LSAT 149 | Section 3 | Logical Reasoning: Q18

LR Question 18 Explanation

QUESTION TEXT: Herbalist: Many herbal medicines work best when…

QUESTION TYPE: Necessary Assumption

CONCLUSION: At least some people who take herbal medicines every day should skip them for a day or two.

REASONING: A lot of herbal medicines work best over several months. But, they can have toxic side effects when taken over long periods.

ANALYSIS: I couldn’t really see a way to prephrase this. I’ll just note that a lot of answers get relationships backwards. It’s important to first narrow down to 2-3 answers, and then think carefully about how the words relate logically.

In other works, judge answers gently on your first pass, but carefully on your second. This can help avoid eliminating the right answer, and then give you more focus to find the true meaning of confusing answers.

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1. CORRECT. If people don’t use herbal medicines for long enough periods to get side effects, then there is no need to skip days. The only reason to skip was to avoid side effects. (The people aren’t using the medicines long enough to have them work best, either, but that’s another story).
   Negation: Nobody who uses uses herbal medicines daily uses them long enough to get side effects.
2. The author’s argument is actually stronger if this isn’t true. The author wants you to avoid side effects, but they presumably would like it if the medicines work. The argument was mostly focussed on avoiding problems from side effects.
   Negation: Every herbal medicine works just as well even if you occasionally skip the usual dose.
3. Like B, the argument is stronger if this isn’t true! The author would prefer if occasional dose skipping was enough to prevent side effects.
   Negation: No herbal medicines have toxic side effects for several months, provided that the usual dose is occasionally skipped.
4. “Anyone”? Watch out for answers this general. We can negate this to “not everyone”, which could be 99.9% of people. The author might agree there is a tiny percent of people who shouldn’t take herbal medicines. Either because they are healthy already, or they are unusually susceptible to side effects.
    
   If an answer negates from 100% to 99.9%, the negation is almost never powerful enough to be correct.
   Negation: 99.9% of people who take herbal medicines should gives them at least several months to influence the body. But a small percent of people shouldn’t do this, if they react badly for example.
5. This just adds an unnecessary condition to the author’s argument. It’s rarely helpful to add a necessary condition to something.
    
   For example, suppose you suddenly have a severe bad reaction to a medicine. Not from a toxic side effect, but from an allergy the first time you take the medecine. This answer would tell you not to skip your usual dose, because the problem is not “a toxic side effect from several months of usage”.
    
   That’s obviously an insane idea! Necessary conditions can never help an argument, they only add restrictions to it, and often produce absurd results.
    
   Anyway, the technical reason this is wrong is that the negation supports the argument. A negation is supposed to destroy the argument!
   Negation: It might be sensible to skip a medication for a day or two if it causes a problem for you sooner than after two months of daily use.

Recap: The question begins with “Herbalist: Many herbal medicines work best when”. It is a Necessary Assumption question. Learn more about LSAT Necessary questions in our guide to LSAT Logical Reasoning question types.