Most experienced students will recognize that to weaken an argument, you should try to think about why the conclusion might be wrong despite the evidence. This approach is definitely a good one, and as a general matter, the right one on the vast majority of Weaken questions.

But on some difficult Weaken questions, it’s important to recognize another way to weaken: showing an inconsistency in the author’s position. Read on to explore this method of weakening.

As a preliminary matter, when weakening an argument, we're primarily trying to weaken the reasoning of the argument. But there are actually several ways by which one can weaken the reasoning of an argument.

One way is to directly counter the assumption. So, take this sample argument:

Penguins are chubby.
So, they're cute.

The assumption here is that if something is chubby, then it's cute. So we can weaken the argument by showing that there's at least one thing that's chubby that isn't cute.

(Now, if your instinct is to say, “But that doesn't mean penguins aren't cute - they can still be cute. And chubbiness can still be one factor that contributes to their cuteness; it can still be a + on the scale of cuteness” - you're right. But we've still weakened the argument by showing that their premise does not automatically prove their conclusion. The author of the argument was assuming that the chubbiness of penguins, by itself, without anything more, would guarantee their cuteness. We've shown that that assumption is wrong, which hurts the argument.)

That way of weakening an argument I think is most natural, and most susceptible to an approach that focuses on "Why could the conclusion be wrong even if the evidence is true?"

But another way to weaken reasoning is by showing that the assumption would lead to an inconsistency in the author's position. And this way of weakening is something that can escape one focused solely on pointing out a "loophole" (to borrow the parlance of the popular book). For example, let's add a bit to the stimulus:

Many animals in the world are not cute. Snakes, skunks, and rhinos, for example.
But, penguins are chubby.
So, penguins are cute.

One assumption of this argument is that if something is chubby, then it's cute. Do you see how we would weaken the argument by pointing out that rhinos are chubby? If we accept the author's assumption, then the author would compelled to think that rhinos are cute - but that contradicts something the author believes. But if we don't accept the assumption, in order to preserve the author's belief that rhinos are not cute, then the author's premise no longer leads to their conclusion about cute penguins. So we've caught the author in a contradiction - their affirmed position on rhinos conflicts with the assumption underlying their argument about penguins. Argument = weakened. Notice how if you go into the answers on this argument thinking "Why could penguins not be cute even if they're chubby?", you won't immediately notice the logic of the answer "Rhinos are chubby."

Now the example above demonstrates what one might call an "indirect" showing that the assumption is false. Rather than directly giving an example of something that's chubby but not cute, we've shown that the author's own beliefs commit them to something that contradicts that assumption - thus, the assumption is false.

Here's another version of "indirectly" showing that the assumption is false by way of inconsistency.

Devi said she'd be in Los Angeles at 9pm.
Thus, we can expect to see her in LA at that time.

We can weaken this argument by pointing out: "She said she'd be in San Francisco at 9pm." This weakens because if we accept the author's assumption that she'll be where she said she'd be, then she'd have to be in both LA and SF at 9pm. Clearly that's impossible, which means the assumption is wrong.

But this idea of inconsistency as a weakener also extends to situations in which we're not actually showing that the assumption is false.

Here's an example:

Sarah says that Paradorn, our new international student, was born in Thailand.
But his accent does not sound like that of someone born there.
Thus, Paradorn was probably born in a different part of Southeast Asia.

Now one way to think about this is to ask, "Why might he actually be from Thailand despite not having an accent that sounds like someone born there?" And if this your question, you'll be naturally disposed to answers that point out that you can be from a country without having an accent associated with speakers of that country. For example, he might have been born in Thailand but moved to a different place when he was young, which would explain his lack of an associated accent.

But what would you think about this answer: "Paradorn's accent does not sound like that of someone born in parts of Southeast Asia outside of Thailand."

This actually does weaken the argument, because the author's underlying assumption is that if one does not have an accent that sounds like that of someone born in a particular area, then that means one was probably not born in that area. By pointing out that his accent is not like that of someone born in SEA, we've caught the author in an inconsistency. According to their own assumption, we'd have to conclude that he probably wasn't born in the non-Thailand parts of SEA, either. But that goes against the author's own conclusion - so either the author's assumption is wrong, or the author's conclusion is wrong. Argument = weakened.

In my experience, this type of answer is a bit difficult to pick up on because most people would be mainly focused on showing why Paradorn actually could be from Thailand despite the lack of an accent. And it's tough to see how this answer gives us some positive reason to think he may have still been born in Thailand.

In addition, the logic of this answer seems to go against our initial instinct to question the relationship between being born in a particular place and having an accent associated with that place. Many of us would think "The lack of an accent doesn't really tell us much about where you were born." And then it would be weird to pick an answer that weakens by seeming to suggest that because he doesn't have an SEA accent, he's not from SEA. But the key is that this answer's logic isn't actually about countering the conclusion of the argument. It's about showing the author's inconsistency - if their assumption is true, then their conclusion doesn't make sense. And if their conclusion is true, then their assumption doesn't make sense. This does weaken the argument even if we have not actually shown that the author's conclusion is false or that their assumption is false. What we've shown is that at least one of them must be false.

This type of weakening logic, I think, is hard to recognize if your approach is limited only to asking "Why could the conclusion be false even if the premises are true?"

-Kevin