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LSAT If-Then Statements

This is the second article on a series of articles dealing with LSAT Formal Logic. This article will explain conditional if-then statements as they are used on the LSAT test.

This lesson series covers the following articles:

LSAT Formal Logic: An Introduction
LSAT If-Then Statements
LSAT Contrpositive 
Best LSAT Logic Tip

LSAT If-Then Statements

Let’s play with a conditional statement: All monkeys eat bananas.

First, we know that we can paraphrase that statement as “if monkey then banana,” or “M –> B.” Those statements all say the exact same thing. If you find a monkey, you can plug him into the “M –> B” equation and find out that he eats bananas. So, given that all monkeys eat bananas, if someone tells you that George is a monkey, you can infer that George eats bananas. 

I know what you’re thinking. Big deal, that’s pretty obvious. But which one of the following could you also infer?

(A)  Mary doesn’t eat bananas, so she must not be a monkey.
(B)   Pete isn’t a monkey, so he must not eat bananas.
(C)   Jane eats bananas so she must be a monkey.
(D)  The only creatures who eat bananas are monkeys.
(E)   If you’re not a monkey, you don’t eat bananas.

Suddenly, faced with some similar sounding answer choices, it’s not as easy. Not that it’s too hard. But if you don’t know what you’re looking for, that’s a lot of potentially confusing stuff to read. If you’re curious, (A) is the only correct inference. It is a special inference called the contrapositive.

The Contrapositive of LSAT If-Then Statements

There are only three things you could do to manipulate statement like, “If you are a monkey, then you eat bananas,” or “M –> B.” Two of them will give you wrong answers, one of them is the contrapositive. First, let’s consider the wrong answers. You could flip the terms:

If you eat bananas, then you are a monkey (B –> M)

If you give this some thought, you can see it isn’t necessarily true. Just because monkeys always eat bananas doesn’t prevent other animals from eating bananas too. If we’re told that George eats bananas, George might be a monkey, or he might be any other banana-loving creature. This is a bad inference.

Technically, this is a logical fallacy called “affirming the consequent,” because it tries to offer the consequent as proof of something. But a consequent doesn’t do anything for you. Many things can lead to a consequent. Consider the statement, “if I have no gasoline, my car won’t start.” The consequent is that my car won’t start. But there could be many reasons why my car won’t start — no gas, no engine, it’s too cold. So knowing that the consequent is true doesn’t prove anything. Who knows why my car won’t start?

Another bad inference you could make involves negating the terms:

If you are not a monkey, then you do not eat bananas.

You can see for the same reasons that this is a bad inference. If someone tells you that George is not a monkey, we don’t know that he doesn’t eat bananas, since many other animals probably eat bananas too. This is a fallacy called “denying the antecedent,” because you negate the first part of the original statement and then assume that the consequent is also false. But again, many roads can lead to the same consequent. Consider again, “if I have no gasoline, my car won’t start.” If you deny the antecedent and say that I do have gasoline, you still can’t prove that my car will start. There could be many other reasons why my car won’t start other than lack of gasoline.

There is only one thing we can infer from a conditional statement and that is the contrapositive. To form the contrapositive, you must do two things: reverse the order of the statement, and negate both sides. For example:

Statement: If you’re a monkey, then you eat bananas (M –> B)

Contrapositive: If you do not eat bananas, you are not a monkey (not B –> not M)

This has to be true. If someone says that George doesn’t eat bananas, then we can positively say that George isn’t a monkey. Why? Because all monkeys eat bananas. Remember, negating an already negative statement gives you a positive statement. For example:

Statement: If I have no gasoline, my car won’t start (not G –> not S)

Contrapositive: If my car starts, I have gasoline (S –> G)

We know this has to be true. If my car starts, it must have gasoline in it (as well as an engine, and keys, etc.)

In the next article, we’ll practice using the LSAT Contrapositive.