Linear Setup Practice Drill #12

[Aleksandrovna]

Hello!

The 12th question in the first practice drill on page 39 in the logic games bible confuses me a little bit.

One of the rules states "R is not scheduled for Thursday unless L is scheduled for Monday." Question 12.1 asks, which one of the following must be true and option A) R is scheduled for Thursday is described as wrong because it violates the rule quoted above.

That doesn't make sense to me at all. I understand option A to be correct because logically (in my head) R would HAVE TO be scheduled for Thursday if L is scheduled for Monday. Unless of course I completely misunderstood the instructions on how "unless" modifies the necessary condition.

Thank you in advance for your help!

[Dave Killoran]

Hi Aleks,

Let me see if I can help you out here a bit. First, you are using a pretty outdated version of the LGB. I significantly revised and expanded the book over four years ago, and the newer version is about 135 pages longer. For students using the current edition, the drill that is being referenced here is now on page 92.

In the newer version, I expanded the explanation of this particular item, but since you do not have it, I'll add it in here:

Although answer choice (A) could occur, it does not have to occur, and thus (A) is incorrect. I believe you may be misinterpreting the last rule by making a Mistaken Reversal, and that is leading you to incorrectly consider answer choice (A). Let’s take a moment to analyze the last rule:

"R is not scheduled for Thursday unless L is scheduled for Monday."

The critical part of the sentence is the word "unless," and when "unless" appears it introduces the necessary condition, and then the rest is negated and becomes the sufficient condition. Hence, the proper diagram for the rule is:

R Th L M

Let’s review that again. To diagram a statement involving “unless” (or its analogues “except,“ “until,” or “without”), take the following two steps:

1. The clause modified by “unless” becomes the necessary condition.

2. The remainder is negated—which normally involves removing or adding a “not”—and becomes the sufficient condition.

Thus, in the rule under consideration, “unless” modifies “L is scheduled for Monday” and that becomes the necessary condition; the remainder, “R is not scheduled for Thursday,” is negated to “R is scheduled for Thursday” and becomes the sufficient condition. Hence the diagram: R Th L M

So, with this diagram in mind for question 12.1, when L is scheduled for Monday, the necessary condition is met. This does NOT mean, however, that R must be scheduled for Thursday (it is possible but not certain). To infer R is on Thursday would be a reversal of the statement and erroneous. Since (A) tries to say that it must be true that R is on Thursday, answer choice (A) is therefore wrong.


Please let me know if that helps. Thanks!