Two ways to weaken an argument
Every LSAT weaken question has an argument with premises and a conclusion. To weaken the argument, you need to make the conclusion less likely to follow from the premises — not prove it false, just reduce its support.
There are two fundamental mechanisms for doing this:
Mechanism 1: Attack a premise. Show that one of the argument's supporting claims is false, incomplete, or less reliable than the argument needs.
Mechanism 2: Introduce an alternative explanation. Accept the premises as true, but show that the conclusion isn't the best or only explanation for the evidence. If something else could explain the same facts, the argument's conclusion is weakened.
Most LSAT students learn Mechanism 1 first and apply it instinctively. Mechanism 2 is less intuitive — it doesn't attack anything in the argument directly, it just shows the argument has a competitor. That's exactly why hard weaken answers use it.
What an alternative explanation looks like
Arguments that are vulnerable to alternative explanations usually have this shape: the premises describe some fact or data, and the conclusion claims that fact is caused by or explained by some specific thing.
Example: 'Sales at the downtown store increased after the new advertising campaign launched. Therefore, the advertising campaign caused the increase in sales.'
The argument assumes advertising is the explanation for rising sales. An alternative-explanation weaken answer might say: 'A competing store that drew away some customers closed permanently two weeks before the advertising campaign launched.' This doesn't attack the premise (sales did increase), but it offers a different explanation — the competitor closed — that could explain the same evidence without crediting the advertising.
The conclusion isn't proven false. It just lost its exclusive claim to explaining the data. That's all weaken requires.
Why alternative-explanation answers seem wrong at first
These weaken answers often feel out of left field because they introduce information the argument never mentioned. Students who are looking to 'attack' the argument find these answers confusing — nothing in them seems to directly contradict a premise.
The reason they're correct is that the argument's implicit assumption is that the stated explanation is the only viable one. If the argument assumes 'advertising → sales increase' and you show 'competitor closure → sales increase' is equally plausible, the argument's confidence in its conclusion drops significantly.
If you're working through weaken questions and an answer introduces something that feels irrelevant, ask yourself: 'Does this give an alternative explanation for the same facts the argument is trying to explain?' If yes, it probably weakens the argument by undermining the 'my explanation is the best one' assumption.
Strengthen is the mirror image
Understanding alternative explanations also helps with strengthen questions. Just as weakening can mean introducing a competitor to the argument's explanation, strengthening can mean ruling out an alternative explanation.
Example strengthen answer for the same scenario: 'No other stores closed in the area around the time the advertising campaign launched.' This rules out the most obvious competitor to the advertising explanation — it removes an alternative cause. That makes it more likely that advertising actually caused the sales increase.
Pairing weaken and strengthen this way — thinking of them as mirror operations — helps you anticipate what kinds of answers can work on each question type before you read the choices.
Spotting alternative-explanation vulnerability before reading the answers
When you read an LR stimulus, ask yourself: does the conclusion make a causal or explanatory claim? If the conclusion says 'X caused Y,' 'X explains Y,' or 'X is responsible for Y,' the argument is vulnerable to an alternative explanation.
Prephrase before reading the choices: what else could explain Y besides X? You don't need to think of the exact alternative the testmaker chose — just prime yourself to recognize an alternative when you see it in the answer choices.
Causal arguments on the LSAT almost always have this vulnerability, and recognizing it in the stimulus lets you evaluate answer choices much faster. You're not just looking to 'attack something' — you're looking for a new explanation for the same evidence.
Frequently asked questions
Does introducing an alternative explanation prove the argument's conclusion is false?
No. Weaken questions don't require you to disprove the conclusion — just to reduce the support for it. An alternative explanation shows the conclusion isn't the only reasonable inference from the evidence, which is enough to weaken the argument even if the conclusion might still be true.
How do I tell the difference between an alternative explanation that weakens and one that's irrelevant?
The alternative explanation has to explain the same evidence the argument is trying to explain. If the argument says advertising caused a sales increase, an alternative explanation about a competitor closing is relevant because it could also cause a sales increase. An answer about the quality of the store's products is relevant only if you can connect it specifically to the timing and magnitude of the sales increase.
Do these two mechanisms — attack a premise vs. alternative explanation — apply to all weaken questions?
Almost all. The specific mechanism depends on what the argument is assuming. Causal and explanatory arguments are most vulnerable to alternative explanations. Argument by analogy or by generalization are more vulnerable to premise attacks. Conditional logic arguments are usually weakened by showing the sufficient condition can happen without the necessary condition following.
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