LSAT Logic: Neither Necessary Nor Sufficient

Sometimes test-takers get so wrapped up in formal logic and necessary/sufficient conditions that they forget something very important.

Some things in the world are neither necessary nor sufficient.

Take the relationship between frequent showering and kissing, for example. (Yes, it's a cutesy PG-rated example. Deal with it.)

Let's assume that:

Evidence #1: Some people are rich and/or famous enough that they could get a lots of kisses without showering frequently.

Evidence #2: However, other people shower all the time but still don't get lots of kisses.

Conclusion: Showering frequently doesn't help one get kisses.

The above conclusion is invalid.

The first piece of evidence tells us showering frequently is not necessary to get kisses.

The second piece of evidence tells us showering frequently is not sufficient to get kisses.

So, we've learned showering frequently is neither necessary or sufficient to get kisses. Does this mean showering frequently has no impact on kiss-getting? Of course not.

Showering frequently can help one get kisses without being enough to get kisses. For example, being good-looking probably helps.

Now look at the following two questions from real LSAT PrepTests:

PrepTest 32 (October 2000), Section 1, Question 11 (page 121 in Next 10)
PrepTest 38 (October 2002), Section 4, Question 21 (page 347 in Next 10)

It's worth noting that the first of these two questions (PT32) is not a flaw question. It's simply recognizing the same point as the showering example above and the question in PT38. We could easily turn the PT32 question into a flaw question by adding the following sentence to the end of the stimulus:

"Therefore, prior experience in foreign affairs does not promote successful foreign policy."

We could then accuse the argument of "failing to consider the possibility that prior experience in foreign policy promotes successful foreign policy without being necessary or sufficient alone to guarantee it."

Photo by stevendepolo / CC BY 2.0