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Just completed this PT, and this is one of the questions I got wrong. After reviewing, I want to make sure I have the reasoning right. This was a real hell of a question, so I wanted to post it here to see if I missed anything.

I mistakenly diagrammed the first sentence of the stimulus (If understanding a word always involves knowing its definition, then understanding a word requires understanding the words that occur in that definition) as two conditionals. This lead me falsely to answer choice A, instead of E.

Instead, I take it I should have approached the question like this:

If understanding a word always involves knowing its definition, then understanding a word requires understanding the words that occur in that definition:

[Understanding --> Knowing the Definition] --> [Understanding --> Understanding Words in Definition]

CP: ~[Understanding --> Understanding Words in Definition] --> ~[Understanding--> Knowing Definition]

We are further told that there are clearly some people, like all babies, that do not know the dictionary definitions of SOME of the words they utter.

Baby --> ~Understand Dictionary Definitions (of some words!)

In Answer Choice E, we are told

If some babies understand all the words they utter, then understanding a word does not always involve knowing the dictionary definition.

So this scenario would mean that Babies (s) <--> Understand Words

combined with

Babies --> ~Understand Dictionary Definitions

And gives us:

Understand Words (s) <--> ~Understand Dictionary Definitions

Meaning: If it were true, you don't have to know the Dictionary Definition all the time!

The conditional statements in this question are a bit of a distractor.

We are told that all babies do not know the dictionary definitions of some of the words they utter. Stated otherwise, no babies know the dictionary definitions of all of the words they utter.

Let's assume that there are some babies who understand all the words they utter (lets say 3 babies out of 10 babies understand all the words they utter), and, as stated above, no babies know the dictionary definition of all the words they utter (0 babies out of those same 10 babies know the dictionary definitions of the words they utter), then understanding a word does not always involve knowing its dictionary definition (choice E).

Choice E must be true because, as per the example above, there are 3 babies who understand the words they utter but do not know the dictionary definition of those words. It's therefore impossible that understanding a word always involves knowing the dictionary definition of that word.