Getting Down to Brass Tacks: LSAT Sufficiency & Necessity
- Mar 24, 2011
- Advice on Logic Games, Advice on Logical Reasoning, LSAT
- Reviewed by: Matt Riley
Getting Down to Brass Tacks: LSAT Sufficiency & Necessity
It’s been a while. Life at Blueprint has been pretty busy, resulting in my extended hiatus from the blog. But it’s good to be back.
First, let me reintroduce myself. Riley. UCLA grad. Co-owner of Blueprint. Scored 176 or higher four times on the LSAT. Taught more LSAT students in the last six years than anyone in the universe (I think). Unfortunately nicknamed the LSAT Ninja.
Great, now that we got that out of the way, let’s talk LSAT. I recently started teaching a course gearing up for the June LSAT. In the hopes of aiding my team (as well as all of the other LSAT students out there), I will be authoring weekly blog posts with LSAT tips.
This week: sufficient and necessary.
I know, not the most exciting of topics, but it really is the only appropriate place to start. You see, these two words really are the foundation for the LSAT. They are akin to addition and subtraction in math. Or being able to stomach pointless small talk on a first date. If you don’t know the basics, things aren’t going to go well.
Sufficient and necessary conditions are the key ingredients to conditional statements, which are the most common logical propositions on the LSAT. Just to be clear, these are simply big words for claims that you use regularly in your everyday interactions.
“If I have one more shot of tequila, I will wake up on the bathroom floor.”
“Unless I get a real job, I will have to start working the corner.”
“I would only sleep with him if he were the last man on the planet.”
Believe it or not, this form of statement will largely determine your success on the LSAT. Weird. But here is the trick. When you are “hanging with the homeys” (new slang I just picked up), you don’t realize that these statements follow very concrete rules. These claims lead to certain inferences, and they are the basis for some tempting fallacies.
So here are a few lessons that I hope will assist you in your battle against conditional statements…
1 Sufficient goes on the left. Necessary goes on the right.
I have devised many methods for helping students understand conditional statements. I’ve helped them rephrase the statements. I’ve urged them to understand the intricacies of the logical relationship. I’ve implored them to grasp the complex nature of sufficiency and necessity.
However, there is one simple piece of advice that always works best: just remember which side to put the sh*t on.
That’s right, just remember where the sufficient goes and where to throw the necessary. The rest will take care of itself. Here are a few examples:
“Anyone who owns a private island is filthy rich.”
Most students will understand that owning a private island, in this example, is sufficient to conclude that someone is rich. It is enough to guarantee that someone is really, really loaded. Simply place that condition on the left side of the diagram and you are good to go: Private Island –> Filthy Rich.
“To be a Justin Bieber fan, you must be under the age of 12 or a pedophile.”
In this one, it is important to notice that they are introducing necessary conditions to be a fan of the boy with the most famous hairdo in Hollywood. One must meet one of these conditions; it is a requirement. So don’t think, just jot down an arrow diagram and throw these conditions on the right: Bieber Fever –> Under 12 or Pedophile.
See, simple as that.
2. There are only two things that matter.
Students commonly give conditional statements much more credit than they deserve. As soon as they see the arrow and variables crossed out, they have bad flashbacks to Ms. Brown’s high school algebra class. But derivatives be damned, these statements are far simpler than that. There are only two things that you have to watch for:
1. Satisfying a sufficient condition
2. Denying a necessary condition
Consider the following claim:
“To take part on Dancing with the Stars, you must be a washed up celebrity.”
As we stated in Part 1, notice that must is introducing a necessary condition, so our diagram would look like this: DWTS –> Washed Up Celeb.
Here are the two inferences that can be drawn from this statement. If a certain person (Bristol Palin, the Situation, or Kirstie Alley, perhaps) is on that fabulous show, then we automatically conclude that they are a washed up celebrity (because they have satisfied the sufficient condition). Also, if someone is not a washed up celebrity (they might never have been a celebrity or their career might still be going strong), then we know that person will not be dancing an awkward merengue on ABC anytime soon (because we denied the necessary condition).
That seems easy enough, but here is where students get thrown. Nothing else matters. For instance, if someone is a washed up celebrity (Danny Bonaduce, Screech, Steven Seagal, Vanilla Ice, or either Corey, for instance), what does that mean? You might be tempted to say that he or she is on Dancing with the Stars, but that would be terribly wrong. In this situation, we have satisfied the necessary condition, but that doesn’t tell us anything.
Also, let’s say we know someone has never appeared on DWTS (doesn’t meet the sufficient condition). Then what? You answer should be the same: so what. That person may or may not be a washed up celebrity.
When you are dealing with conditional statements, always keep your eyes peeled for the two things that matter. Other than that, in the immortal words of Slim Shady, you just don’t give a fu*#.
Until next week, may your logic be sound and your diagramming powerful.
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