LSAT 155 - Section 1 - Question 08

This is a Parallel Flawed Method of Reasoning question.

The argument makes the oldest mistake in the book: sufficiency-necessity confusion.

The argument lays out a rule: if no high school diploma and no demonstrated competence, then no license.

/hs and /dc → /license

The argument then applies this rule to Marie but makes a logical mistake in doing so.

Marie has both a high school diploma and a demonstrated competence. That means she has failed the sufficient condition of the rule. That means the rule goes away. We cannot draw a conclusion about whether Marie will be licensed.

Yet that’s just what the argument does. It concludes that Marie will be licensed.

/hs and /dc → /license

hs_marie and dc_marie

________

license_marie

That’s the sufficiency-necessity confusion.

Answer Choice (A) says without having either an excellent ear or exceptional manual dexterity, a person cannot play the piano well. "Without" is group 3, negate sufficient.

If no excellent ear and no manual dexterity, then no play piano well.

If (A) wants to be the correct answer, it just has to apply this general rule to Paul in the same sufficiency-necessity confused way that the rule was applied to Marie. (A) needs to show that Paul has an excellent ear and he has manual dexterity. Then (A) needs to conclude that Paul plays the piano well.

But (A) doesn’t do that and so (A) is wrong.

Instead, (A) has Paul fail the necessary condition by showing that he plays the piano well. That means Paul must have either an excellent ear or exceptional manual dexterity. That would be a valid conclusion. Yet (A) invalidly concludes that he must have both an excellent ear and exceptional manual dexterity.

Correct Answer Choice (B) says it's not possible to be an effective foreign language teacher without being fluent in at least two languages. "Without" is group 3, negate sufficient.

If not fluent in at least two languages, then not effective.

Already we encounter a trick. (B) tries to hide itself. We’re expecting a conjunctive sufficient condition, yet (B) gives us just a single sufficient condition. (B) is hoping that we would just move on.

But (B) is the correct answer, and it's correct because it commits the same sufficiency-necessity flaw.

Take this general rule and apply it to Professor Y. It says that Professor Y is fluent in Greek, English, and Persian. So Professor Y is fluent in at least two languages. She fails the sufficient condition just like Marie failed her sufficient condition. (B) just needs to invalidly conclude that Professor Y will be effective in order to be the correct answer choice. That’s exactly what (B) does.

/at-least-2 → /effective

at-least-2_prof-y

________

effective_prof-y

Answer Choice (C) says a person cannot be a licensed plumbing contractor without having completed an apprenticeship. "Without" is group 3, negate sufficient.

If licensed plumbing contractor, then completed apprenticeship.

Take this rule and apply it to Martin. Martin is a licensed plumber. I'm assuming that to mean the same thing as licensed plumbing contractor. Therefore, Martin completed his apprenticeship. This is valid.

Answer Choice (D) says no one can be an effective mayor of a major industrial seaport without a thorough knowledge of both national and international affairs. “Without" is group 3, negate sufficient.

If effective mayor of a major industrial seaport, then have thorough knowledge of national and international affairs.

Take this rule and apply it to Mayor L. Mayor L is an effective mayor. Does that trigger the sufficient condition? No, because we don’t know if Mayor L is an effective mayor of a major industrial seaport.

Okay, so (D) assumes that being an effective mayor is the same thing as being an effective mayor of a major industrial seaport. That’s not a reasonable assumption but it’s also not a sufficiency-necessity error. If we patch up that assumption, then (D) is valid. (D)’s conclusion would follow.

Answer Choice (E) says the only way to make a delicious vegetable soup is to use fresh vegetables. "The only" is group 1.

If delicious vegetable soup, then used fresh vegetables.

This particular vegetable soup is delicious, which means it satisfies the sufficient condition. That means (E) can validly draw the necessary condition as the conclusion. That’s exactly what (E) does.