati, then not finding them in Tasmania doesn't prove that they are extinct. Without either of those things, the argument won't work. They are necessary assumptions, so we go into the answer choices looking for something on one of them.

Answer choice A, sheep farming drove the last Tasmanian tigers to starvation by chasing them from their natural habitat. Now this would be helpful to know, but it's not required. The argument doesn't require any particular way that the sheep farmers killed off the Tasmanian tigers. It doesn't need to be starvation from chasing.

It could be by exposing them to diseases. It could be by shooting them in the head. We don't need it to be any one particular way. Just we we do need the tigers to not be there anymore. So answer choice A is not the answer. Take a look at B.

Some scavengers in Tasmania are capable of destroying tiger carcasses without a trace. Well, this would actually be harmful to the argument. If there are scavengers that can destroy tiger carcasses, then the naturalist not finding tiger carcasses doesn't really prove anything. So this would be bad for the argument.

It's not required for the argument. So go to answer choice C, every naturalist working in the Tasmanian tiger's natural habitat has looked systematically for evidence of the tigers survival. Be careful of extreme answer choices in assumption questions. We don't need it to be every naturalist who's looked. There could be a couple of slacker naturalists, and that wouldn't be a problem for the argument.

We also don't need everybody to have looked systematically. We do need it to not be just one guy who looked around his apartment to see if there was a Tasmanian tiger there. But it doesn't have to be every single naturalist for us to believe that naturalists haven't found any evidence of them. So C is not our answer.

Go to answer choice D. The Tasmanian tiger did not move and adapt to a different region in response to the loss of habitat. Now, this is something that we have to believe, because if they move to a different region, then the fact that we haven't found them in this region doesn't prove anything.

Necessary assumption is something that needs to be true for the argument to work. If this isn't true, the argument won't work. So D is my answer. Just glance at E to find out what's wrong with it. E says that those who have reported sightings of the Tasmanian tiger are not experienced naturalists.

The people who've reported sightings, we don't need anything in particular to be true of them other than that they are wrong. So we don't need them to be rank amateurs. We don't need them to be amateur naturalists. We don't need them to be semi-skilled naturalists, and we don't need them to be or not be experienced naturalists.

Just as long as they're wrong, it's fine. So answer choice E is not something we need, and D is the answer.

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