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 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.
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.
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?
In the following samples conditional statements, the if-clause is italicized:
Take a moment to try and come up with your own examples.
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.
In the following sample conditional statements, the then-clause is bolded:
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.
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.
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:
What you don’t know:
Want to learn more about this topic? Watch Ben discuss the difference between sufficient and necessary here.
Using the rule below, answer each question:
If you sell your soul to a big law firm, then you will wallow in misery.