PrepTest 141, Section 4, Question 25
Roberta is irritable only when she is tired,(26 more words not shown)
The reasoning above is flawed(3 more words not shown)
- Ainfers from a(9 more words not shown)
- Bassumes the conclusion(6 more words not shown)
- Cgeneralizes on the(5 more words not shown)
- Dtakes a necessary(10 more words not shown)
- Etakes a necessary(10 more words not shown)
Reading the argument
Roberta can be tired without being irritable.
The two rules make tiredness a requirement for different things. If she is irritable, she must be tired. If she loses things, she must be tired. Neither rule says that whenever she is tired, both things happen.
Losing her keys therefore gives us a firm reason, within this argument, to say she's tired. We don't even need the yawning to get that far. But we can't keep going from tired to irritable. She could be tired, misplace her keys, and remain perfectly calm. Everything in the evidence would be true without the conclusion following.
What we’re looking for
The flaw question asks us to describe that extra step. The author takes something required for irritability—tiredness—and treats it as enough to establish irritability. The important comparison in the choices will be which of the two rules the author misuses.
Working the answers
- A
The conclusion isn't that tiredness causes yawning. Yawning is mentioned as evidence about Roberta's condition, but the argument's destination is irritability. A causal claim about why she yawns doesn't describe the move from tiredness to irritability.
- B
The premises don't already tell us Roberta is irritable. They give conditional rules and observations about yawning and losing keys, from which the author tries to infer her mood. The inference fails, but it doesn't fail by simply assuming that mood as a premise.
- C
No broad generalization is drawn from a single instance. The conclusion concerns Roberta's present condition, not a rule that all people who lose keys are irritable or that she is always irritable. C describes a different kind of reasoning.
- D
The losing-things rule is used correctly: losing her keys establishes tiredness because she loses things only when tired. The author doesn't start with tiredness and use it to prove she lost something; the loss of the keys is already given. D attaches the necessary-versus-enough mistake to the wrong condition.
- ECorrect
Tiredness is required for Roberta to be irritable, but that doesn't make it enough. Knowing that she meets a requirement doesn't establish that she has the resulting trait. The author correctly gets to tiredness from the lost keys, then makes the unsupported move from tiredness to irritability.
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