3.21 - Common Flaws

We're closing out our Logical Reasoning chapter exploring some of the LSAT's most common flaws.

You'll encounter these fallacies regularly during your LR practice. Learn to spot them, object to them, and rephrase them in plain English. You'll be well on your way to perfecting Logical Reasoning.

In this lesson we'll:

• List common flaws you'll see in Logical Reasoning
• Provide examples of how these flaws look on the test

Let's dive in.

Quick note: These flaws aren't listed in any particular order.

Sufficient vs. Necessary

Confusing sufficient and necessary conditions is the most common flaw you'll encounter on the test. It's LSAT 101.

This flaw's so important on the LSAT that we did an entire lesson about it.

Here are a few examples of how sufficient versus necessary might show up in the answer choices:

• [This argument] mistakes a condition sufficient for bringing about a result for a condition necessary for doing so
• [The argument] confuses a condition's being required for a given result to occur in one case with the condition's being sufficient for such a result to occur in a similar case
• [The argument] treats a requirement for a product to be popular as something that ensures that a product will be popular

We won't beat this dead horse any further. Go check out the lesson linked above for an in-depth walkthrough on sufficient and necessary.

Correlation vs. Causation

If sufficient vs. necessary is LSAT 101, correlation vs. causation is LSAT 102.

Here's what correlation / causation flaws can look like in the answer choices:

• The argument assumes that a correlation between two phenomena is evidence that one is the cause of the other.
• [This argument] infers a cause from a mere correlation

We beat this one up early on as well. Go check out our correlation / causation lesson if you need a refresher.

Problems with Groups, Numbers, and Proportions

These concepts are LSAT 103. Hence, why we covered them earlier too.

That lesson dived deep on various flaws associated with numbers and groups. Check it out for an in-depth refresher.

Ad hominem

Ad hominem characterizes any kind of attack on an argument's source rather than the argument being put forth.

They're super common in political discourse and popular media, where pundits try to "score points" rather than focus on logic or reasoning.

On the LSAT, we really only get to attack the gap between evidence provided and conclusions drawn.

You know you're dealing with an ad hominem argument whenever the author attacks their opposition rather than their opposition's current argument. This could take the form of attacking their character, things they've said or done in the past, their motivations or biases, or even attacking arguments they made in the past.

Here are a couple examples of how ad hominem might look in an answer choice:

• [The argument] treats evidence of apparent bias as evidence that the claims made are false
• [The argument] casts doubt on the reliability of a study by questioning the motives of those reporting it.
• [The argument] rejects a position merely because the proponent of the position has other objectionable views

Bad Samples & Hasty Generalizations

LSAT arguments often provide study and survey data as evidence, sometimes from unrepresentative or insufficiently sized samples. You need to call BS whenever you catch it.

For example, an argument might conclude that our city needs to build a bridge because a surveyed neighborhood voted in favor of the bridge 50 to 20. That's 70 people. US cities average between 5,000 and 50,000 residents. These 70 folks aren't going to speak adequately for the whole city.

Similarly, we don't get to jump to conclusions based on a handful of examples.

For instance, imagine I argued the following: One time, at band camp, two kids went for a hike in the woods and were never seen again. Therefore, you should never hike in the woods. Sure, it sucks to be those kids, but that doesn't mean the average walk through the woods isn't safe and pleasant.

Good conclusions need sufficiently large samples.

Here are examples of how these might look in answer choices:

• [The argument] draws a conclusion based solely on an unrepresentative sample of XYZ's products
• The argument draws a general conclusion about a group based on data about an unrepresentative sample of that group
• [This argument] relies on a sample that is too narrow
• [The argument] draws a general conclusion on the basis of just one individual case

Bad Appeals to Authority

When arguments make appeals to authority figures, it's important that they appeal to relevant experts.

Imagine I made this argument: My mechanic knows his way around my vehicle and he always gives sound advice about how to care for my car. Therefore, I should follow his advice about what medications I should take.

Not so much. And I'm not picking on mechanics, here. I wouldn't hold my doctor's opinion higher than my mechanic's on matters related to automotive care.

Here's what bad appeals might look like in answer choices:

• The argument relies on the testimony of experts whose expertise is not shown to be sufficiently broad to support their general claim
• [This argument] relies on a source that is probably not well-informed

Part to Whole

Just because part of group has a certain characteristic doesn't mean the larger group must share that same characteristic.

Steph Curry's one of the most prolific shooters in NBA history. He plays for Golden State. That doesn't mean every player on Golden State's a great shooter.

This also works in the opposite direction (whole to part). An individual sharing a characteristic with the members of a larger group doesn't mean that individual must be a part of that same group.

Think Steph Curry again: The NBA Hall of Fame is full of prolific shooters. Steph's a prolific shooter, so he must be in the Hall of Fame. Even though Steph will almost definitely end up in the Hall of Fame, he hasn't retired yet.

Here are some examples of what a part to whole flaw might look like in the answer choices:

• The argument infers that a property belonging to large institutions belongs to all institutions
• [The argument] generalizes, from the fact that one species with a certain characteristic survived certain conditions, that all related species with the same characteristic must have survived exactly the same conditions

Equivocating Terms

You don't get to change something's definition mid-argument.

Here's an argument containing a clumsy equivocation:

All banks are financial institutions. So the edges of a river must also be financial institutions.

Obviously river banks and the place you deposit your paychecks aren't the same thing.

Answer choices involving equivocations might look like this:

• The conclusion is based on a shift in meaning of a key term from one part of the argument to another part.
• [The argument] illicitly relies on two different meanings of a critical term

Bad Argument ≠ False Conclusion

On the LSAT, we must attack bad arguments. But bad logic doesn't necessarily mean a false conclusion.

For instance, imagine I said, "I had a great workout today. Therefore the sun will rise tomorrow." Your gut reaction ought to be that the quality of my workout has absolutely no bearing on the sun rising tomorrow. But you don't get to conclude that the sun won't rise tomorrow as a result.

In other words, bad arguments can be right by mistake.

And here's what this tends to look like in the answer choices:

• [The argument] rejects a position on the grounds that an inadequate argument has been made for it
• [The argument] rejects the possibility that the professor is indeed disruptive on the grounds that she wasn't proven to be disruptive.

Circular Reasoning

Circular reasoning's pretty rare on the LSAT, but it does pop up from time to time.

Circular reasoning assumes what it's trying to conclude, so it sticks out like a sore thumb. But it's rarely the correct answer choice on Flaw questions.

Here's an example: Everything Big Brother tells us is true because Big Brother says so. Or: Everything in this religious text is true because it say so in the text.

It'll look like this in the answer choices:

• [The argument] restates as a conclusion a claim earlier presented as evidence for that conclusion
• [The argument] assumes the conclusion that it sets out to prove.

Other Common Flaws

There are some other common flaws that you'll see less often as awkwardly worded answer choices.

We'll touch on them briefly so you know what they are and how to spot them.

False Dichotomies

How many times have you gagged at someone saying, "There are two types of people in the world..."?

False dichotomies set up similar scenarios: There's a this and a that, and that's it.

Sometimes this is fair: There are people who can swim and people who can't. Principally, every person falls into one group or the other.

Sometimes it's BS: There are two types of pets—cats and dogs. If you're a bird- or lizard-owner, you probably rolled your eyes at that. Good!

Don't let the author paint false dichotomies.

Bad Analogies

Studying for the LSAT is like a marathon—you must train diligently and prepare to perform for a sustained amount of time on test day. Right?

Well, sort of. And that's the point. All analogies break down at some point.

For instance, there are tons of ways studying for the LSAT is nothing like training for a marathon.

Be prepared to push back on conclusions by analogy.

Possibility vs. Certainty

Possibility (even probability) doesn't equate to certainty.

Recall from our Could Be True vs. Must Be True lesson that, while must contains could, could doesn't have to contain must.

Even if an argument establishes an overwhelming probability that something will occur, we can't conclude it will definitely happen without actually proving it.

Red Herrings

You've probably been involved in an argument where someone changes the subject. That's a red herring, and the LSAT's chock full of them.

Sometimes authors will start you down one path, only to conclude something that's seemingly related but actually tangential to the issue at hand.

Consider this example:

To be a successful entrepreneur, one must innovate, take risks, and serve customers well. Sarah has won several awards for her service to the community. So Sarah's at least part of the way to becoming a successful entrepreneur.

The red herring here centers around service. We know successful entrepreneurs must serve customers well, and we know Sarah has been commended for serving her community, but the latter is a distraction that doesn't actually qualify for the former. So the conclusion is unsupported.

Old / New Isn't Necessarily Best

Past success is no guarantee of future success. And change does not equal progress.

Sometimes arguments will try to convince us that because we've had X number of the same results, we'll definitely get that result again. Cue the negative buzzer sound.

We can't assume that maintaining the status quo is always in our best interest. Conversely, we can't assume that changing it will benefit us.

If an argument tries to convince you that changing conditions will invariably lead to superior results, you need to push back.

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There you have it—Logical Reasoning in the books! Next up, we're diving into Reading Comprehension with a section overview and some general tips. See you there!

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