Logical reasoning PrepTest 131 · Section 3 · Question 19

Incorrect. Since the conditional doesn't apply to the babies (without more info), we can't use it to reach this conclusion.

Incorrect. We know that there are people who say words without knowing the dictionary definition, but we don't know if they understand them, and the conditional doesn't apply to that case.

Incorrect. If words can be understood without knowing their dictionary definitions, then babies could understand some words they say, but it's not guaranteed, so this answer is too strong.

Incorrect. "any other word" is out of scope of this argument, as it's not mentioned in any premise.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
(Understanding → Knowing dictionary definition) → (Understanding → Understanding words in definition)
Some people (e.g., babies) say words they don't know the dictionary definition of.

Answer Anticipation:
Some of our instructors regard this as the hardest LR question that has ever appeared on the test. It's certainly up there!

One of the reasons that this question is so difficult is that the conditional in that first sentence is nested—the sufficient and necessary conditions are themselves conditional statements! While it's not too rare for a single condition of a conditional statement to be a conditional itself, this might be the only time when both are conditionals.

The reason this makes things so difficult becomes apparent when we see that the example raised—babies who say words without knowing the dictionary definition—negates the necessary condition of that nested sufficient condition. How does that play into the conditional?

Well, to figure that out, we have to talk about the contrapositive of that monster conditional. Your first instinct might be to contrapose the nested conditionals, and then contrapose the whole conditional, but that's not correct. Instead, we just take the contrapositive of the broader statement—we flip and negate it, which means we need to know how to negate a conditional.

To negate a conditional, we don't negate the sufficient condition, necessary condition, or both—instead, we negate the connection between the sufficient and necessary condition. To use a simple example, consider the conditional: Dancing requires music. The negation of that is: Dancing doesn't require music. In other words, the negation of a conditional is, abstractly, that the sufficient condition no longer guarantees the necessary condition.

So the contrapositive of the monster conditional is (written in common English to make it easier to follow):

If understanding a word doesn't require understanding the words in the definition, then understanding a word doesn't require knowing the dictionary definition of that word.

And, from there, the babies example doesn't trigger the rule because we don't know if the babies understand the words they're saying. So, at the end of all of this work, we've determined that the example raised and the conditional don't speak to the same situation (without us knowing whether babies understand the words they say), and so we're left with a very complex conditional and a specific example, and we need to head into the answer choices understanding these statements but without a specific anticipation.

Answer Explanation:
Babies say words without knowing their dictionary definitions. If they understand these words, then it's possible to understand a word without knowing its dictionary definition, as this answer states. It's correct—and note that it didn't rely on that monster conditional.

Key Takeaway:
Don't get too wrapped up in the conditional here—something like this is unlikely to repeat, and the correct answer didn't even rely on it. Instead, remember how to negate a conditional, as that has shown up on the LSAT—negating a conditional means that the sufficient condition doesn't guarantee the necessary condition.