Q8 - Linguist: Only if a sentence

[unclefester2013]

For some reason, this type of question is the bane of my existence. It should be easy, but for some reason, I can't diagram the Sufficient and Necessary correctly to arrive at the correct answer. I'm missing a key piece or understanding of the conditional reasoning. I've read the Superprep explanation, but I just can't get it to sink in. HELP!

[lsatlifer]

For some reason, this type of question is the bane of my existence. It should be easy, but for some reason, I can't diagram the Sufficient and Necessary correctly to arrive at the correct answer. I'm missing a key piece or understanding of the conditional reasoning. I've read the Superprep explanation, but I just can't get it to sink in. HELP!

I pay attention to words that indicate necessary conditions. The stimulus starts off with a necessary condition key prompt: "only if".

Sentence 1: G Only if D ( G -----> D; Contra: ~D ---> ~G)
Sentence 2: If G then R (G ----> R; Contra: ~R ----> ~G)
Sentence 3: D
Sentence 4: R

The flaw occurs in sentences 3 and 4 because the linguist argues that D is sufficient to guarantee R, when there is nothing in sentences 1 and 2 that suggests ANYTHING about what follows from knowing that a sentence is diagrammable (D). What the linguist is attempting to do is this:

D -----> G -----> R

but notice that D ----> G is a mistaken reversal of sentence 1. the knowledge that a sentence is diagrammable is not enough (sufficient) to guarantee that it is grammatical.

So in looking at your answer choices, you're searching for an answer choice that will point out that at least some diagrammable sentences are not grammatical. Why? Because if all grammatical sentences are diagrammable (G ---->D, as in sentence 1) and all diagrammable sentences are grammatical ( D ----> G,the assumption in the argument), then the linguist's argument is valid. We need to logically dismiss or negate that assumption.

So the logical negation of "ALL diagrammable sentences are grammatical" is "NOT ALL diagrammable sentences are grammatical" or in other words, some diagrammable sentences are ungrammatical.

Hope this helps.

To practice, I would study the key words and phrases that indicate sufficient and necessary conditions.

[unclefester2013]

That was a BRILLIANT explanation!! Thank you very much!

[wjvlautin]

Why do we have to assume that the linguist is trying to say D-->G-->R? Couldn't the linguist try to justify D-->R independent of G?