Conditional Diagramming: Mastering “Only” in LSAT Questions

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We are continuing our Conditional Diagramming discussion today with the word “only” which is used frequently in LSAT questions. “Only” is a necessary condition indicator, and its usage is often easy to parse. Here’s an example:

Only doctors carry malpractice insurance.

“Only” modifies “doctors,” and thus the proper diagram is:

Carry malpractice insurance → doctor

However, in a number of instances, “only” is used in a way that is more deceptive. In those cases it can be difficult to determine the proper relationship. The classic example is when “only” modifies a term that refers to a later, separated portion of the sentence; when this occurs, “only” doesn’t modify the term physically closest to it in the sentence. Consider this example:

The only way to the top of the mountain is the switchback trail

In this case, “only” modifies “way,” and thus whatever the “way” is will be the necessary condition. Of course, “way” modifies “trail,” and thus the correct diagram is:

Top of mountain → switchback trail

So, if you see someone on top of the mountain, then according to the author they took the switchback trail. On the other hand, if someone took the switchback trail, does that mean they got to the top of the mountain? No, so switchback trail is not a sufficient condition. Note how tricky this is: “only” is actually physically closest to the term “top of the mountain,” but that’s not the term that “only” is modifying. So, just because “only” is close to a condition doesn’t mean it is actually attached to that condition; you need to make sure that it actually refers to that condition.

Here’s another example:

The only people who drink are those who smoke:

“Only” modifies “people,” and “people” in this case refers to those who smoke. Therefore, the correct diagram is:

Drink → Smoke

If this is still confusing, one way to think about this presentation is that the phrase “the only” is equal to a sufficient condition. In all other cases (that is, when “the” is not in front of “only”), “only” operates as a necessary condition indicator.