2.10 - Good / Bad Arguments

Distinguishing between good and bad arguments is crucial to your LSAT success. Here, we're shedding light on the types of arguments you'll encounter on the LSAT and how to evaluate them.

In this lesson, we'll:

• Define valid and invalid arguments
• Use examples to distinguish between the two
• Learn when to attack arguments and why
• Play around with the relationship between validity and truth

Defining Valid and Invalid Arguments

Let's start with some definitions.

Valid arguments prove their conclusions. These are good arguments. They win.

Invalid arguments err in their reasoning—their conclusions do not follow from their premises. These are bad arguments. They lose.

These terms differ ever so slightly from their formal logic definitions. In day-to-day life we're free to (and should) call BS on arguers' mistaken premises and bad conclusions.

On the LSAT, however, we must accept even the strangest premises as fact and focus on attacking poorly drawn conclusions.

The LSAT Loves Bad Arguments

This probably comes as no surprise, but the LSAT loves to hit us with bad arguments. The vast majority you'll encounter will make no sense.

Why? Because they test your ability to identify logical flaws and analyze gaps in evidence. High-level LSAT performance depends on your ability to recognize an argument's validity (or lack thereof).

Spotting Bad Arguments

Bad arguments err in reasoning. In other words, the evidence provided doesn't prove their conclusion.

Here's a bad argument that confuses sufficient for necessary:

Premise: If you get a high LSAT score, you will earn a scholarship.
Premise: You earned a scholarship.
(Flawed) Conclusion: You must have gotten a high LSAT score.

Recall from our Sufficient vs. Necessary lesson how this argument commits a mistaken reversal. That is, you met the requirement for getting a scholarship, but that's not enough on its own to guarantee you'll get one.

Here's another example, this time confusing correlation for causation:

Premise: Children who play video games have better hand-eye coordination.
(Flawed) Conclusion: Playing video games causes better hand-eye coordination.

If your reaction was, "Wait, what?", then good! You probably read our Correlation vs. Causation lesson. If not, check it out.

What if good hand-eye coordination causes children to be better at (and therefore probably enjoy) video games? What if good hand-eye coordination and video game playing are both caused by having great spatial cognitive skills? These what-if alternatives poke holes in the argument's flawed reasoning.

These examples make different mistakes, but they reach the same result: conclusions that don't logically follow from their premises.

Spotting Good Arguments

What do good arguments look like, then? Their conclusions logically follow from the premises.

Let's look at another example:

Premise: All cats have tails.
Premise: I have a cat named Luna.
Conclusion: Luna has a tail.

Before you bite my head off about your tail-less cat, remember: on the LSAT, you must grant the premises. In this context, it's an undeniable fact that all cats have tails.

Cats have tails. Luna's a cat. She's got a tail. Easy peasy—valid argument.

Here's another:

Premise: If it rains, then the ground gets wet.
Premise: It's raining.
Conclusion: The ground is getting wet.

We start with a conditional: when it rains, the ground gets wet. We follow up with a weather report: it's raining. The result? Some soggy ground. Boom! Valid.

When to Attack Arguments (and Why)

Remember, most arguments on the LSAT suck.

In fact, as you improve your understanding of validity, you'll start to realize just how infrequently the LSAT uses valid logic. Watch your LSAT teachers—they'll be surprised when they read a valid argument.

And it makes sense, right? Pointing out and objecting to nonsense is what lawyers do. They catch the mistake. They find the loophole.

So when should you attack arguments? Short answer, pretty much always.

When I say "attack" arguments, I mean that it's your job to call out BS conclusions. Every time.

Posture plays a big role here.

By posture, I mean that you want to approach each argument with an extreme degree of skepticism. In practice, this looks like you saying, "Okay, author... I'm willing to grant you that the sky is blue. But why should that mean the sky will be red tomorrow?" It means your inner monologue fighting back against every conclusion that doesn't make sense.

Treat arguments like they just cut you off in traffic. Flip them the figurative bird. Tell them why what they did was wrong.

Adopting an "attack mode" mindset will significantly improve your Logical Reasoning performance.

Playing Around with Validity and Truth

The "accept premises as true" concept can be tough for some LSAT students, especially ones with illustrious academic backgrounds who want to tell the test all the ways it get "the facts" wrong.

Before you label me a snob, know that I used to make this same mistake all the time. So I thought it would be fun (and informative) to play around with these ideas.

Valid (But False IRL)

Consider the following argument:

Premise: All birds can fly.
Premise: Ostriches are birds.
Conclusion: Ostriches can fly.

You don't need to visit your local petting zoo to know that ostriches can't fly. This argument's based on a premise that's false in reality—that all birds can fly. But in LSAT terms, this argument is 100% valid. That is, if we grant the premises, the conclusion logically follows.

Before you start to worry that you'll frequently encounter stuff like this on the test, relax. You won't. I can't recall any anecdotal examples of obviously untrue premises from official questions off the top of my head.

This exercise was just meant to have some fun while deepening our understanding of validity.

Invalid (But True By Accident)

Now, let's rework our ostrich example. Imagine I made this argument:

Premise: All birds can fly.
Premise: Ostriches are birds.
Conclusion: Ostriches can't fly.

Again, we're in an LSAT mindset. We're granting premises and attacking conclusions. 

If we run into an argument like this on the test, it's incumbent on us to recognize the gap between the evidence and the conclusion. But we must also recognize that this argument, despite its bad logic, ends up being true. The author was right by accident. We can't conclude that someone's bad logic prevents them from being right.

You'll encounter flaw questions that make this mistake on the actual test. So remember, bad logic doesn't necessarily lead to an untrue conclusion.

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That's a wrap on good and bad arguments. I encourage you to go practice attacking some bad arguments while these ideas are still fresh.

But what would you add here? What would remove? Leave me a comment to help me help you.

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Next, we're tackling another critical concept that students often mess up: knowing what the question wants you to do. See you there!