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# Mistaking sufficiency for necessity vs mistaking necessity for sufficiency

Alum Member
128 karma

I'm a bit confused about the technical differences between the errors of (1) mistaking sufficiency or necessity and (2) mistaking necessity for sufficiency. I realize the instances in which the errors arise may be different (i.e. mistaken negation and mistaken reversal, respectively), but aren't the 2 mistakes essentially the same error described in two different ways? If you were to switch the necessary and sufficient conditions of a conditional statement (mistaken reversal) would you not be able to describe the error using either one of those descriptions?

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• Alum Member Sage
edited September 2017 13286 karma

Ignore this - my explanation did not make sense...sorry

• Alum Member
edited September 2017 3197 karma

Necessary would be something that MUST be present for something to happen whereas Suffency Guarantees it WILL happen...

An argument might say you need a #2 pencil to take the LSAT, an LSAT score is required to be admitted to Yale Law. Adam has a #2 pencil, so Adam will go to Yale Law.

A #2 pencil is necessary to take the LSAT

But a #2 pencil is not sufficient to guarantee you'll get into Yale Law...

It is true you can't get into Yale without a pencil but the pencil is required (necessary), but not sufficient to get you into Yale.

When a Sufficiency/Necessary issue takes place, its saying something that must be present becomes something that 100% causes the thing to happen.

• Alum Member 🍌🍌
8700 karma

I do not think that the difference as far as flaw questions go on the LSAT is something necessarily we would be asked to differentiate: at least for the vast majority of sufficient/necessary flaws. If an argument gives us:
A---->B

a sufficient necessary conflation would be an argument that on the basis of:
A---->B
concludes:
B--->A
or
A---->B

As you pointed out, these are essentially/logically the same mistake. There has been one parallel flaw question I can think of that did indeed hinge on our ability to tell the difference between these two variations of the same mistake. That question is: Pt 51 Section 1 Question 20. That is the only question I can recall doing that. With full disclosure, I have yet to do beyond or BR PT 56.

In my estimation, what we should instead focus on with the appearance of a sufficient/necessary conflation on the LSAT are the clever ways the LSAC describes that flaw. Our first pass through the answer choices obviously should be to look for the words "sufficient/necessary" in the descriptor for the flaw question after we spotted the mistake, but sometimes they aren't there, and an answer choice that goes deeper into the meaning of what the flaw actually is is present. But, that is a discussion for another time

David

• Alum Member
edited September 2017 128 karma