The LSAT's dirtiest word

My mom's less than thrilled about it, but my LSAT class, LSAT blog, and LSAT book are all filled with dirty words. I find that sprinkling in a few f-bombs keeps students awake, which is job one when you're teaching 4-hour LSAT classes and spilling tankers of ink on LSAT logical reasoning. (I'll let Mom off the hook here: She certainly didn't teach me to swear. Nope, that would have been my dad. Or more accurately, my dad and his golf buddies, who swear like sailors when they're not at church. Mom, Dad, and Spring Creek Golf & Country Club's many delinquents--I love you all.)

Poor Ralphie.

The purpose of this post is to demystify the LSAT's very dirtiest word, which has a tendency to shock even the hardiest of law school aspirants. It's not four letters--it's six. Cover your ears: The word is "unless."

Let's take a statement from a test I covered last night in class. The September 2006 Official LSAT contained a Logic Game that included the following rule:

"F cannot be selected unless Z is also selected."

Rules that are formulated in this way cause no end of frustration for many LSAT students. My hope is, by the end of this post, to convince you that the word "unless" isn't really all that foul. If you practice enough, you're going to get the hang of it. No need for us all to wash our mouths out with soap.

The rule "F cannot be selected unless Z is also selected" can really only mean one of two things:

Candidate 1:  F-->Z (if F is selected, then Z is selected.) Contrapositive:  Z-->F (if Z is not selected, then F cannot be selected.)

or

Candidate 2: Z-->F (if Z is selected, then F is selected.) Contrapositive:  F-->Z (if F is not selected, then Z cannot be selected.)

I don't expect this to be immediately obvious to everyone. But we do use the word "unless" in everyday conversation, so I don't think the use of this word on the LSAT needs to strike such fear in our hearts.

Maybe taking it out of the abstract would help a bit. We don't know F and Z. But we do know Fabio and Catherine Zeta-Jones (the sexiest F and Z I could think of).

Yeeeeeah, I'm looking at YOU, Fabio. You sexy man. ...  Wait, what?

Okay, imagine if the rule had said "Fabio cannot be at the party unless Catherine Zeta-Jones is at the party." So NOW what do you think the rule means?

Candidate 1:  Fabio is at the party --> Catherine Zeta-Jones is at the party. Contrapositive:  Catherine Zeta-Jones NOT at party --> Fabio NOT at party.

or

Candidate 2:  Catherine Zeta-Jones is at the party --> Fabio is at the party. Contrapositive:  Fabio NOT at party --> Catherine Zeta-Jones NOT at party.

Again, the statement was "Fabio cannot be at the party unless Catherine Zeta-Jones is at the party." Time to choose... do you think it's Candidate 1 or Candidate 2?

If Candidate 1 is your answer (F-->Z; Z-->F) then it's okay for Catherine to be at the party by herself, but it's NOT okay for Fabio to be there by himself. (If Fabio's there, then Catherine's there too.)

So this would be okay:

Yeah... that would definitely be okay.

and this would also be okay:

Ooooooh, I didn't think it could get better, but it did.

This, on the other hand, would NOT be okay:

Sorry Fabio, you can't get in without Catherine.

If Candidate 2 is your answer (Z-->F; F-->Z) then it's okay for Fabio to be at the party by himself, but it's NOT okay for Catherine to be there by herself. (If Catherine's there, then Fabio's there too.) So this would be okay:

Nice.

and this would be okay too:

Yeah, that's VERY nice.

But this would NOT be okay:

Where's the sexy man with the vest?

One last time:  The rule is, "Fabio cannot be at the party unless Catherine Zeta-Jones is at the party."

Time for the answer:

The winner is Candidate 1. The statement "Fabio cannot be at the party unless Catherine Zeta-Jones is at the party" simply means "if Fabio is there, then Catherine has to be there too":  F-->Z; Z-->F. Catherine's presence is necessary for in order for Fabio to be there. And if Fabio is there, that's sufficient information for us to know that Catherine has to be there too.

It's okay if this is still a bit puzzling--I'll be back in my next post with a boatload of additional "unless" examples. For now, see if personalizing it (as I've done above, taking it out of the abstract by using well-known celebrities) helps you understand an "unless" statement. I really don't think this is the kind of thing that can be understood by any formula or memorization technique. We use the word unless all the time in real life. Just like swear words, it's nothing to get worked up about. I'll be back soon with more.