Understanding Conditional Statements on the LSAT

A misunderstanding of conditional statements is the most common source of error on the LSAT. Conditional statements are at the heart of logical reasoning. They’re also at the heart of how human beings make arguments in regular, day-to-day discussions. Here are some examples of conditional statements at work in daily life:

If I increase my LSAT score by a handful of points, I’ll be able to go to law school for free.
I want to go to bed early tonight; I don’t want to be too tired tomorrow morning.
If I fail this exam it’ll hurt my GPA.
He agreed to get us a table if he arrives at the restaurant first.

If you’ve never studied formal logic, or if you hate it with a passion, don’t worry. Conditional statements are common sense. You just don’t know it yet.

What Is a Conditional Statement?

A conditional statement is just an if-then statement like this one:

If you drink that poison, then you will get sick.

The if-clause is called the “sufficient condition,” and the then-clause is called the “necessary condition.” If nothing else sticks from this lesson, remember this distinction.

What Is a Sufficient Condition?

LSAT test-writers refer to the if-clause as the “sufficient condition.”

It’s sufficient because it’s enough to guarantee that the then-clause will happen. You don’t need anything else. If you drink that poison, then, according to this rule, you will get sick.

In real life, we might think of cases when people drink the poison and don’t get sick. There are always exceptions in life. But when you read an if-then statement on the LSAT, you have to take it at its word. It didn’t say that you might get sick or that most people get sick; it said that you will get sick.

Remember: Accept premises, but attack conclusions!

The if-clause is sufficient for triggering the then-clause. If the if-clause is triggered, then the then-clause has to happen. See what we did there?

Examples

In the following samples conditional statements, the if-clause is italicized:

If the sun spots continue to grow, then we will lose all our crops this year.
‍If the car costs more than $2,000, don’t buy it with cash.
We can sell all our spiders if we clean out their cages.

Take a moment to try and come up with your own examples.

What Is a Necessary Condition?

Test-writers refer to the then-clause as the “necessary condition.”

Above, you learned that if the if-clause is triggered, then the then-clause must happen. The then-clause is necessary. If you drink the poison in the example above, you necessarily will get sick. There is no world in which you drink that poison and don’t get sick.

Notice, however, that the reverse is not true. If you get sick, we can’t say for sure that you drank the poison. You could have. But you also could have gotten sick for some other reason.

The then-clause is not a sufficient condition because it does not guarantee that anything happened.

For the same reason, the if-clause is not a necessary condition because it doesn’t always have to happen when something else happens. Keep reading to learn about how to avoid confusing a necessary condition with a sufficient condition.

Examples

In the following sample conditional statements, the then-clause is bolded:

If the sun spots continue to grow, then we will lose all our crops this year.
If the car costs more than $2,000, don’t buy it with cash. (Note: The word “then” doesn’t need to be explicitly stated. The word “if” is enough to signal a sufficient condition in the first clause, so the second clause must be the necessary condition.)
We can sell all our spiders if we clean out their cages. (Note: The sufficient condition doesn’t always come first! Again, focus on the word “if.”)

Confusing Necessary with Sufficient

This is one of the LSAT’s most common flaws, so make sure to learn it inside and out. Now that you understand the difference between a sufficient condition and a necessary condition, you should be able to spot instances when an argument confuses the two.

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The names “sufficient” and “necessary” are important because most people think about if-clauses incorrectly. Consider the following silly if-then statement:

If you eat at Chipotle, then you will get a special tax credit.

What do you need to do to get that “special tax credit”?

If your gut reaction is “eat at Chipotle,” you’re not alone, but you’re misinterpreting the word “need.” Eating at Chipotle is the sufficient condition—eating there guarantees that you’ll get the special tax credit. But is that the only way to get the credit?

Eating at Chipotle might be the only way to get the credit, but there could be any number of other ways. Maybe you can get the credit by eating at McDonald’s or by just calling the IRS. There is not enough information in the argument to know. All you know for sure is that eating at Chipotle is one guaranteed way of getting the credit.

So, what do you need to do to get the credit? The short answer is that it’s unknown. The rule doesn’t tell us what’s necessary to get the tax credit. It tells us only what is sufficient to get that credit. Eating at Chipotle is sufficient for getting the tax credit. But the reverse is not necessarily true. Eating at Chipotle might not be necessary for getting the credit. You might be able to get the credit some other way.

If you initially answered “eat at Chipotle” to the question above, you mistakenly turned the sufficient condition into a necessary condition.

Known Unknowns

When faced with a conditional statement, it’s important to be as aware of what you don’t know as you are of what you do know.

What you do know:

You know what is sufficient to trigger the necessary clause (eating at Chipotle is enough to get the credit).
You know the guaranteed result (getting the tax credit) of the sufficient clause (eating at Chipotle).

What you don’t know:

You don’t know what is sufficient to trigger the sufficient clause. Try to fill in the blank: “If ____, then you will eat at Chipotle.” You can’t! There might not even be a sufficient condition for eating at Chipotle. If you tried to fill the blank with “you get a special tax credit,” you mistakenly turned the necessary condition into the sufficient condition.
You don’t know what is necessary to trigger the necessary clause. Try to fill in the blank: “If you get a special tax credit, then ____.” Again, you can’t fill in this blank! If you tried to say, “you ate at Chipotle,” you mistakenly turned the sufficient condition into the necessary condition.

Want to learn more about this topic? Watch Ben discuss the difference between sufficient and necessary here.

Pop Quiz

Using the rule below, answer each question:

If you sell your soul to a big law firm, then you will wallow in misery.

What is a sufficient condition for wallowing in misery?
What is a necessary condition for wallowing in misery?
What is a sufficient condition for selling your soul to a big law firm?
What is a necessary condition for selling your soul to a big law firm?
Which clause—if-clause or then-clause—is necessary for the other clause?
Which clause—if-clause or then-clause—needs the other clause to happen?

Answers:

Selling your soul to a big law firm. This condition is a sufficient condition because if it happens, then you will definitely wallow in misery. (In the real world, of course, this is not true for every attorney—just, well, the vast majority.)
We don’t know. The rule never says what is “necessary” for wallowing in misery; it only tells you what is “sufficient” for such wallowing. The if-clause is a sufficient condition. And although it could be a necessary condition as well, you simply don’t know because there might be other ways to get yourself to wallow in misery.
We don’t know. The rule never says what is “sufficient” for selling your soul to a big law firm; it only tells you what is “necessary” for such a sale. The then-clause is a necessary condition. And although it could be a sufficient condition as well, you simply don’t know because the fact that you’re wallowing in misery does not guarantee that you sold (or will sell) your soul to a big law firm. Maybe your significant other just broke up with you after saying, “It’s not you. It’s me.”
Wallowing in misery. This condition is a necessary condition because it has to happen if you sell your soul to a big law firm, at least according to this rule.
Then. The then-clause is a necessary condition. It has to happen if the if-clause happens. The if-clause, however, doesn’t have to happen if the then-clause happens.
If. This is tricky. Because the if-clause guarantees that the then-clause will happen, it needs the then-clause to happen. If you eat at Chipotle, then you have to get that credit. If you don’t get that credit, there’s no way that you could have eaten at Chipotle. The then-clause, however, doesn’t need the if-clause to happen. You could get the tax credit some other way.

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