LR Question 22 Explanation, by LSATHacks
QUESTION TEXT: The return of organic wastes to the soil is a good…
QUESTION TYPE: Flawed Parallel Reasoning
CONCLUSION: Small farms ➞ Returning waste good
REASONING: returning waste good ➞ nontoxic AND not much energy
Small farms ➞ nontoxic AND not much energy.
ANALYSIS: This argument make an incorrect reversal. The argument gives us necessary conditions for saying that returning waste is good. And we know small farms meet those necessary conditions.
But you can never prove something with necessary conditions. To prove that returning waste is good, we would need a sufficient condition. And the argument hasn’t given us a sufficient condition.
The conclusion reverses the evidence and assumes that anything that meets the two conditions is good. So we’re looking for an argument:
- A conditional statement
- A situation that meets the necessary conditions
- An incorrect reversal of the statement
___________
- This is a good argument. The conditional statement gives a sufficient condition for thriving, and greenhouses meet that condition.
- This is a good argument. It provides a sufficient condition.
- CORRECT. This matches. We have necessary conditions for viability. The argument reverses this and assumes they are sufficient conditions.
- This is a good argument. It gives a set of sufficient (and necessary) conditions, so meeting the conditions proves the conclusion.
- This is a bad argument. It would be good if it showed the meal had no carbohydrates and protein. But this argument didn’t do that – the 20% remainder of the meal might have had both carbs and protein.
This is failure to meet a condition. It’s not an incorrect reversal, which was the error in the argument.
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I apologize if this isn’t a place where I can ask questions but if it is– how is (D) a good argument? I thought it did exactly what (C) did in reversing the necessary and sufficient. Isn’t (D) saying “eligible -> 19 years of age (x), secondary school (y), have played in sport (z)” therefore “since x y z -> eligible” ?
Good question. D shows both a necessary condition AND a sufficient condition. It says “those and only those”. The first those is showing a sufficient condition.
So meeting the three conditions is both necessary and sufficient.
I agree with the analysis above that says that C is only a necessary condition, as in the stimulus.
Although “if and only if statements” are associated with necessity, I do not believe that this formulation is a necessary condition.
Let’s say that there are 5 criteria to be eligible. But, as in the example, there are only four listed. It would still be correct to say that “Those competitors–and only those competitors who meet all of the following criteria are eligible…”. Only those who met the first four are eligible. But this would be a necessary condition and not a sufficient one. ???