Chapter Three, Tutorial Four
Logical Equivalence in SL

Consider this pair of sentences:

Bob will attend law school only if he does well on the LSAT's.    (B>W)
If Bob doesn't do well on the LSAT's, he will not attend law school.    (~W>~B)

Don't these sentences sound like about the same thing?

They should because they are logically equivalent. Roughly, that is, they mean the same thing.

Let's try to recall just how we defined logical equivalence back in chapter one. Two members of a pair of sentences are logically equivalent if and only if ______?

If and only if what??? Select the correct way to fill in the blank:

  1. both sentences could not possibly be false.
  2. the two sentences are identical.
  3. it is not possible for one of the pair to be true while the other is false.