Sufficient vs. Necessary Assumptions

[FK00144]

pg. 5-1 in the course book

is sufficient and necessary assumption related to the sufficient and necessary in conditional reasoning or is it just referring to the answer choice being sufficient/ necessary for the conclusion to be drawn?

[Dave Killoran]

pg. 5-1 in the course book

is sufficient and necessary assumption related to the sufficient and necessary in conditional reasoning or is it just referring to the answer choice being sufficient/ necessary for the conclusion to be drawn?

It's both actually! The nature of the answer choice (in relation to the conclusion) is reflective of those conditions. Does that make sense? It should after the lesson, but if not, let us know!

[James Finch]

Hi FK,

Absolutely! The difference is that Sufficient Assumptions (Justify Questions) can act as sufficient conditions to the conclusion's necessary condition: if the sufficient assumption is true, then the conclusion must be true. This makes them a bit easier to deal with than Necessary Assumptions (Assumption Questions).

Necessary assumptions act differently, in that they are necessary to the argument in the stimulus, but not sufficient to make it true. This means that the conclusion is the sufficient condition, while the assumption would be the necessary condition. This requires us to use the Assumption Negation technique (essentially the contrapositive) to test whether an answer choice is really necessary to an argument or not.

To recap, the logic behind a Sufficient Assumption looks like:

Assumptiontrue Conclusiontrue

while a Necessary Assumption would diagram to:

Conclusiontrue Assumptiontrue

requiring us to use the Assumption Negation technique (the contrapositive) to test for the correct answer choice:

Assumptiontrue Conclusiontrue

Hope this clears things up!

[Jay]

Dear Powerscore

Hello.

when using negation test, for Necessary Assumption Questions, does the negation HAVE TO DESTROY the argument completely?

What if the negation partially weakens the initial argument, but does not destroy the argument as a whole?

Would this fall under " Strengthen " (or Weaken) answer?

Thank you!

[KelseyWoods]

Hi Jay!

The negation does not have to completely destroy the argument, but it needs to weaken the argument pretty strongly--at least more strongly than the negations of each of the other answer choices.

Hope this helps!

Best,
Kelsey

[Dave Killoran]

Would this fall under " Strengthen " (or Weaken) answer?

Hey Jay,

An Assumption answer, when negated, then falls under the Weaken category. So, as Kelsey rightly says, it definitely does not have to destroy the argument but will usually hurt it somewhat significantly.

We did a podcast on this that you might find helpful. It's episode 121 at https://www.powerscore.com/lsat/podcast/.

Thanks!

[abhola]

Hi Powerscore team,

From the discussion above, I understand the role of negation test in validating the necessary assumptions.

But how do we use negation test with sufficient assumptions? A bigger question is, is the negation test even useful when dealing with sufficient assumptions?

Thanks.

[Dave Killoran]

But how do we use negation test with sufficient assumptions? A bigger question is, is the negation test even useful when dealing with sufficient assumptions?

You don't It does not reliably apply to Sufficient assumptions/Justify the conclusion questions. For those we use the Justify Formula.

Thanks!