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How to Diagram Ornery “Only” Logical Reasoning Questions

If only the LSAT would stick with easy-to-diagram conditional statements like “if it’s a carrot, then it’s a vegetable”, or “if I get Mike Tyson’s tattoo, I’ll forever regret it.”

Alas, your Logical Reasoning section will rarely be quite so friendly. You’ll be nailed with parallel flaws, double negatives, “EXCEPT” questions and, most of all, lots of diagramming. So, to perfect your diagramming skills, we’re launching a series of articles that will cover some of the trickier elements of conditional statements.

Up first: “Only” Questions.

If memorization is your forte, then remember simply that “only” always introduces a necessary condition. As in “the only time you’ll see ‘only’ on LR is when it is introducing the proposition that is guaranteed by the sufficient condition.”

For those less memorization-inclined, however, let’s make this concept a little more intuitive. As an example, consider the following:

“Only Apple Inc. has the patent rights to rectangles with rounded edges”

What does the ‘only’ refer to in this case? It refers to Apple — meaning exclusively Apple; meaning none other than Apple; meaning that if it isn’t Apple, then it doesn’t have patent rights to rectangles with rounded edges. Essentially, if there exists something that has the patent, it’s necessary that that thing be Apple Inc. You would diagram it as:

patent rights to rectangles —> Apple Inc.

Unfortunately, not all questions are so straightforward. The most difficult involve conditional statements in which the referent (that which is referred to by ‘only’) does not immediately follow the word ‘only’. For instance:

“The only city that requires marijuana dispensaries to provide free weed to indigent patients is Berkeley”.

Tricky. Here the referent does not immediately follow the ‘only,’ so it can be a little more difficult to locate the necessary condition. But if we rephrase the conditional statement, it’s clear that it is necessary to be Berkeley in order to be a city with this policy. To say that having this policy is necessary in order to be Berkeley is 1) very silly and 2) the fallacy of the converse.

A little tip that has always worked well for me: I only (wink wink) diagram “only” statements in the contrapositive. For those unfamiliar with this term, it describes the relationship between “if A then B” and “if not B then not A” (the contrapositive being the latter). So when I diagram the above conditional I think of it in terms of:

“If it’s a city other than Berkeley, then it doesn’t require dispensaries to provide free weed”.

Thus, “not B —> not FW”

Tinker around with this a bit, and figure out what approach will make you the most comfortable, accurate, and efficient on LSAT test day. To get your mind working, check out the following examples.

The only bad pie is rhubarb pie.

Rhubarb pie is the only bad pie.

Both of these statements are diagrammed:

BP —> RP

This is because in both cases the “only” refers to that godforsaken nadir of pastries that is rhubarb. It is the limiting condition of the sentence — the property upon which badness rests.

Here’s another:

Only Kobe Bryant compares to Michael Jordan as a basketball player.

The only basketball player who compares to Michael Jordan is Kobe Bryant.

As with the previous example, both of these are diagrammed:


This is because the “only” refers to the Black Mamba. He’s necessary to make comparisons to Michael Jordan.

If you’re struggling with diagramming “only” statements, take solace in the fact that you are not alone. Many find them difficult. However, as long as you keep in mind that you’re looking for what the “only” refers to, your ‘only’ (last one, I promise) concern going into the LSAT will be how to celebrate after.