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Identifying Assumptions: *Ask* Yourself About New Concepts in the Conclusion

Logical Reasoning

I guess I was assuming the argument already said that ...

Does that sound like you?

If so, you might benefit from adding another step in your process to Necessary Assumption questions (and other assumption-related questions, like Flaw, Strengthen/Weaken, Sufficient Assumption):

Ask whether there is a "new" concept in the conclusion.

This is because one of the most important aspects of identifying assumptions is noticing concepts in the conclusion that are not mentioned or logically covered in the reasoning.

If the premises are about one kind of investment being more secure than another, but the conclusion talks about the investment being “secure”, then you just have to notice that shift from a relative concept (“more secure”) to an absolute one (“secure”).

If the premises say that the students are spending less time in class, and the conclusion says that they’re spending less time at school, then you need to recognize that being “in class” and “at school” are different. You can be at school without being in class—sitting at the cafeteria tables, playing soccer in the school’s field, for example.

The key idea is that if there is a "new" concept in the conclusion, then the argument must be making some kind of assumption related to it. There may be other assumptions, too, related to gaps between premises, but you can be sure that at least one of the assumptions must be about that new concept in the conclusion.

As good LSAT students, you probably are already familiar with the idea described above. But a lot of people seem to rely mainly on passively noticing new concepts rather than actively thinking about this as a step in solving questions.

Let's work through some examples that increase in difficulty.

Example 1:

person using pencil

Rooney graduated with the highest GPA in the history of our law school.
Thus, she must be good at writing law school exams.

What are the concepts in the conclusion? (1) Rooney and (2) being good at writing law school exams. Are either of these things left out of the premise? Yes - do you see that "good at writing law school exams" is not mentioned in the premise? That means the author is making an assumption about the relationship between having the highest GPA and what that tells us about being good at writing law school exams.

Example 2:

Oftentimes students just fail to notice the difference between two concepts - they make the assumption that the argument itself is making, which is why it's hard to spot that assumption.

Our new neighbor, Xander, was convicted of over fifty murders and has been referred to by local historians as one of the worst serial killers in the United States.
So, we were living next to a murderer this whole time and never knew it!

What are the concepts in the conclusion? (1) We, (2) Living next to, (3) a murderer, (4) not knowing that we were living next to a murderer.

Are any of these new? “We” and “living next to” are covered by the phrase “our new neighbor”. But you might notice that the idea of "not knowing" our neighbor is a murderer is new - the evidence never provides anything related to what we knew about Xander. So the argument is assuming something about our lack of knowledge. What if we actually knew he was a killer before he was found out?

Did you also see that the concept of "being a murderer" is also new? The evidence just refers to being "convicted" of murders and "being referred to by historians" as a serial killer. None of those is the same as being a murderer - what if he's an innocent person who was wrongly convicted and tarred as a serial killer?

Example 3:

Golden Gate Bridge during daytime

Another issue is that sometimes students don't realize something is a new concept because they think that the fact that it was mentioned elsewhere in the stimulus means that it's not new. But in reality, the concept can still be "new" if it's not mentioned in the reasoning that supports the conclusion.

In addition, you might have to translate the conclusion if it uses referential language. You can't identify new concepts in the conclusion unless you've spelled out exactly what the substance of the conclusion is.

Some social theorists claim that San Francisco's large homeless population could be reduced by implementing policies that condition the provision of free food and medical services to the homeless on their staying off drugs and actively looking for a job. However, most of the homeless do not react to incentives in the same way that the average non-homeless member of society would react. Thus, the social theorists' claim is false.

If you break down the argument to premise and conclusion, here's what we get:

Premise: Most of the homeless do not react to incentives in the same way that the average non-homeless member of society would react.

Conclusion: SF's large homeless population cannot be reduced by conditioning the provision of free food/medical services to homeless on the requirement that they stay off drugs and actively look for a job.

Notice that the first sentence about the social theorists' claim is not a premise - it's simply referred to by the conclusion as being wrong. So in my understanding of the argument, the first sentence just disappears - we've translated that into the substance of the conclusion, and that first sentence has nothing to do with the reasoning of the argument. Now we can properly think about new concepts in the conclusion.

First, what are the concepts? (1) SF’s homeless population (2) can’t be reduced (3) by a policy of conditioning free food/medical on staying drug free and looking for a job.

None of these are mentioned in the premise. So, the argument’s assuming something about each of them.

1. San Francisco's homeless? They weren't mentioned in the reasoning. Maybe they are different from the "most of the homeless" in the premise. The argument is assuming that San Francisco's homeless do not react to incentives in a significantly different way from "most" homeless. (Notice that if the premise said "All homeless..." then SF's homeless wouldn't technically be a "new concept" because they would be logically covered by the premise, even if the words "San Francisco" are new.)

2. The whole idea of policies that condition food/medical services on requiring them to stay off drugs or look for a job—where is that coming from? The premise doesn't say anything about it. So the argument must be assuming that these kinds of policies relate to how the average non-homeless person reacts to incentives. It's assuming that having the conditions of staying drug free or getting a job would be things the average non-homeless would react to differently from most homeless. If this weren't true—if the homeless and non-homeless reacted in the same way to these conditions, then the premise would have nothing to do with the conclusion because they'd be talking about two different things.

3. Reducing homeless populations? Does the premise say anything at all about reducing homeless populations or what is required for that? No. So the argument is making some kind of connection between the different reactions that homeless people have to incentives and the reduction of homeless populations. It's assuming that the policies in question—conditioning food/medical services on drug-free/look for job—can reduce homeless populations only if they work through incentivizing the homeless in some way. If the policies could reduce homeless populations in a way that didn't relate to incentivizing them, then the premise (which was only about incentives), would have nothing to do with proving the conclusion. What if, for example, the policies could reduce homeless populations by stirring the moral fiber of SF's populace, who would find the policies draconian and cruel and as a result band together to provide housing to SF's homeless? The argument is assuming that this isn't a possibility.

I hope this helps if you're having trouble with assumptions and always find yourself thinking "there's no way I would have noticed that..." Maybe one reason you're not noticing it is because you're not explicitly identifying key concepts in the conclusion and asking whether they were mentioned or logically covered by the premises?

If you're reading quickly and uncritically, the difference between QOQOOQOQ and QOQOQOOQ might not stand out. But if you actually examine each set of letters and explicitly ask "Are these the same?" Then it's a lot easier to see where the difference is.

Future LSAT Reading Comp. Passages

This section of the post collects interesting online articles that you might see edited down to an actual LSAT RC passage in the distant future. Read these recommendations every week and I can guarantee that your RC score will improve or you'll learn something interesting about the world, or both.
🧠 Psychotherapy is meant to help. But what about when it harms? | Aeon Essays
📚 Literary festivals: sheer hell in a tent | Alexander Larman | The Critic Magazine
🧟‍♀️ The Problem with Zombie Constitutional Amendments | Harvard Law Review
🦣Why ‘De-Extinction’ Is Impossible (But Could Work Anyway) | Quanta Magazine