Necessary assumptions | Video lesson | Logical Reasoning | LSAT | Khan Academy

To determine what kind of question this is, let's jump below the passage to the question itself. Which one of the following is an assumption required by the economist's argument? This idea of an assumption being required is our signal that we are dealing with a necessary assumption question. So the answer is gonna be a claim that has to be true in order for the argument to have a chance to work. The other four choices will be claims that just aren't necessary to the argument's survival. Either because they're not relevant or they're too strong to be necessary or some reason like that. So pause your video now if you'd like to try this question on your own. Otherwise, let's move on to the explanation. Okay, let's read the stimulus together. When reading for a necessary assumption, you'll want to really understand the main conclusion and evidence of the speaker's argument. And I want you to see if you can detect any kind of gap between the conclusion and evidence. So the economist says, our economy's weakness is the direct result of consumers' continued reluctance to spend, which in turn is caused by factors such as high-priced goods and services. This reluctance is exacerbated by the fact that the average income is significantly lower than it was five years ago. Thus, even though it's not a perfect solution, if the government were to lower income taxes, the economy would improve. Let's mark our conclusion first. We have a pretty nice keyword here with us. And just to double-check that it's actually our main conclusion, we can confirm that everything coming before it is meant to lead up to or support this conclusion. So the economist makes a sort of conditional prediction that if the government lowered income taxes, the economy would improve. Okay, why does the economist believe that this would happen? Well, the answer to that question is our evidence. Now, when we're identifying evidence, we don't want to just robotically regurgitate everything that we see in the rest of the passage. We wanna really simplify it and understand it for ourselves. So basically, one piece of evidence is this sort of chain reaction that high prices leads to people not wanting to spend, and that leads to our economic weakness. The second piece of evidence is that that reluctance to spend is made worse by the lower incomes. So to find a gap in an argument, it can be really helpful to look for an idea or concept that kind of comes out of nowhere. So look at our progression here. On the evidence side, we're talking about lower incomes, high prices, reluctance to spend, economic weakness. Okay, what's the star player of the conclusion though? The spotlight of the conclusion is on this recommendation to lower income taxes. Well, wait a minute. We never talked about lowering income taxes in the evidence, but the economist doesn't seem to notice that. That means that the economist is assuming or taking for granted that lowering income taxes is somehow related to something discussed in the evidence. So the economist here believes that if the government lowered income taxes, something from the evidence would happen. What is that thing? Well, income in the evidence is connected to reluctance to spend. We see that in the second sentence. So the economist is assuming that if the government lowered income taxes, consumers would be more willing to spend. Because think about it. If they weren't more willing to spend, this conclusion that the economy would improve doesn't have a good leg to stand on. So it's necessary for this relationship to exist in order for the argument to work. We have a super strong prediction here, and that's a great goal to have for necessary assumption questions. If you have a super strong prediction, you won't get lost in all the choices. So let's find our match. A, increasing consumer spending will cause prices for goods and services to decrease. That's not a match for our prediction, so we can get rid of it. As an extra note, this relationship doesn't work because in the passage, we learn that high prices cause reluctance to spend, but this choice is that willingness to spend causes lower prices. That relationship is backwards. B, if consumer spending increases, the average income will increase. This doesn't match our prediction either, and it's actually backwards. The economist is assuming that a higher income will cause higher spending, but this choice has that relationship backwards, saying that higher spending will cause higher income. C, if income taxes are not lowered, consumers' wages will decline even further. This doesn't match our prediction either, so we can cross it off. Once again, the economist is assuming that a higher income will cause higher spending, but this choice is basically saying that if income taxes don't get reduced, people's income will keep falling. That doesn't have to be true for the argument to work. Let's actually negate this choice and see what happens to the argument. Let's say that if income taxes are not lowered, consumers' wages won't decline even further. So we didn't lower taxes, but people's wages stopped dropping. That doesn't hurt the argument because we still don't know what would happen now that people's wages stopped dropping. Will they spend? Will they not spend? Since this doesn't affect the argument, it can't be a necessary assumption of the argument. Just a quick note of clarification on negation before we move to the next choice. If you negate a choice for a necessary assumption question and negating that choice weakens the argument, then you have the answer. What it means is you took away something that was necessary to the original argument. That's why the argument was weakened. If you negate a choice and it does nothing to the argument or it strengthens the argument, then that's not the answer. Think about it this way. Let's say that I believe that water is necessary for a plant's survival. If I wanted to test whether that's really true, what I would wanna do is take water away and see what happens. If that plant is weakened, then yes, water must be necessary. If nothing happens to the plant, then I guess water wasn't necessary after all because I took water away and the plant was still okay. So that's the idea behind testing necessary assumption choices by negating them and then seeing what the impact is on the argument to do so. Okay, let's move on. D, consumers will be less reluctant to spend money if income taxes are lowered. This is a match. This is almost exactly what we predicted. And to test it, let's negate this choice. What if consumers wouldn't be less reluctant to spend money if income taxes are lowered? In other words, let's say they still just don't wanna spend money even though their income taxes were lowered. In that case, the economist's argument falls apart because the lowering of income taxes wouldn't improve this economy that's weak because people aren't spending. So choice D is a necessary assumption and we can select it on test day. For the record, E is irrelevant to the argument because it focuses on government spending, which isn't something that the economist ever addressed. So an idea that's irrelevant to the argument can't be a necessary assumption. It can't be the claim that keeps the argument alive. So to recap, for necessary assumption questions, it's helpful if you can phrase the conclusion and the evidence simply and accurately on test day. Then you can look for the gap between the evidence and the conclusion and make a prediction for what must be true but wasn't stated in order for the argument to work. If you wanna test a choice, you can negate it and see what happens. If negating the choice does nothing to the original argument or makes it better, then that choice is wrong. If negating a choice makes the original argument fall apart, then you've got your answer.