The Logical Reasoning section of the LSAT contains a number of frequently repeated argumentative structures. One of the most common involves an author using the given evidence to reach a definitive conclusion without considering alternative factors. Here’s an example of such an argument:
Last night, I planted a bed of roses in my front yard. This morning, I noticed that the roses had been dug up and spread around the yard. My next-door neighbors have a dog that enjoys digging up flowers, so it must have been their dog that dug up my new roses.
In this argument, the author discovers a possible factor in the flowery destruction: a dog with a penchant for digging up flowers. While it sounds like a reasonable explanation, the author unfairly concludes that this “must” be the only explanation. Such extreme wording often indicates that the author has failed to consider alternative possibilities – and that’s certainly true in this case.
With this kind of argument, there are a lot of questions the LSAT could ask. For starters, the LSAT could ask you what the “error in reasoning” is or what makes the argument “vulnerable to criticism.” In these questions, you merely have to point out the flaw. The answer could be presented in a fairly straightforward manner (e.g., the author fails to consider that some other animal could be responsible for digging up the roses) or may be dressed up in slightly abstract wording (e.g., the author treats one possible explanation for a particular occurrence as if it were the only explanation for that occurrence).
The LSAT could also ask you to weaken or undermine the argument. Since we know that the author is overlooking possible alternatives, a quick way to weaken the argument is by offering another explanation. For example, a correct answer may suggest that the author lives next to another neighbor who owns a rabbit known for digging up flowers.
Finding the correct flaw or identifying information that could weaken the argument stems from recognizing the author’s primary assumption: that the neighbor’s dog is the only animal that could be responsible for the roses being dug up. This assumption is just one more piece of information the LSAT could ask for. However, like with flaw questions, you have to be prepared for multiple variations on wording the correct answer. On less difficult questions, the assumption could be written clearly (e.g., no other animal could have dug up the roses). On more difficult questions, the correct answer could consider a specific alternative and deny the possibility of that alternative happening (e.g., the property surrounding the author’s home is not inhabited by gophers, which are known for digging up flowers).
These answers are the most difficult to spot because they often bring up information that wasn’t provided in the original argument. It wouldn’t be unusual for your first reaction to be, where did the gophers come from? The argument was talking about a dog! However, if the author’s home was near flower-digging gophers, that would weaken the argument by providing a possible alternative explanation. Therefore, the author must assume that these gophers aren’t in the area.
As you prep for the LSAT, you will come across many arguments in which the author overlooks alternative explanations. The more easily you recognize them, the more efficiently you’ll be able to answer the questions based on those arguments. Also, remember to be flexible when evaluating the answer choices; the LSAT can present you the correct answer in many different ways.