The BriefA Blog about the LSAT, Law School and Beyond
If you're having trouble with understanding what a contrapositive is on the LSAT, then watch this short video and read the rest of the post.
CONTRAPOSITIVES WILL SAVE YOUR LIFE.
Contrapositives are a life-saver on the LSAT. Often, you’ll think you got an answer choice right. “Duh, they want me to infer that ‘All business school students are greedy.’ Hmm… but I don’t see it in the answers. WHAT IS GOING ON LSAT?!” Well, that’s because the right answer choice says “If you’re not greedy, you’re not a business school student.” See, same thing! I mean that too. Contrapositives are logically equivalent statements. You can think of them as being genetic twins. They’re the same.
So which one is the contrapositive? Actually, they each are contrapositive of the other.
Group 4 is made up of the following terms:
- Not both
All the words in this group follow this translation rule:
You pick either idea, then negate that idea, then make the idea you negated the necessary condition.
Let’s try it, in six simple steps:
Group 3 is made up of the following terms:
These are my favorites, because they’re notoriously confusing for students. I’m not sadistic or anything. That’s not why I like them. I like them because of how easily this apparent difficulty can be overcome by sticking to your translation mechanism.
This game is pretty hard. It's the one about Anastasia and whether she parks in lots X, Y, or Z on Monday through Friday and how much each lot costs - $10, $12, or $15. It's from LSAT PrepTest 44, October 2004, Section 3, Questions 18-22, Logic Game 4.
Watch the video lesson below and learn how to make the key inferences that will help you solve this game.
For more Logic Games explanations like this one, hop over to our Logic Games page. There, we've recorded video explanations for every Logic Game going back over a decade. All in HD, with variable playback speed, and you get to ask questions. Oh, the best part: it's completely free.
Confused by how to negate statements on the LSAT, and not sure how to use them for contrapositives? Watch this video.
Negation is contradiction. They mean the same thing. Remember we’re not talking about opposites (hot v. cold). We’re talking about logical negation, about contradiction (hot v. not hot). The easiest way to get the contradiction of any sentence is to tag the clause “It’s not the case that...” in front of the sentence. But, asking you to use that method, I’m appealing heavily to your intuitions on understanding what a negation is. In case your intuition isn’t very helpful, I’m going to walk you through a couple of commonly problematic types of sentences and show you how to negate them.
This game is from LSAT PrepTest 45, December 2004, Section 3, Questions 1-6, Logic Game 1. It's about sequencing Patterson's meetings with five clients - Reilly, Sanchez, Tang, Upton, and Yansky - and her workout at the gym.
Watch the video lesson below.
When you're taking your timed, practice LSATs, you must practice skipping the hard LR/RC/Games questions. That seems to go against intuition: How am I supposed to get a high score on the LSAT if I don't even attempt some of the questions?
I'll answer with something cryptic, which the rest of this lesson will further explain: you're making a bad assumption. You're assuming that you will actually get everything you answer correct, which is, for 99.99% of the people, false.
The way you want to approach your LSAT is to embrace a principle borrowed from Economics: the low hanging coconut.
When we say an argument is "good," what do we really mean? Well, recall that an argument is just premise(s) plus conclusion. Premises supports the conclusion and the conclusion is supported by the premises. So, “good” really just describes how well the premises support the conclusion. In other words, “good” describes the strength of the support relationship between the premises and the conclusion. Just how “good” can the relationship be? Until it becomes perfect, of course. When that happens, it is said that the argument is valid. Turn your attention to the chart below:
To weaken any argument on the LSAT's Logical Reasoning section, you have to understand one thing.
This thing is not tangible nor obvious. It’s instead abstract and subtle – which means it’s hard to understand. Here it is: You have to take away the support. What support? The support that the premises give to the conclusion. Please read that again. It may sound very obvious, but you’d be surprised at how often you will fall prey because you didn’t heed this warning. It’s only nature since our instinct when arguing is to deny our opponents’ premises. He asserts that there are bugs in the kitchen and therefore an exterminator visit is in order. You deny his premise by asserting that there are no bugs in the kitchen. But what would be the point of asking a question like this? All you did was to say “no.” Much more interesting is it to concede to your opponent her premises and then try to argue that despite your concession, her conclusion still doesn’t follow from her premises. That’s what Weaken (weakening) questions test you on. It most certainly does not say that you are to contradict, attack or weaken a premise nor does it ask you to contradict, attack or weaken the conclusion. It says you are to deprive the support that the premises give to the conclusion, as if with magic. Let me illustrate. Did you watch Dragon Ball Z growing up? Remember Goku’s Kamehameha?
Necessary assumptions can wreck an argument.
First, let’s understand what a necessary assumption is. It’s an assumption (definition: unstated premise) that is necessary for the argument. “Necessary” here means the same thing it does when we talk about a “necessary condition” (as opposed to a “sufficient condition”). Let’s put the relationship between argument and necessary assumption in Lawgic and then run the contrapositive.
Argument (valid) –> Necessary Assumption (true)
/Necessary Assumption –> /Argument
In English, this means that when we negate the necessary assumption, the argument falls apart. No necessary assumption, no argument.
Necessary Assumption questions present you with an argument (premises + conclusion) where in order for the conclusion to be valid, there is a necessary (critical) assumption not stated in the argument. Without this particular assumption, the argument falls apart. Your job is to find this sucker, a necessary assumption, in the answer choice.
There are two types of Necessary Assumptions (NA).