Q10 - A recent study suggests

[timsportschuetz]

I have a major problem with one of the answer choices of this question! Answer choice E seems to be an extremely attractive answer and I actually chose this over D.

Let me explain my reasoning: The conclusion states that Alzheimer's may be caused by a virus. The support for this conclusion is that a study, in which blood of Alzheimer's patients was injected into rats, found that those rats developed a disease exhibiting symptoms similar to those of Alzheimer's. I understand that D is an attractive answer. However, I inherently believe that E is a much better strengthener! The conclusion is of causal nature. Thus, in order to strengthen this conclusion, we could state that when the cause is absent, the effect is absent as well. E states that when rats that do not have this disease (we can infer that these same rats do not have the injected Alzheimer's virus in their blood nor another virus that may naturally cause the Creutzfeldt-Jakob disease in rats) produced no symptoms of the disease in the other injected rats. This effectively means that when the cause was absent, the effect did not occur! Thus, strengthening the argument!

This is very similar to PT 45 Section 1 #12.

[christine.defenbaugh]

Thanks so much for posting such an interesting question timsportschuetz! However, I'm not sure that I completely follow your explanation.

I think you're saying that the rats in (E) did not have any disease. And that may be true for the rats getting the injection. But the rats that the blood is taken from clearly have Creutzfeldt-Jakob disease, which we already know is caused by a virus. So what 'cause' are you suggesting is absent here? We can't actually say there are zero viruses here.

I'd also be cautious, as a general matter, of using a single instance of 'absent cause, absent effect' as a strengthener for cause. Let's say that I claim that my wicked rain dance causes it to rain. It's not a great strengthener for me to say "Well, you remember last Tuesday, when it didn't rain? I didn't do my rain dance that day." If I told you that every.single.day it didn't rain I didn't do my rain dance, then that starts to sound like a strengthener (though a very weak one). But a single instance doesn't really strengthen.

Let's tackle this question from the beginning.

Since this is a strengthen question, we want to start with the core:

Premise: blood from Alzheimer's patients was injected into rats
Premise: those rats then got C-J disease
Premise: C-J is caused by a virus
Conclusion: Alzheimer's might be caused by a virus

The only other mention of virus before the conclusion is the fact that C-J is caused by one. So to strengthen the conclusion, we need to strengthen the connection between Alzheimer's and C-J. (D) helps us get there: if Alzheimer's and C-J are essentially the same thing, that makes it more likely that Alzheimer's is caused by a virus too!



These Don't Help!
(A) If Alzheimer's in rats is NOT caused by a virus, that would make it a lot LESS likely that Alzheimer's is caused by a virus!
(B) This brings up a difference between C-J and Alzheimer's, weakening the connection between them.
(C) C-J's effect on humans is not relevant to the connection between C-J and Alzheimer's.
(E) C-J infected rats give blood to clean rats, and the clean rats don't catch C-J. This suggests that C-J is not, in fact, blood transmittable. Which is strange, given that C-J appeared after a blood injection in the premises. Regardless, this has no bearing on the relationship between C-J and Alzheimer's, and thus doesn't make it more likely Alzheimer's was caused by a virus.



I would caution you not to force every argument that mentions something about cause into a strict paradigm where the answer rests on the absence or presence of the proposed cause or effect. That model is only applicable occasionally. This question, for instance, really rests on the relationship between a thing we know the cause of (C-J) and the thing we don't (Alzheimer's). As such, it's a completely different structure than PT45-S1-Q12.

Please let me know if this answers your question, or if you have additional thoughts on this!

[timsportschuetz]

Thanks for your quick response! I want to clarify that my mentioning of the method of "absent cause, absent effect" was not intended to mean that this is the only way to strengthen/weaken an argument. It is simply one of the valid methods that can be used based on formal logic within the logic perimeters established by LSAC.

If an argument's conclusion is of causal nature (whether it denies or supports causation), then any answer that uses the above method is credited (of course, it must fit within scope and relevancy).

I believe that I understand your point! It is actually a flawed assumption to interpret answer [E] as stating that rats without the virus causing C-J disease also do not have the blood and/or virus that causes Alzheimer's. As you stated, it could be true that the disease in rats is caused by something other than the supposed virus causing Alzheimer's. I had wrongly assumed that since the rats did not have C-J, that they also do not have the Alzheimer's virus. However, they could have the Alzheimer's virus but do not have the C-J virus (which is stated as fact in the answer choice).

Thanks! Also, I believe the question I mentioned in my original post is extremely useful in the analysis of this particular question. It shows how tricky a credited answer choice can be when required to strengthen/weaken causal arguments.

[WaltGrace1983]

In my opinion, this is definitely a question in which it is easy to get turned around on. However, I think this one can be more easily attacked my thinking about the basic structure of the reasoning.

Rats with Alzheimer's diseased blood show symptoms of C-J disease
+
C-J disease is caused by a virus
→
Alzheimer's is caused by a virus.

The assumption here is that there this some connection between C-J and Alzheimer's! A correct answer would most likely mention the word "Alzheimer's" to show this connection.

As for (E)...

I think it does say that there is an absence of a cause (the blood from rats without C-J) produces an absence of the effect (the C-J disease itself). However, how does this relate to Alzheimer's?! It seems that (E) just fortifies the idea that C-J is caused by a blood virus but DOESN'T fortify the idea that there is some connection between C-J and Alzheimer's. Someone let me know if I am wrong here.