Which is the Necessary Condition or Sufficient Condition?

[george12]

I am struggling with the first part of chapter six on conditional reasoning. I am still unclear how you know which one is the necessary condition and which one is the sufficient condition. To me it seems like they are somewhat interchangeable and I got a significant amount wrong on the mini-drill and do not understand the explanations. Could you give me further explanation or are there any other resources that might help me with this?

[Dave Killoran]

Hi George,

Thanks for the question. Which term is necessary and which is sufficient is actually determined by the language used in the stimulus. It sounds to me like what you are trying to do is actually figure each relationship out based on what you know of the world when you read each sentence. In the real world that would work fine, but in the LSAT world that causes a lot of problems because LSAT speakers can make up ridiculous and nonsensical statements. So, the best way to handle conditionality is to simply use the indicator words in the sentence and respond only to those. The two lists at the bottom of page 124 are critical and you must memorize each one. Then, use those words to help you figure out which part is sufficient and which is necessary. Here's an example:

Sentence: The sun rises only if I wake up in the morning.

If you reason this out using the real world, you would probably make "sun rises" the necessary condition because it is ridiculous to think that the sun would depend on whether I woke up or not. However, I wouldn't even try to think about it that way. Instead, I'd would see the term "only if," know that that introduced a "necessary" term, and immediately realize that "I wake up" is necessary and thus that "sun rises" is sufficient, leading to the following diagram:

Sun rises --> I wake up in the morning

The meaning of that diagram and relationship is ridiculous from a real world standpoint, but, nonetheless, that is exactly what the author said in the sentence.

At first, rely just on the indicator words, and then over time you will start to see the nuances of the relationship and get a much better handle on which condition is which. Good luck and please let me know if you have any further questions!

[Fish]

Hi,
I am currently working the Necessary and Sufficient and also having trouble. When you say to memorize the words on page 124 do you mean the Cause and Effect ones... I think I'm getting very confused between the two.
Thanks for your help.

[Dave Killoran]

Hi Fish,

Thanks for the question. You will need to know both the Causality words and the Conditionality words, so memorize all of them. That will help alleviate your confusion over which form of reasoning you are looking at.

Memorizing them first is critical because it gets you up to speed on recognizing when certain forms of reasoning are present. But, remember, that the ultimate goal should be to understand the reasoning behind how both forms work, so once you can recognize when causality or conditionality is present, focus thereafter on understanding what is actually occurring (because there will come some points where one of the reasoning forms are present but there is no obvious indicator word).

Please let me know if that helps. Thanks!