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Three critical LSAT logical terms you MUST memorize

  • by John
  • Apr 06, 2011

Most of the conditional logic that appears on the LSAT is fairly straightforward, of the “if A then B” variety. However, there are three LSAT logical operators that you must memorize in advance of the test. They have been showing up more and more on newer LSATs; if you have a logic game that involves “unless” logic and you don’t know what to do with it, you risk missing the entire game.


Unless statements require a three-step process which you should commit to memory.

Statement: “You’ll come to a bad end unless you change your evil ways.”

The rules for interpreting “unless” statements are as follows:

  1. Whatever term(s) is/are modified by “unless” become the necessary conditions and go to the right of the arrow. This is the statement that follows unless. In the example, it’s “change your evil ways.”
  2. Whatever is left is the sufficient condition and goes to the left of the arrow…
  3. And the sufficient condition (left of arrow) is negated. This is the part that students tend to forget, but it’s critical.

So, the statement can be rendered: No bad end –> change your evil ways.

Sometimes the contrapositive can be easier to understand: didn’t change evil ways –> came to a bad end.

Only if

Only if statements essentially reverse the standard conditional relationship. Take the following statements:

Statement 1: You’ll succeed in life if you take your father’s advice.

As usual, the term modified by “if” is sufficient and goes to the left of the arrow, yielding the statement “TFA –> SiL”

Statement 2: You’ll succeed in life only if you take your father’s advice.

Note here the “only if” operator. What does this indicate? It doesn’t say that if you take the advice you’ll succeed. It says instead that in the world in which you’ve succeeded, you must also have taken your father’s advice. (Remember, conditional statements don’t imply a causal relationship. We’re not saying that succeeding means that you will then take the advice.) So, this statement is outlined “SiL –> TFA.” The “only if” operator reverses the conditional relationship.

If and only if

“If and only if” operators actually indicate two relationships at once: “if,” and “only if.” Therefore, the first step is to write out both statements.

Statement 3: You’ll succeed in life if and only if you take your father’s advice.

This breaks down as:

TFA –> SiL (this is the “if” statement)

SiL –> TFA (this is the “only if” statement)

These statements can be combined into a double-sided arrow if you like: TFA <—> SiL

Bottom line: You MUST memorize these three operators before you’re equipped to work modern LSAT logic games. The past several tests have involved at least one and often more rules that necessitate understanding this logic.

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