#25 - Roberta is irritable only when she is tired

[Adam Tyson]

Good point, Henry! The fact that she lost her keys IS sufficient to prove that she must be tired, so that is NOT a flaw. The only flaw is the Mistaken Reversal, assuming that because she is tired she must be irritable.

[queenbee]

Hi
here is how I diagrammed the logic:

If R = I --> Tired
If R = L --> Tired

The stimulus said, R lost her keys (R = L), so I took that as R = L --> Tired. So we know that she lost her keys, therefore she must be tired.
It then says, she must be irritable. "Must be" is the indicator of a necessary condition. the only way that could be a necessary condition is if there is a mistaken reversal where If Tired --> R=I.

Based on the diagraming above and on the language used, I selected D.

Answer choice (E) is saying that it took R=I as a necessary condition to be a sufficient condition. But R=I was already a sufficient condition based on the diagraming above.
Would you please help clarify my mistake?
Thank you

[Paul Popa]

Hi Queen,

Great question! So, we're told that regarding Roberta:

I --> T
and
LT --> T.

We're then told that she lost her keys, so we definitely know that she is tired. Her yawning is irrelevant; having lost her keys is sufficient enough to conclude she's tired. But then the argument claims she is "almost certainly irritable." In other words, the argument is incorrectly concluding that because she is tired, then she is probably irritable. But this is a mistaken reversal of the first statement. Robert might not be irritable at all, even though she's tired. Answer choice (E) describes this perfectly: being tired is a necessary condition for being irritable, but the author treated it is a sufficient one.

D is close, because it describes a mistaken reversal, but it refers to the wrong term. The author doesn't assume that because Roberta's tired, that that is why she lost her keys. The author only commits a mistaken reversal once, when they claim that because Roberta is tired, she is probably irritable as well. Hope this helps!

[pineapplelover18]

Hi guys, just want to ensure I understand this AC bc the wording is a bit tricky. I understand what's wrong but the AC throws me off.

SO by "taking a necessary of Roberta's being irritable to be a necessary" the author means that the flaw takes the "necessary" (which is tired) and places it in the place of "sufficient" (which is the irritable) thereby doing a mistaken reversal?

I understood if Irritable -- Tired, and if Loses things --- Tired, and the Flaw goes loses things --- irritable (which is both sufficient conditions) but places irritable as a necessary. But does this mean then if irritable was used as the necessary in the conditional then tired has to be the sufficient? or is it Loses things --- tired --- irritable? if so, how exactly did we get to this chain?

[Delanoking1]

This conclusion is flawed for two reasons. First it assumes that since Roberta has been yawning and just lost her keys, then she must be tired.

Why is this a flaw? The stimulus says Roberta "loses things only when she is tired." I think it's a legit inference that if she lost her keys, she must be tired.

I was thinking the same thing as well; her losing things is a sufficient condition to infer that she is tired. But there is still a mistaken reversal on the irritable inference.

[Rachael Wilkenfeld]

Great pineapple and delano!

We know that irritable--->tired as well as loses things---->tired. Roberta lost things, so we can conclude she's tired. Tired is the necessary condition. It's also the necessary condition for irritable. But the author makes a mistake here. The author says well, since we know that Roberta is tired, that's ENOUGH to say she's irritable. That treats the necessary condition (tired) as if it were a sufficient condition. That's the flaw, and that's what answer choice (E) describes.

Answer choice (D) is similar, but the author doesn't take tired as sufficient to show she's losing things It recognizes that she loses things, therefore, she is tired. The author doesn't incorrectly conclude that she is losing things, the author incorrectly concludes that she is irritable. That's the bit we aren't able to prove.

Great work!

Hope that helps!