LSAT Logical Reasoning – Understanding the Fundamentals

LSAT Logical Reasoning – Understanding the Fundamentals

Every now and then, I get inquiries from folks who have taken a TestMasters class or read the PowerScore Logical Reasoning Bible, and they really detest LSAT logical reasoning. Somewhere along the line, logical reasoning went from being a simple exercise of evaluating arguments to a confusing mess of question types, diagrams, and funky arrows.

Regaining composure can be difficult, but I have a simple exercise that can help. In 20-30 minutes, with a fair amount of follow-up and practice, people can usually step back out of whatever LSAT mire they’ve sunk into, and get back on track. Your goal is to stop answering questions defensively, and to master the test by thinking like the test-makers. This requires an ability to 1) read an argument, 2) understand its logical flow, and 3) perceive its logical soundness or its logical flaws. More and more, I’m doing this exercise with everyone (not just TestMasters defecters), because it’s helpful in understanding how arguments work on a fundamental level. I’ve attempted to reproduce it here as a conversation – just put yourself in place of the student and read along. Please use the space in the comments with any questions.

The exercise goes something like this:

Me: “So, if you had to drive a fuel-efficient vehicle, what would you drive?”

Student (let’s call her Laura): “I don’t know… a Prius?”

Me: “Okay. What color?”

Laura: “Probably blue.”

Me: “Great. A blue Toyota Prius. And how many miles per gallon do you think you’d get?”

Laura: “50?”

Me: “Sure. So what if I made this argument:

Laura drives a blue Toyota Prius.

Laura gets 50 miles per gallon.

Therefore, if Kyle buys a blue Toyota Prius, Kyle should also get at least 50 miles per gallon.

Me: “So, on a scale of 1 to 10, where 1 is really weak, and 10 is really strong, how strong or weak is my argument?”

Laura: “Eh, somewhere around a 4. It’s not too good.”

Me: “Yeah, there are a lot of unknowns, right?”

Laura: “Sure.”

Me: “So let’s stop and look at what we know.

First, we have some facts. You drive a blue Prius. You get 50 miles per gallon. These are called premises – they are the facts that aren’t really in dispute. Now, we might not know in real life whether a Prius can get 50 miles per gallon, but when it comes to the LSAT, real-world information doesn’t matter. Your job isn’t to worry about factual accuracy, it’s to worry about logical accuracy. So we take the premises (facts) as true – you drive a blue Prius, and you get 50 miles per gallon.

Then we have my opinion, which is that I can buy a car of the same color, make, and model, and get the same gas mileage. This is what we call aconclusion. A conclusion isn’t just a “summary” of the facts – it’s what the author thinks is a logical consequence that follows from (or because of) the facts. If the conclusion seems like it is very likely to follow based on the facts given, we call that a strong argument. If the conclusion seems pretty unlikely, we call that a weak argument. Arguments like the one we’re dealing with are called ‘inductive’ arguments because we can potentially bring in (‘induce’) more information that would help us determine how likely the conclusion is. (We’ll talk about ‘deductive’ arguments later, they’re a little different).

Back to our inductive argument about the Prius. We already said that this is pretty weak argument. What are some of the factors that make it weak?”

Laura: “Well, you don’t know about driving habits, A/C, engine size… a bunch of stuff.”

Me: “Exactly. If you drive entirely in the country, and if I only drive in traffic downtown, would that strengthen or weaken the argument?”

Laura: “Weaken. I’d probably get better gas mileage.”

Me: “Absolutely. What else would weaken the argument?”

Laura: “If I don’t use the A/C, and if you use it on full blast.”

Me: “Great, what else?”

Laura: “If you tow a bunch of weight. Oh, but you wouldn’t do that with a Prius.”

Me: “It doesn’t matter. LSAT questions will ask you to strengthen or weaken arguments. The question will ask you what, if true, would strengthen or weaken. You don’t have to worry about whether a statement actually is true in real life – all that matters is whether, if it were true, it would strengthen or weaken the argument. Here, knowing that I tow a boat, and that you don’t, would certainly weaken the conclusion. All of the statements you’ve given me weaken the conclusion of the argument because they make it less likely. On our 1 to 10 scale, each of these brings down the strength of the argument from 4 to 3 to 2 down towards zero. If I’m towing a boat downtown with the A/C on full blast, odds are I’m not going to get the same mileage as you if you’re cruising in the countryside with the windows down. Right?”

Laura: “Makes sense.”

Me: “Now, what if we wanted to strengthen the argument? One way to do that would be simply to rule out these weakeners. What would that sound like?”

Laura: “Kyle doesn’t use A/C?”

Me: “Well, you’re right, that would strengthen the argument, but that’s going a little further than just ruling out the weakener. How about ‘Kyle doesn’t use any more A/C than Laura does’? That still strengthens the argument, right?”

Laura: “Yeah, it makes A/C a non-factor.”

Me: “Exactly. A lot of times, you’ll strengthen an LSAT argument by ruling out a possible factor. We might also say something like, ‘Kyle’s city-to-highway driving ratio is not higher than Laura’s,’ or ‘Kyle does not tow more weight that Laura.’ Either of these statements rules out a weakener and thus strengthens the argument. Make sense?”

Laura: “Sure.”

Me: “Now, that’s one way to strengthen an argument. Basically, what we’re doing there is putting words to whatever the author has to be assuming when he makes the argument. If I’m making this argument – if I really believe that I’ll get the same mileage as you based just on the color, make, and model, then I have to be assuming that these other factors are non-issues. If I thought that our driving habits or A/C use differed greatly, I probably wouldn’t make this argument, would I?”

Laura: “No, probably not.”

Me: “So these statements are what the LSAT calls necessary assumptions. Given what few facts we know, if I’m going to conclude that I’ll get the same gas mileage as you, it is necessary for me to assume that things like driving habits and A/C won’t affect my conclusion. If we write out these assumptions and add them into our argument, they strengthen the argument. Likewise, if we add in sentences that disprove our assumptions, that would weaken our argument.”

Laura: “Cool.”

Me: “We can also strengthen an argument by adding in things which, even though they aren’t necessary, they would still make the conclusion more likely. That’s where your statement comes in: ‘Kyle doesn’t use any A/C at all.’ Now, I don’t have to be assuming that in order to make this argument, but if that turns out to be true, it certainly helps my argument.”

Laura: “I see.”

Me: “Sometimes, you’ll see answers on the LSAT that just seem crazy. They sound extreme. A lot of the prep courses tell you to avoid extreme language, but the catch is, if you’re strengthening, weakening, or justifying an argument, you might want extreme language. We can talk about ‘question types’ later, but what if I said, ‘All blue cars get the same gas mileage’?”

Laura: “Well, that would strengthen…”

Me: “Yes, it would. And again, real-world information doesn’t matter, right? All that matters is that if we know all blue cars get the same gas mileage, then we can logically conclude that I’ll end up with the same mileage as you. In fact, adding in this sentence strengthens the argument so well that our conclusion becomes inescapable. On our 1 to 10 scale, this argument just became a 10. So here’s the key: any statement that would make your conclusion inescapable like this is what we call a sufficient assumption. It’s not something the author has to assume. But if you bring in the fact that all blue cars get the same gas mileage, then your argument becomes perfect, doesn’t it?”

Laura: “Yeah.”

Me: “So these kinds of statements are still strengtheners, but we can also call them sufficient assumptions. Can you think of any other statements that would be sufficient assumptions here?”

Laura: “What if… all Toyotas got the same gas mileage?”

Me: “Exactly. You could also say something like, ‘The only factor affecting gas mileage is a car’s model’ – that would have the same effect. Often, answers to sufficient assumption questions (and some strengthen questions) are worded with this kind of language on the LSAT. Most of the time, you’ll want to be wary of extreme language, but when it comes to these questions, bring it on. Remember, you’re bringing in additional, outside premises and your goal is to make the conclusion more likely – strong language will certainly do that better than vague language.”

Laura: “Got it.”

Me: “Great, so for homework, why don’t you come up with 2 arguments kind of like mine. Each one should have a couple of premises and a conclusion like mine does. For each argument think of some things that would weaken it. Then, show me different ways of strengthening the argument: statements that would strengthen by ruling out weakeners (necessary assumptions) and statements that would strengthen by making the conclusion inescapable (sufficient assumptions). You might have 1 or 2 strengtheners that don’t fit neatly into “necessary assumptions” or “sufficient assumptions” as well, like “Kyle doesn’t use any A/C” in our example above. Do the best you can, email me what you come up with, and we’ll go from there. Hopefully this will be helpful in understanding the big picture – how arguments work, and what the LSAT is getting at when they ask you to strengthen, weaken, justify, or identify assumptions.”



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