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# NA vs SA

Core Member
74 karma

What is the primary difference between Necessary Assumption Qs and Sufficient Assumption Qs? Both are required to make the argument valid but I'm not quire sure how the answer choices differ.

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• Member
1058 karma

Here are some of the differences between SA and NA:

-Sufficient Assumption questions are asking you to find the information that will guarantee that the conclusion in the stimulus is true. While Necessary Assumption is asking you what must be true given that the premises and conclusion in the stimulus are true.

-Sufficient Assumption is like asking you to strengthen the argument except much stronger than most Strengthening questions. While Necessary Assumption is more similar to a Must be True question. You need to find the AC that can be proven from the information given to you in the stimulus.

-Generally, Necessary Assumption answers will sound weaker because they have to stay provable while Sufficient Assumption will be strong.

• Alum Member
edited April 2020 2042 karma

The first and most major way that they are different is that sufficent assuptions make the argument valid or very close to valid BUT a necessary assuption does not.

An assumption is something unstated that our speaker left out during the argument. To find a necessay assumption you are looking for the answer choice that must be true if the argument's conclusion is true. You can also ask yourself would the speaker have to agree with this answer choice? If the speaker would disagree or can disagree and the argument still makes since than the than the answer choice is not necessary.

A sufficent assumption provides the most support to the argument. It lends so much support that it generally makes the argument valid or very close. The sufficent assumption proves that the conclusion is true.

Language can also help you distinguish between the two. Sufficent assumptions are powerful. They will use words like: always, only, never, must, requires. Sufficent assumptions do not have to be true but if they are they bring the strength of the argument to validity or very close to it.

Necessary assumptions are provable, they will use words like: Sometimes, possibly, there could be one, not the only one ect. Necessary assumptions have to be true, but they simply help the argument survive.

• Alum Member 🍌🍌
8684 karma

The way that I like to look at it is that the premise and conclusion exist together with some varying level of support. Imagine a continuum where on one side the conclusion was really well supported by the premise(s): think validity or near validity and another end where the conclusion existed completely separated from the premise as far as support goes: we might call this a non sequitur or a conclusion that comes out of left field.

I look at our job on the LSAT with SA questions as to take an argument (premise(s) and conclusion) and add an assumption to that argument that together with the existing structure brings the argument as a whole towards the really well supported area of that continuum mentioned above: ie making the argument valid/air tight or really really close to being valid. One of the most common ways that we do this on the LSAT is to state with an answer choices something that says: If premise then conclusion. This tells us that the premise leads by the laws of logic to the conclusion (remembering here, as my students who might be reading along right now undoubtably recall me stating as fundamental to this process: the contrapositive is always fair game.) There is an enormous amount to discuss here, but this is the nucleus of what we are doing with SA questions. In a sense, I use the following phrase to describe what we are doing with SAs: we are using what we have in the stimulus along with what we provide with the answer choice to get to where we need to be: an argument where the conclusion follows from the premise and the inclusion of the answer choice. We are making the argument better!

For NA questions we are often not making the argument great, but rather, we are preventing it from being worse off! The above commenter's post is really helpful in this regard, we are often with an answer choice mentioning something that MBT. in that sense we can look at an NA as: if this conclusion follows from this premise(s) then what must be true in the background of that argument? Sometimes these answer choices are stated in the affirmative, sometimes they are stated in the negative (a blocking necessary assumption). Notice here that these type of answer choices on NAs don't guarantee or push the argument in its totality too much towards that air tight/logically valid side of the continuum, instead, stating these answer choices prevent the argument from collapsing in on itself towards the non-sequitur or no support side of the continuum.

For NA questions our job might include the possibility of an assumption very much similar to that of SA: where we are in a specific sense bridging premise to conclusion. In that sense we might have some overlap between the job we are doing on NAs and the job we are doing on SAs. One specific example of this is when an argument has one single core assumption: that assumption could be both sufficient and necessary. More than likely when folks speak about bridging NAs they are talking about a bridge from premise to conclusion in a way that is necessary and not sufficient.

I hope this helps.
There are a ton of times here where more detail is required*

• Alum Member
401 karma

An NA is something that has to be true for the conclusion to follow from the premises. SA is anything that allows the conclusion to follow from the premises. A NA is a lot more stringent, an answer choice could be so strong in an NA that it is no longer made 'necessary,' because there could be a weaker variant that would still allow the conclusion to follow from the premises. With SA you can go all out it can be as strong as possible so long as it allows for the conclusion to follow from the premises.