Confusing Necessary for Sufficient

[elbism]

Hello,

I was just wondering, if in an LSAT stimulus the author confuses the necessary for the sufficient, then, in the same breath, do they confuse the sufficient for the necessary? Is it the logical equivalent? because of course, when you confuse N for S you are trading spots, so S has to move to N at the same time. I guess what I'm asking is, for example, if the variables that are mentioned are confused (and the two unmentioned variables have of course also switched place amidst this confusion) can the correct answer refer to the unmentioned variables?

For example,
if you have to be smart to get into Yale

Y --> S

and the author assumes if you're smart you'll get into Yale:

S --> Y

This would be confusing the necessary for the sufficient (Smart)
But as we see, the Y has moved from S to N but that's not the variable the author is emphasising. Despite that, can the correct answer still be "confuses sufficient for necessary" Even though this confusion was just a byproduct of the mistaken assumption?
I guess what i'm asking is - does it always work both ways despite what the author is specifically addressing?

Thanks.

[Nikki Siclunov]

Hi elbism,

The Mistaken Reversal is indeed a logical fallacy that confuses a necessary condition for a sufficient condition:

• A B
  Therefore, B A

The author assumes that a condition (B) necessary for a certain event (A) to occur is sufficient for that event to occur. In other words, he confuses the necessary condition (B) for a sufficient condition.

By contrast, a Mistaken Negation is a logical fallacy where the author confuses a sufficient condition for a necessary condition:

• A B
  Therefore, A B

By stating that the absence of the sufficient condition guarantees the absence of the necessary condition, the author is basically assuming that the sufficient condition (A) is necessary for B (i.e. B A). In other words, the author confuses the sufficient condition (A) for a necessary condition.

Hope this clear it up! Also, check out the two most recent blog posts on conditional reasoning, which you may find helpful:

Conditional Reasoning Redux: The Only Cheat Sheet You Need on the LSAT

Beyond "Unless": Advanced Conditional Reasoning on the LSAT

Good luck!

[elbism]

Hi elbism,

The Mistaken Reversal is indeed a logical fallacy that confuses a necessary condition for a sufficient condition:

• A B
  Therefore, B A

The author assumes that a condition (B) necessary for a certain event (A) to occur is sufficient for that event to occur. In other words, he confuses the necessary condition (B) for a sufficient condition.

By contrast, a Mistaken Negation is a logical fallacy where the author confuses a sufficient condition for a necessary condition:

• A B
  Therefore, A B

By stating that the absence of the sufficient condition guarantees the absence of the necessary condition, the author is basically assuming that the sufficient condition (A) is necessary for B (i.e. B A). In other words, the author confuses the sufficient condition (A) for a necessary condition.

Hope this clear it up! Also, check out the two most recent blog posts on conditional reasoning, which you may find helpful:

Conditional Reasoning Redux: The Only Cheat Sheet You Need on the LSAT

Beyond "Unless": Advanced Conditional Reasoning on the LSAT

Good luck!

No I know.. thank you..But my question is whether it's the same thing since mistaken reversal and mistaken negation are logical equivalents? EG if the error in the stimulus is that the author confuses a necessary for a sufficient, can the correct answer be "confuses a sufficient for necessary" since that's the logical equivalent too?

[Nikki Siclunov]

The MR confuses a necessary condition for a sufficient condition; the MN confuses a sufficient condition for a necessary one. The two flaws are very similar to each other, and it is certainly possible that a MR be described as an assumption of MN, just like it's possible that a MN be described as an assumption of MR. The second scenario is more common than the first. I have never seen a MR description serve as an incorrect, decoy answer to a correct MN answer, or vice versa. This is because both answer choices would be technically correct, even if one describes the logical fallacy slightly more precisely than the other.

Hope this helps!

[elbism]

The MR confuses a necessary condition for a sufficient condition; the MN confuses a sufficient condition for a necessary one. The two flaws are very similar to each other, and it is certainly possible that a MR be described as an assumption of MN, just like it's possible that a MN be described as an assumption of MR. The second scenario is more common than the first. I have never seen a MR description serve as an incorrect, decoy answer to a correct MN answer, or vice versa. This is because both answer choices would be technically correct, even if one describes the logical fallacy slightly more precisely than the other.

Hope this helps!

thank you!!