#22 - Further evidence bearing on Jamison’s activities

[maximbasu]

Hello,
I chose C as the correct answer while the correct answer was B.

How would you diagram this question? Answers A and C seemed to also parallel the stimulus' validity, conclusion, reasoning. Why is C correct?

Thank you, Maxim.

[Clay Cooper]

Hi Maxim,

Thanks for your question. I would diagram the stimulus like this

Old evidence ~prove his guilt

But we know that his guilt has in fact been proved. So, it stands to reason that there must be new evidence:

New evidence proved guilt.

Answer choice C does not follow the same structure; instead, it looks like this:

~Norris authorized it ~was authorized

But, we are told, there is no doubt that she did authorize it. In other words, we are told that the necessary condition of the contrapositive is in place; what we have proved is that she lied, not that she did in fact authorize it (we are given that information as evidence). Thus the evidence we are given is different than that in the stimuls, and what we use that evidence to prove is not parallel.

Does that make sense?

[Johnclem]

Hello,
I read the above explanation but I'm having trouble diagraming the stimulus and the correct answer choice.

1) This is how I diagramed the stimulus : ( it's not matching with the diagram above , but Isn't the conclusion supposed to be on the right side of the arrow?

Old evidence --> ~ not have been able to prove
We are able to prove --> New evidence


2) the correct answer choice has the word "until" so I diagramed it like so.. ( why are we not negating the sufficient )?

If she took her usual train --> arrived afternoon
c: did not take her usual train


Thanks
John

[Adam Tyson]

I agree with your diagram, John - good job! This goes to show that there is more than one way to skin an LR question.

As to the "until" in answer B, that one's a bit tricky, but in this case it isn't actually indicating a necessary condition, but is instead embedded inside the necessary condition. The sufficient condition here is "had she done so" (which, as you correctly surmised, means "if she had taken her usual train"). The necessary condition is "she wouldn't have arrived until the afternoon". You could paraphrase that claim and use the unless equation, as you were thinking you might need to do, and say something like "IF she had taken her usual train she WOULD have arrived no earlier than the afternoon (negating the "wouldn't", rather than the "had"), and you would have gotten to the same answer choice, I expect.

Good eye, and good question. When in doubt, try asking yourself if the way you have diagrammed the question makes sense in keeping with the stimulus or if it conflicts with it or confuses it somehow. That's a more intuitive approach, less mechanical, but it can be a useful way to check yourself. You seem to be doing that already, so just keep that up and trust your instincts. Use the mechanical approaches we teach to your advantage, but don't subordinate your good instincts while doing so.

[jlam061695]

I still don't understand how the reasoning in B matches that of the stimulus at all.

Copying Clay's diagram for the stimulus:

old evidence proved guilt
__________________________________
C: new evidence proved guilt

This was my diagram for B:

taken usual train to N arrived at N at noon
arrived at N at noon (since she was seen having coffee at N at 11 A.M)
_________________________________________________________________
C: taken usual train to N

The two diagrams and premises just don't seem to match up at all for me.

[Adam Tyson]

Try this, which is a variation on Clay's diagram:

Premise: Old Evidence -> prove guilt

Premise: guilt proven

Conclusion: old evidence

Implication of conclusion: new evidence (If we couldn't do it based solely on old evidence, but we have now done it, there must be more than the old evidence, i.e. there's new evidence)

That should match up nicely with your diagrams.

More important, though, is that it matches up with your understanding of both arguments. Put aside the diagrams for a moment and just consider the logical relationships at play, the structures of the arguments. That's where you should get a warm fuzzy feeling about the correct answer, because it feels the same as the stimulus. Something could not have happened if a certain thing was true, but it did happen, so that thing must not be true. We can diagram the arguments all day long, and find several different ways to do it, but none of that matters if the arguments don't boil down to the same basic structure.

Step back from the diagrams and do a gut check on this one. See what you think based on that approach, them come back and play with the diagrams again if you feel the need.

Keep at it!

[jlam061695]

Thanks for re-diagramming it for me, it makes more sense now! I now realized that both the stimulus and the answer choice are playing around with contrapositives.

[LSAT2018]

Can you look over these for me? This question was really tough!

Stimulus:
Previously Available Evidence → Not Proven
Proven → Not Previously Available Evidence (Contrapositive)
Correct Answer:
Take Train → Not Arrive in Nantes
Arrive in Nantes → Not Take Train

Other
(A) Old List → Not Purchase (No Contrapositive)
(C) Invalid Reasoning
(D) Library → See Cancellation Notes (No Contrapositive)
(E) Make Remark → Feel Badly Treated (No Contrapositive)

For A, D, E they are not parallel because they have one conditional statement? And C is not a conditional statement because of the 'likely' part? Would this be considered an invalid statement?

[Adam Tyson]

Your analysis of all the answers, correct and incorrect, looks good to me! Yes, "likely" takes a claim out of the realm of true conditional reasoning and into the realm of Formal Logic, a close cousin. Well done!

[grunerlokka]

Why is A not a good answer to this question? ("Smith must not have purchased his house within the last year. He is listed as the owner of that house on the old list of property owners, and anyone on the old list could not have purchased his or her property within the last year.")

It seems to me just as sound as B, the correct answer: ("Turner must not have taken her usual train to Nantes today. Had she done so, she could not have been in Nantes until this afternoon, but she was seen having coffee in Nantes at 11 o’clock this morning.")