There is one surefire way to see if you have the right assumption. It’s called the denial test. If you assume the premises and the conclusion to be true, the assumption must also be true. Just because the author didn’t write it out explicitly, it’s there. The author takes it for granted.

This is the third in a series of lesson articles introducing you to LSAT Assumption Questions. The series includes:

The Denial Test

An argument is a three-legged table. It depends on all three legs-premises, conclusions, and assumptions-to stand. If you cut one of the legs off, the table will fall. If you cut off the central assumption, the argument will fail.

Where the LSAT gets tricky is in the wrong answer choices. A wrong answer is like a decorative leg glued to the table. It looks good, but it doesn’t hold the table up. You’ve already seen how to recognize and throw out the most common wrong answer choices:

  • scope errors
  • right answers for wrong questions
  • distortions
  • contradictions
  • bad connections
  • wrong tone

The test makers use all the these wrong answer styles to add decorative legs to your table.

What happens if you cut off a decorative leg, a leg that isn’t doing anything to hold up the table but is just stuck there looking pretty? Nothing. The table still stands. What happens if you cut off one of the three legs that actually support the table? The table falls down.

The denial test cuts legs off the table. When you cut the right leg off-the assumption-the table will fall down and you’ll know you got the right one.

To use the denial test, take the statement you want to test and negate it (reverse its meaning). If the argument stops making sense or becomes impossible, then the original statement must be a central assumption.

Using the same argument, “all dogs are carnivores, so Mike is a carnivore,” we know that the central assumption is that Mike is a dog. Let’s apply the denial test to that statement and see what happens to the argument. First, negate (reverse the meaning of) the statement.

Mike is not a dog.

Second, plug that into the argument:

All dogs are carnivores

+

(Mike is not a dog)

=

Mike is a carnivore

Does that make sense? I guess Mike could still be a carnivore, but that would be pure luck that the conclusion was right. If Mike isn’t a dog, how do we know. All we know is that dogs are carnivores.

What happens if you apply the denial test to something that isn’t an assumption?

Consider the statement “Mike likes to run.” If this were an answer choice, you would recognize that it is out of scope and cross it out. But if you didn’t see that it was out of scope, you could apply the denial test to see if the argument needs it to stand. First, negate the statement: “Mike hates to run.” Second, plug it into the argument.

All dogs are carnivores

+

(Mike hates to run)

=

Mike is a carnivore

You should look at that and say, “so what?” The conclusion isn’t any more reasonable if Mike likes to run than if he hates to run. If the argument is just as good after we reversed the statement, we must not have started with a central assumption. Remember, the argument requires the central assumption. If it works just as well with the opposite statement, the argument doesn’t need it in the first place.

Now let’s look at some LSAT practice assumptions.

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