Q22 - Further evidence bearing on Jamison's

[ManhattanPrepLSAT1]

Evidence
Without further evidence we wouldn't have been able to establish Jamison's involvement, and we have established Jamison's involvement.

Conclusion
We must have found more evidence.

This is simply an argument that reasons by use of a contrapositive. So let's go find another argument in the answer choices that does the same thing.

(B) is an exact match and relies on a contrapositive.
It says if Turner had taken her usual train, she wouldn't have been in Nantes. But she was in Nantes. So she didn't take her usual train.

Incorrect Answers

(A) is wrong in that it reasons by positive argument structure.

Positive Argument Structure looks like
A --> B
A
----------
B

Contrapositive Argument Structure looks like
A --> B
~B
----------
~A

(C) is wrong in that it doesn't rely on the application of a conditional relationship.
(D) is wrong in that it reasons by positive argument structure - the same as answer choice (A).
(E) is wrong in that it doesn't rely on the application of a conditional relationship.

[peg_city]

I'm having a hard time figuring out why B is right here.

[timmydoeslsat]

The stimulus can be seen as this:

Basis of previously available evidence alone ---> ~Possible to prove

We have a situation where something has now been proven.

Using the contrapositive, we know that it was ~Basis of previously available evidence alone.

This is the conclusion of this argument, the necessary condition of our contrapositive: Further evidence must have come to light.

So we would like to see in our answer choice, a situation where the contrapositive gives us a concluding statement.

Also, this is a valid argument.

A) Old list ---> ~Purchase property within last year

Smith on old list.

Argument concludes Smith must not have purchased within last year.

Valid argument, but does not exhibit the same sort of reasoning.


B)

Taken usual train to Nantes ---> ~Nantes until afternoon

Was in Nantes before afternoon

Concludes did not take usual train to Nantes.

Looks good. It has the contrapositive reasoning structure to conclude the necessary condition.


C) Uses the phrase likely to have forgotten as a premise and concludes "must." Eliminate.


D) In library ---> Saw cancel notices

W was in library

Concludes must have known. This is an invalid argument. The notice could have been in a different language or it could have been worded in such a fashion that W did not know class would be canceled.

Even if the argument concluded that W must have saw the cancel notices, this would still not be our answer for this question as it does not reason by the contrapositive.


E) Kind of remark ---> Felt bad

L made remark

L must have deeply resented.

Mismatch of conclusion from premises here. Also not the contrapositive.