Sufficient → Conclusion → Necessary

yayu.zheng

Does this mean that the sufficient alone is enough for the conclusion to be drawn and once the conclusion can be drawn, the necessary is a must?

Chris Nguyen

Yup!

Essentially, sufficient assumptions are enough to make the conclusion of an argument 100% true. Necessary assumptions must be true for the conclusion to be true.

KevinLuminateLSAT

This might be a bit nitpicky, but I think the term in the middle should be "Valid Argument" rather than "Conclusion". This is because when talking about whether a statement is a sufficient assumption and/or a necessary assumption, we're really speaking about whether the statement is sufficient for the argument to be valid and/or necessary in order for the argument to be valid. If you just focus on the conclusion itself without a solid understanding of how the premises are designed to support the conclusion, then you may not recognize when something is a sufficient and/or necessary assumption. This can be a problem if people start mistakenly thinking that "If the necessary assumption is false, then the conclusion must be false."

For example, let's say you know that this is the conclusion of the argument:

"We will visit Japan this year."

Is the following statement a sufficient and/or necessary assumption? "I will get a 180 on the LSAT."

You actually can't tell - you can't evaluate whether something is a sufficient/necessary assumption without knowing the premises. Let's bring in the full argument:

"If I get a 180 on the LSAT, we will visit Japan this year. Thus, we will visit Japan this year."

Now we can tell that "I'll get a 180" is both sufficient to make the argument valid, and it's necessary for the argument to valid, too.

Someone who's purely thinking about the conclusion might not realize that getting a 180 is sufficient and necessary for the argument's validity. For example, they might think - "In order for the claim that we will go to Japan to be true, does it have to be true that I will get a 180? Maybe I don't get a 180 and we still decide to go to Japan this year. How does negating the idea that I'll get a 180 falsify the claim that we'll go to Japan? I don't think it's necessary for the truth of the conclusion."

Notice that this analysis misses the mark - the reason that getting a 180 is necessary for the argument is because if it were not true, then the premise that was given would have nothing to do with supporting the conclusion anymore. The author of the argument gave us the premise because she was assuming that it would be triggered, which is how we'd reach the conclusion that we're going to Japan. But if you focus only on the truth of the claim "We're going to Japan this year", you might not see this analysis.