Students sometimes take critical thinking to help them prepare for standardized tests such as the GRE and LSAT. They are right to: Although no one course can transform low scores into high ones, critical thinking explicitly lays out many of the ideas and methods we use without realizing it, and so makes it easier to apply them to logical and analytical puzzles.

(This is not to say that students should take courses merely as cram sessions. But, given that critical thinking is bound to have effects on your reasoning that go far beyond exam scores, there's nothing wrong with going to it for an immediate practical purpose.)

The "Logical Reasoning" section of the LSAT bears an especially close relationship to the topics covered in critical thinking. Logical Reasoning questions include questions about a passage's main idea or conclusion, about assumptions made in the passage or inferences that can be drawn from it, and about fallacies or other flaws exhibited in the passage. We have covered these matters in earlier discussions of clear assertions (Chapter 2), argument (Chapter 7), and fallacies (Chapters 5–6). But Logical Reasoning can also ask about the structures of arguments; and here Chapter 8 can help.

Consider the following: Truck drivers are seasoned professionals. So some truck drivers will be aware of your presence when you're in their blind spot. Which is the necessary assumption for the conclusion to follow logically?

1. All seasoned professionals will be aware of your presence when you're in their blind spot.
2. Truck drivers are the only seasoned professionals.
3. All seasoned professionals who remain unaware of your presence when you're in their blind spot are truck drivers.
4. Some seasoned professionals will be aware of your presence when you're in their blind spot.
5. Some truck drivers will remain unaware of your presence when you're in their blind spot.

It is the work of a moment to write the given premise and conclusion as:

All T are S.

Some T are A.

Every term in a categorical syllogism must occur exactly twice. So before even thinking about validity, you know that the missing premise must contain S ("seasoned professionals") and A ("people who are aware of your presence when you're in their blind spot"), and nothing else. Already this narrows down the possible answers to A and D.

Now try out both possibilities. The first answer gives you:

All T are S.

All S are A.

Some T are A.

Valid. The second answer gives you:

All T are S.

Some S are A.

Some T are A.

Invalid. (If you're using the rules, you notice that the middle term has not been distributed in either premise.) So the answer is A.

The last part of the "Additional exercises" below contains a few more such questions. When you have mastered categorical logic, you can fly through such problems much faster than if you have to stop and think about what each sentence contributes to the argument.