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A unless B

 
“A unless B” is a rather complex structure. Though surprisingly we use it with little thought or confusion in our day-to-day speech.

To see that “A unless B” is equivalent to “~B—>A,” consider the following situation:
 
Biff is at the beach unless it is raining.

Given this statement, we know that if it is not raining, then Biff is at the beach. Now if we symbolize “Biff is at the beach” as B, and “it is raining” as R, then the statement can be diagrammed as
 
~R—>B

Example: (A unless B)

Example
Melinda can become a lawyer unless she does poorly on the LSAT or does not get a scholarship.
 
Which one of the following statements cannot be validly drawn from the above statements?
  1. Melinda is lawyer. So she must have both done well on the LSAT and gotten a scholarship.
  2. Melinda is a lawyer and she did well on the LSAT. So she must have gotten a scholarship.
  3. Melinda did poorly on the LSAT. So she will not become a lawyer.
  4. If Melinda does not become a lawyer, then she did poorly on the LSAT or could not get a scholarship.
  5. If Melinda does poorly on the LSAT and does not get a scholarship, then she will not become a lawyer.
Solution
This argument says that two things stand in Melinda’s way—performing poorly on the LSAT and not getting a scholarship. That is, if Melinda does well on the LSAT and gets a scholarship, then she can become a lawyer.
 
Since Melinda is a lawyer in choice (A), she must have overcome the two obstacles—the LSAT and the scholarship. Hence, (A) is valid. This eliminates (A).
 
Next, (B) essentially expresses the same thought as (A). This eliminates (B).
 
Next, (C) says that Melinda didn’t meet one of the two criteria, so she won’t become a lawyer. Hence, (C) is valid. This eliminates (C).
 
You should notice that the conclusion in (D) is too strong. Melinda may do well on the LSAT and get a scholarship yet decide not to become a lawyer.
 
The answer, therefore, is (D).
 





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