Cannot Be True Questions and Could Be True Answers

[Jon Denning]

Every so often I get a question directly from a student that I feel is either interesting enough, or broadly concerning enough, to warrant reposting here for all to see. Yesterday I received just such a question from a student just starting out and enjoying his prep with the Logical Reasoning Bible thus far, but still struggling with Cannot Be True questions. Specifically, confusion arose over the difference between answer choices that seemed like new info but that could in fact be known from the stimulus, versus answer choices with legitimately new info that were therefore unknowable (could be true, as I explain below).

Here is my reply:

Hi XXX – thanks for the question and I’m thrilled to hear you’re enjoying your prep—the biggest improvements come from people who find a way to have some fun with this, so that bodes well for you!

Cannot Be True is a tricky (or at least somewhat odd) idea at first, so I’m glad you took the time to seek some clarification. Hopefully this helps

I suppose the easiest way to think about this “new info” idea is to consider how it worked for Must Be True questions earlier in the book: the correct answer choice must address something that can be legitimately known from the info in the stimulus, meaning answers that present brand new information (information that doesn’t directly repeat, or follow from, the stimulus) are impossible to confirm or deny with any certainty. The best you can say about novel info is that it’s possible, i.e. it could be true.

That makes answers wrong in Must Be True for the exact same reason it makes answers wrong in Cannot Be True: we need things we can validate/verify, one way or the other, and brand new details are impossible to determine definitively.

So what about the right answer in Cannot? Well first it needs to address the facts/details of the stimulus (as noted above) so that we can at least confidently say something about it. That’s where the new info trap occurs. Second, it needs to present information that is contradicted by the information in the stimulus, in much the same way right answers in Must contain facts that the stimulus confirms. This won’t be “new information” as it has to follow from what you’ve already been told; instead it will feel like a continuation of the details provided in the stimulus, where the answer choice info runs contrary to what we know or can infer.

It would be like if I told you all I had to eat for lunch today was a sandwich. I can then say related things that are true:

I didn’t have pizza for lunch today (note pizza is “new” but this is still a fact we can determine from
what we’ve been told)
Today I’ve eaten at least once
I sometimes eat lunch

Or that are false:

I skipped lunch today
I never eat sandwiches
All I ever have for lunch are salads (again, not new in the unknowable sense)

Or that are genuinely new and can’t be known:

I also had a sandwich for lunch yesterday
I usually eat lunch at noon
My favorite type of sandwich is turkey
For dinner I’ll be having lasagna
I really need a haircut
(this list goes on forever)

The key for both Must and Cannot is distinguishing what you can know, and thus what doesn’t count as new info, and what goes beyond the bounds of the information provided in the stimulus and is thus a mere possibility. Answer choices that never escape the realm of possibility (whether for new info or some other reason) can be immediately dismissed for Cannot Be True, just as they can for Must Be True.

I hope that helps!