LSAT Question Explanation
PT 105, Section 4, Question 8
Identify the FlawArgument structure
Speaker X's sentence will be recognized as grammatical.
Only if a sentence can be diagrammed is it grammatical. Any grammatical sentence is recognized as grammatical by speakers of it's language. Speaker X's sentence can be diagrammed.
Explanation
This stimulus has some tough wording to work through, but it helps to recognize that this is conditional logic. The first two sentences tell us that a sentence being grammatical guarantees two things: that the sentence can be diagrammed, and that the sentence will be recognized as grammatical.
P1: Grammatical → Diagrammable
P2: Grammatical → Recognized
The third sentence tells us that Speaker X's sentence can be diagrammed. But a sentence being diagrammable doesn't guarantee anything, because diagrammable is a necessary condition. Not a sufficient condition.
So the conclusion, that X's sentence will be recognized as grammatical, isn't proven. To make this conclusion we would need to know that X's sentence is grammatical, because that was the sufficient condition for recognizable. But we don't know that, because we only know that X's sentence is diagrammable, which is the necessary condition for grammatical. The necessary condition doesn't guarantee anything, some diagrammable sentences might not be grammatical or recognized as such.
Answer choices
This is outside the scope of the argument. We're just talking about if a sentence is diagrammable (if it's possible to be diagrammed), not if most people can do it.
This points out the flaw. The author tried to claim that because X's sentence is diagrammable it must be recognized as grammatical. But some diagrammable sentences may not be grammatical and may not be recognized as such. Diagrammable doesn't guarantee anything based on the evidence.
For those who like to diagram, the flawed reasoning from the Linguist went like this:
XDiagrammable → XGrammatical → XRecognized
This is illogical, it swaps the conditions from premise 1.
Whether this is true or not doesn't affect the conclusion, so it doesn't point out a flaw. This treats diagrammable as a necessary condition, but the Linguist tried to use diagrammable as a sufficient condition to make their conclusion. We need an answer choice that points out a situation where a diagrammable sentence may not be grammatical and/or recognized as such.
This just restates the first sentence.
This may be true but it doesn't affect the conclusion. The Linguist seems to be trying to claim that X's sentence is diagrammable, grammatical, and recognized as grammatical. The characteristics of ungrammatical sentences as described here aren't relevant.