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Difficulty: MediumNecessary vs. Sufficient Condition Flaws

Urban planner: Establishing a comprehensive toll zone in the city center is a guaranteed method for achieving a substantial drop in the number of commuter vehicles. However, the city council recently voted to permanently reject the mayor's toll zone proposal. Therefore, it is certain that there will be no substantial drop in the number of commuter vehicles this year.

The urban planner's argument is most vulnerable to criticism on the grounds that it

  1. treats a condition that is sufficient to produce a specific outcome as though it were necessary to produce that outcomeAnswer
  2. B
    assumes that if a condition is necessary to produce a certain result, then it must also be sufficient to produce that result
  3. C
    concludes that a causal relationship exists between two phenomena merely because they are frequently observed together
  4. D
    relies on an ambiguous interpretation of the key phrase 'substantial drop' throughout the course of the reasoning
  5. E
    incorrectly infers that because a policy is effective for the city as a whole, it will be equally effective for each individual district

Answer

The correct answer is the option stating that the argument treats a condition that is sufficient to produce a specific outcome as though it were necessary to produce that outcome.
The planner's argument explicitly establishes that a toll zone is a sufficient condition for reducing traffic ('guaranteed method'). However, it erroneously concludes that the failure to implement the toll zone guarantees traffic will not be reduced. This assumes that the toll zone was the *only* way to reduce traffic, thereby treating a sufficient condition as if it were a necessary condition.

Step-by-Step Solution

1
Analyze the conditional logic in the premise.
Premise: Toll Zone -> Substantial Drop in Vehicles.
The phrase 'is a guaranteed method' introduces a sufficient condition. Establishing the toll zone guarantees the drop in vehicles.
2
Analyze the conditional logic in the conclusion.
Conclusion: No Toll Zone -> No Substantial Drop in Vehicles.
The planner takes the rejection of the toll zone (the absence of the sufficient condition) to conclude that the result will absolutely not occur.
3
Identify the logical disconnect between the premise and conclusion.
The planner commits an Illegal Negation.
Just because one guaranteed method is rejected does not mean alternative methods (like better public transit or carpool incentives) couldn't also produce the same substantial drop. The argument mistakenly treats a sufficient way to achieve a goal as the only (necessary) way.
4
Evaluate the answer choices to find the one that describes an Illegal Negation.
The correct choice identifies that the argument treats a sufficient condition as a necessary condition.
This precisely matches the abstract description of the flaw committed in the stimulus.

Key Concept

Necessary vs. Sufficient Condition Flaws (Illegal Negation)
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Topics

Necessary vs. Sufficient Condition Flaws

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