Defining Sufficiency & Necessity without Indicators: Help

Comments

• 3 Likes

  clealosdennis Core Member

  July 21 edited July 21 22 karma

  While not perfect, thinking of them as Sufficient=Guarantee and Necessary=Requirement makes them less abstract for me.

  Said another way:
  Think of the sufficient condition as something that, if it happens or is true, guarantees or forces the other thing (the required/necessary condition) to happen or be true. It's enough on its own. (If/then)

  Think of the necessary condition as a prerequisite or a requirement for something else to happen. You can't have the second thing without the first. (crawl, then walk, then run)

  When given a biconditional statement that has no indicator words, the first thing I do is pull out the two pieces involved without worrying about their role. Then I plug them into A / B, and test as follows:

  To test if A is sufficient for B (A → B ): Try to imagine a scenario where A happens, but B does not. If you can, then A is not a guarantee (not sufficient) for B. If you cannot imagine such a scenario, A is a guarantee (sufficient) for B. (Pay close attention to how the question is phrased, esp. things like "if true" language in the stem).

  To test if B is necessary for A (still A → B ) : Try to imagine a scenario where B does not happen, but A does. If you can, then B is not required (necessary) for A. If there isn't a scenario that matches this pattern, B is required (necessary) for A. (This is where that extra context info that may seem like fluff often comes in handy.)

  Here's an example: Citizenship in this country entails the right to vote.

  A = Being a Citizen, B= Right to Vote

  Ask: "If someone is a citizen in this country, does it guarantee they have the right to vote?"

  Answer: Yes, the word "entails" strongly suggests this is a guaranteed outcome of being a citizen. If citizenship entails voting rights, then having citizenship is enough to ensure you have those rights. (I'm about to contradict this for the sake of the example, but bare with me... Keep in mind that the LSAT questions will tell you what to take as true and where you should question, and usually provide the context you need to question it with.)

  The opposite would be to ask: "Can someone be a citizen in this country without having the right to vote?"

  Answer: Are there citizens who don't have the right to vote? Again, keep the info given to you in the question top of mind, and don't let your personal knowledge take you too far afield. If the question stem tells you that it's TRUE that being a citizen means you can vote, then assume they issue voting cards to infants! BUT for arguments sake, let's use our own knowledge that children are citizens but can't vote. That would be a scenario that invalidates our requirement pattern. You can keep going, but as soon as you find one loophole in the requirement, you've solved it.

  This means that being a citizen is sufficient to vote, but having the right to vote is NOT necessary for being a citizen.

  Hopefully this helps, and doesn't add to the confusion!
• sarahramsaroop18 Core Member

  July 21 12 karma

  @clealosdennis thank you!!!
  I have been struggling with this concept for a little over a week and have tried every explanation possible to make it make sense, and I think you have cracked the code in my head! It finally makes sense, especially the use of a question to consider the two ideas presented.
• natemanwell1 Core Member

  July 21 322 karma

  There are always indicators. Would focus more on understanding the argument so the indicators become clear.