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The Deal with Principles in Logical Reasoning

Principles come up in a few different contexts in Logical Reasoning on the LSAT. Often, the word “principle” makes LSAT students think that there’s something weird, different or special about a question. Questions involving principles are a tiny bit different, but it’s really not a big deal. So let’s work out how to do these questions.

What is a principle?

A principle is just a general rule. That general rule can be applied to known facts about a situation to reach a judgment. Principles typically function as premises in an argument. The judgment is a conclusion, while the facts to which the principle is applied are also premises.

Principles are frequently conditional statements, so questions involving principles will often test your diagramming skills. The distinction between sufficient and necessary conditions is often critical.

Strengthen questions frequently involve principles.

Now let’s go over the types of questions that most commonly feature principles. The most common place for principles is in Strengthen questions. You’ll get a prompt along the lines of,

“Which of the following principles, if valid, would most help to justify the reasoning above.”

They’re asking you to take the principles in the answer choices as true. One of those principles is supposed to “help to justify,” or in other words strengthen, the argument. So it’s a strengthen question. If you treated it like any other strengthen question, you wouldn’t go wrong.

The presence of the principle changes things a little bit, but not in the sense of making things weird or hard. The real difference is that the answer is a bit more predictable than on your average Strengthen question. Let’s say that the premises tell you that Claude, while at the dentist, noticed a closet full of medieval torture devices. The conclusion is that Claude should run like hell.

The job of the answer is to help you get from the premises to the conclusion. You can anticipate an answer along the lines of “if you see torture devices at the dentist, you should run.” It’s really that simple. Still, the actual answer could be something like,

“A dentist’s patients should remain in the office only if all devices in the office are suitable for modern dentistry.”

That answer may sound a little different but it’s more or less the contrapositive of what we anticipated. The contrapositive is always valid, so we’re good to go.

The answers are often conditional statements, so conditional indicator words are really important. Let’s say an answer starts,

“A dentist’s patient should run from the office only if…”

That answer is wrong, no matter what follows. The correct answer needs to help us get to the conclusion that Claude should run. This answer gives necessary conditions to run. No matter what those conditions might be, they’re not going to help justify the conclusion that Claude should run.

We also often see principles in Soft Must Be True questions.

Another common type of question with principles is a Soft Must Be True question. These questions ask something like,

“Which of the following judgments most closely conforms to the principles stated above.”

These questions are really the same as the Strengthen questions above, except with the stimulus and answer choices flipped. In these questions, you’ll take the principles in the stimulus as true, and one of the principles will help strengthen an argument in the correct answer.

Start with the principles in the stimulus. If they can be diagrammed, diagram them. You usually can diagram. Then, look at your principles. You want them to lead from facts to judgment. Say the stimulus states,

“Anyone who witnesses a crime should report it. One should report a crime one has heard about second hand only if the source is extremely credible”

The first principle is “Anyone who witnesses a crime should report it.” We should diagram that as:

Witness → Report

That one already leads from a factual matter (you witnessed the crime) to a judgment (you should report it). It’s good to go.

But the other principle is not arranged correctly to begin with. The second principle is “One should report a crime one has heard about second hand only if the source is extremely credible.” We should diagram that as:

Report from hearing second hand → Source extremely credible

The judgment, or what you “should” do, is on the sufficient side. This principle isn’t going to help reach a judgment that someone should report a crime. The arrow goes the wrong way. But it can support a different judgment — just take the contrapositive. If the source isn’t credible, you shouldn’t report it. Boom. Facts to judgment.

Now for the answer. It’ll take one of two forms: Someone witnessed a crime and therefore should report it, or someone heard about a crime from a source that wasn’t extremely credible, and so shouldn’t report it. That’s really it. Expect the converse and inverse as wrong answers. For example, that someone didn’t see the crime and so shouldn’t report it or that someone heard about a crime from an extremely credible source and so should report it. Those answers don’t follow the arrows. They go the wrong way, and so they’re wrong.

There will sometimes be principles in Parallel questions as well.

The third common type of question with principles is a Parallel question. They’ll ask, say,

“Which of the following arguments most closely conforms to the principle underlying the argument above.”

These questions are much like the type we just discussed, but with a critical difference: instead of just giving you the principle, they give you an argument that follows the principle. So your job is to extract the principle. What general rule did the argument follow to get from its premises to its conclusion? Put it into words, and then that’s your motto. The correct answer is an argument that follows the same rule.

This isn’t really all that different from any other Parallel question. There is one important distinction, though. When they ask you to parallel an argument based on the principle it follows, you don’t need all the nitty gritty details to match as well as in a regular Parallel question. The answer just needs to follow the same basic principle.

You got these!

So there you have it. You’ll see principles in a few other contexts, too. If you do, remember that there’s nothing terribly weird about it. A principle is just a general rule that can be used (or misused) to try to draw judgments. Don’t classify it as a principle question. Ask what the answer is supposed to do and go from there.

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