LSAT 74, Logical Reasoning I, Q25

LR Question 25 Explanation

QUESTION TEXT: Student: If a person has an immunity to infection…

QUESTION TYPE: Flawed Parallel Reasoning

CONCLUSION: Many people must be immune to staphylococcus.

REASONING: Many people are exposed to staphylococcus and don’t develop symptoms. Immunity is one possible cause for not developing symptoms.

ANALYSIS: This argument mistakes a sufficient condition for a necessary condition. It gives us this statement:

Immunity to staphylococcus –> symptoms

The argument then shows that some people met the necessary condition: they were exposed and had no symptoms.

The argument incorrectly went backwards. You can’t go backwards with conditionals. We can’t conclude that some who were exposed had immunity. (Maybe they had good immune systems in general, but with no specific immunity to staphylococcus).

To parallel the argument, look for these elements:

1. A conditional statement.
2. A statement that says some cases met the necessary condition.
3. A conclusion that goes backwards and incorrectly concludes the sufficient condition.

___________

1. This isn’t the same. This argument has a conditional statement, and also a “some” statement that adds a new term. That is enough to make this the wrong answer.
   The “some” statement in this argument does allow a valid deduction, though the argument doesn’t make it.
   Moral –> Just
   Serve interest SOME Just –> Moral
   Deduction: Serve interest SOME Moral
   The conclusion of this argument is wrong. It’s an incorrect negation of the valid “some” statement above. The conclusion was: Serve Interest SOME Moral
2. There are no “some” statements given in this answer, just two conditionals. And the conclusion says “probably”, which is structurally different from the absolute conclusion in the stimulus.
   Note that the conclusion is wrong: it incorrectly assumes that anyone who tries to persuade is probably an advertiser. (This isn’t quite an incorrect negation, because of the “probably)
   Advertiser –> Persuade
   Fiction –> Advertiser
   Conclusion: Fiction (probably)–> Persuade
3. This argument ignores a possibility. It’s possible Isabel took the medication, and it alleviated her symptoms even though she’s still quite sick. You can be somewhat better, even though your situation is still bad. You’d need to prove that there was no cure AND no alleviation before you said that Isabel didn’t take the medicine.
   So, the author misunderstands how compound necessary conditions work. That doesn’t match the reversal error in the stimulus.
   Diagram: Medicine –> Cure or Alleviate
4. CORRECT. This exactly matches the incorrect reversal.
   Taxation –> expansion
   The author states the necessary condition occurred, and then incorrectly goes backward to say that there must have been taxation.
5. This argument is just wrong. There are plenty of reasons doctors might wash their hands less than other health professionals.
   1. Doctors are lazy and careless, OR
   2. Doctors are in fewer situations where hand-washing is required.
   Who knows? In any case, this argument is completely different from the incorrect reversal in the stimulus.

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