LSAT Kung Fu Blog / An Introduction to LSAT Logical Reasoning

An Introduction to LSAT Logical Reasoning

Stevie Wonder

Like Stevie Wonder, you too can become an expert in LSAT Logical Reasoning.

So, you’ve heard somewhere that the LSAT is a test that demands that you successfully weigh evidence against claims. OK, sure. But what does that mean

First, let’s establish some ground rules: 

A. The LSAT is a test of reasoning, not of research. You will be asked to attack, defend, analyze, and identify the claims made in the arguments provided on the LSAT. You will never be asked to attack, criticize, or question the evidence provided in those arguments. In other words, we will stipulate the truth of everything offered as evidence on this test. Thus, if an argument claims that Melissa is from Venus on the basis of evidence that some girls are from Venus, then we may demonstrate that the claim is unproved by the evidence, but we must also happily assent to the idea that there are some girls from Venus (even if we know in our hearts that that isn’t true, really). Take as true every piece of evidence you are offered on the test. 

B. The purpose of every argument on the LSAT is to prove some claim completely by appeal to some evidence. An argument’s success, then, is measured not by how convincing or persuasive it is; an argument on the LSAT is successful if but only if its evidence proves its central claim. Analytically speaking, a claim has been proved if but only if the following criterion has been met: That it is impossible for the claim to be false if the evidence used to support it is true. 

C. We can usefully divide the world into two groups: 1. Good (or successful) arguments, and; 2. Everything else. 

[Editor’s note: In the interest of presenting a clear and comprehensible course of materials, we will generally eschew the traditional jargon of philosophy and of formal logic. Therefore, informally, we will use the term “good argument” to denote a correctly stated – that is, a logically sound and valid – deductive argument. We will also rather loosely take the phrase “bad argument” or “weak argument” to include all flawed deductive arguments, but also all inductive arguments. While some arguments on the LSAT are fair examples of inductive reasoning and others are more like deductive arguments that are missing key deductions, for our purposes we can safely put aside this distinction and proceed by discussing the salient aspects of all “good” – or deductive – arguments. Thank you for your cooperation. You are very forbearing.] 

So what makes a good argument? 

In conversation, we use the phrase “chain of reasoning” to refer to the method by which a person’s argument coheres. Let’s take that metaphor and run with it for a minute. 

Consider a tow-chain. Imagine that it is made of thick steel links, each one wrapped around the next, each one welded strong in a circle. Can you see it? 

Now, take a link out of it, anywhere along its distance – it doesn’t matter where. And you don’t have to remove a whole section to see how this works – just take out one link. What can you tow with this chain now? 

The answer is blindingly obvious – a chain that’s missing even one link is useless as a tool. And it’s not that the missing link is somehow more important than the other links. It’s just another link. Or it would be, if it were there. But without it? Well, unless a chain is joined together by each and every one of its links, it’s a flawed chain. If it’s missing even one link along its distance, it is broken and is therefore absolutely worthless. 

OK. An argument is exactly like that chain. In order for a tow-chain to be of service, it must meet two basic criteria – it must be strong, and it must be complete. On the LSAT, we’ve already covered the strength criterion: If we’re told something, then it is true. Each link in an LSAT argument is strong. 

It’s that second part that’s the problem with arguments on the LSAT. They are almost never complete. So we’ve established that in order to work, an argument must be complete. Well, what does that mean? Basically, that means that above all, a good argument is a boring argument. 

A good argument makes no leaps, takes no chances, leaves nothing to the imagination. A good argument moves inexorably, point-by-point, step-by-step, from each premise to a conclusion that is, indeed, foregone. 

The conclusion of a good argument, then, MUST BE TRUE. For a good argument, if the premises are true, so also must be the conclusion. And on the LSAT, the premises are always true. It’s just that there are never enough of them. 

Consider this example: 

1. Poodles are Dogs.
2. Dogs are Animals.
3. Animals are not Vegetables.
C: Therefore, Poodles are not Vegetables. 

See what we mean? Boooring! A real yawner. But it is indisputably true. Now let’s make it look more like your typical LSAT argument: 

1. Poodles are Dogs.
2. Dogs are Animals.
C: Therefore, Poodles are not Vegetables. 

And the thing here is, the test writers are preying upon your natural inclination, just by virtue of being a human, to fill in that obvious gap by thinking to yourself that “Gee, it’s obvious – animals aren’t vegetables.” It is precisely the fact that the gap is so obvious that trips us up! 

Tht fct is dmnstrtd by ths xmple – yu arn’t hvng any trble rdng ths nnsnse sentnce, r yu? 

Yet, almost all of the vowels were missing. The amazing human brain, right? And the writers of the LSAT know this, and they’re counting on you to fill in these clear, obvious gaps in logic by supplying the missing details yourself. They know you want to. 

And that’s just what they want you to do. Remember this, because it’s important – We know nothing unless we’re told. We know nothing unless we’re told. Nothing. 

And you know how our true, good argument was deathly boring, how it told us so very little, how it was soooo anti- climactic? That’s what a good argument should look like. 

Now, you try it. Which of the following are good arguments, and which are not? When they’re not, how could you make them cohere? (And remember, if it’s stated as a premise, it is true). 

A)    1. Cats are made out of rubber bands and used car parts. 

    2. Nothing containing used car parts as a component needs to be fed. 

    C: Cat owners don’t spend much on cat food.

B)    1. Pickles taste like salmon. 

    2. Anything that tastes like salmon is green. 

    C: Pickles are green.

C)    1. Razorblades are sharp. 

    2. Dangerous things are good for children. 

    3. All things that are sharp are dangerous things. 

    C: Razorblades are good for children.

D)    1. God is love 

    2. Love is blind 

    3. Stevie Wonder is blind 

    C: Stevie Wonder is God. 

While we’ve tried with the above examples to add evidence until the claims follow logically, in practice, our primary job on the LSAT is to determine why and how the arguments we’re given fail. That is, we must be prepared to determine where an argument is vulnerable and to articulate that vulnerability. We will then be prepared to exploit its weakness to our advantage (you may enjoy practicing on your friends, relatives, and neighbors).

Questions? Comments? Complaints? Post them below, or shoot me an email.

Be good to one another, for we need it now more than ever,


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