## PrepTest 73, Logical Reasoning 1, Question 8

### Transcript

Question eight, when a question asks for what the argument requires, assuming, you're doing a Necessary Assumption question, which means that you have to find something that the argument needs but doesn't say already. A missing piece of the argument that is needed to help connect the evidence to the conclusion. Something that if it's not true, the argument won't work.

Of course, in order to find that, you have to understand the argument, which means knowing what it is trying to prove and the evidence it gives to prove it. Its conclusion and its evidence. The conclusion here comes at the end after the big long conclusory key phrase. This example shows that the thing that it shows is the conclusion which is eyes are adapted only to an animal's needs rather than to some abstract sense of how a good eye would be designed.

So they're trying to prove that eyes only evolve as much as they need to evolve to meet the animals needs. But the evidence for that is the case of the box jellyfish. They have eyes that have lenses that are able to make very sharp images. The retinas are bad. They can only give them blurry details.

So there is a gap between the evidence and the conclusion here, we know that their eyes only produce blurry details, but the conclusion is that the eyes are adapted to their needs. So then the open question is, is what they need a blurry image. If it's not, then the eyes aren't adapted to their needs. If it is, then this evidence could help prove that conclusion.

So the assumption is that the box jellyfish don't need fancy vision that they can get by with the blurry images. So that's what we'll be looking for in the answer choices. Glance down in answer choice A, box jellyfish are the only kinda jellyfish with retinas that do not focus clearly. Well, whether there are other jellyfish that have similar problems wouldn't be relevant.

What matters is here, the jellyfish that we're using as an example. Is it an example of an animal whose needs are met by his eyes? So A's not the answer. B, box jellyfish have a need to detect prominent features but not fine details. Notice they put the blurry part at the end, but this is what we were looking for. Their needs are not fine details, the blurry images.

They tried to trick us by putting it at the end, but we didn't fall for it be as what we were looking for. What's going on with the other answers? Well, C is about what would happen if they had better retinas and it actually would hurt the arguing. If they could use better eyes, if better eyes would be better for their needs, then they aren't evolved to meet their needs.

C would be bad for the argument. It's not something the argument requires assuming. Answer choice D, this is about where the jellyfish came from, where they came from, doesn't matter. What matters is are they as far evolved as they need to be? And then finally, E, exactly how the jellyfish use their eyes doesn't matter.

We need them to use them in whatever way meets their needs. So if they use their eyes to hunt, great, if they use their eyes to navigate, great, if they use their eyes for something else, fine. What matters is that the way that they're using them is for their needs. So E is adding a specification on to the argument that we don't need, which means that answer choice B was our answer.