Sufficient Assumptions

In a previous post (see https://mbetutorial.blogspot.com/2022/12/necessary-assumptions-lsat.html) I wrote about necessary assumptions. There, I said that a necessary assumption in an argument is an assumption that is needed for the conclusion to follow from the premises provided. The reason why the negation technique works so well when dealing with necessary assumptions is because if you negate an answer choice and the conclusion no longer holds as true, that means that that answer choice (prior to negation) was a necessary assumption of the argument. 

Sufficient assumptions work a bit differently. A sufficient assumption is not an assumption that is necessary to the validity of the argument. In other words, it's possible that the sufficient assumption doesn't hold but the argument still does. For that reason, negating answer choices doesn't work as a technique here. 

On the other hand, although sufficient assumptions are not necessary for the validity of the argument, if the sufficient assumption is true, it guarantees that the conclusion of the argument is also true. So, when faced with a sufficient assumption question on the LSAT, rather than asking whether an answer choice must be true for the argument to be valid, ask whether an answer choice, if true, will render the argument valid. 

For example:

The french fries at this restaurant are unhealthy, because to make them appealable to customers, this restaurant deep fries them in oil. 

The conclusion here is that the french fries at this restaurant are unhealthy. 

Imagine an answer choice asking for an assumption that reads: "any food deep fried in oil is unhealthy."

That answer choice doesn't need to be true for the conclusion to hold here. If I were to negate that answer choice it might say something like "not all foods deep fried in oil are unhealthy." But that wouldn't, necessarily, render the conclusion false. The french fries deep fried in oil might still be unhealthy even if there's a food that's not unhealthy when deep fried in oil. In other words, I don't need for this assumption to be true for the conclusion to hold. Thus, it's not a necessary assumption.

But what if it the assumption is true? What if it's true that any food deep fried in oil is unhealthy. The french fries at this restaurant are deep fried in oil. If that assumption is true, then my conclusion that the french fries deep fried in oil are unhealthy holds. If the assumption is true, then the conclusion is true, and the argument is valid. The assumption is therefore sufficient for the conclusion to be true. 

I'll write more on this in future posts since the distinction between necessary and sufficient assumptions is an essential one on the LSAT!