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Difficulty: HardNecessary vs. Sufficient Condition Flaws

Historian: Any medieval fortress that successfully withstood a prolonged siege must have been situated on elevated terrain and equipped with an internal water supply. The recently excavated Dunhollow Keep was constructed on a steep, rocky promontory and contains a massive, well-preserved internal cistern system. Therefore, Dunhollow Keep undoubtedly withstood at least one prolonged siege during its active history.

Which of the following most accurately describes a flaw in the historian's reasoning?

  1. takes for granted that because a structure satisfied the prerequisites for a particular historical event, it must have actually experienced that event.Answer
  2. B
    treats a set of conditions that are sufficient to guarantee a particular outcome as though they were merely necessary for that outcome.
  3. C
    infers a causal relationship between the geographical location of a fortress and its historical resilience based solely on a correlation.
  4. D
    concludes that because the individual architectural features of Dunhollow Keep were formidable, the fortress as a whole was unconquerable.
  5. E
    relies on an implicit shift in the meaning of the phrase 'prolonged siege' between the general principle and the specific application.

Answer

The correct answer is the option stating that the argument assumes a structure experienced an event just because it met the prerequisites for that event.
The historian's argument commits a classic conditional reasoning flaw known as an illegal reversal. The first premise establishes that elevated terrain and an internal water supply are necessary conditions (prerequisites) for a fortress to successfully withstand a prolonged siege. The argument then observes that Dunhollow Keep possessed these necessary conditions and erroneously concludes that the fortress must have actually withstood a siege. This reasoning treats conditions that are merely required for an event as if they were sufficient to guarantee that the event occurred. The correct option accurately describes this error by pointing out that the historian takes for granted that meeting the prerequisites for an event means the event actually took place.

Step-by-Step Solution

1
Analyze the logical structure of the historian's argument.
Premise: Withstanding a siege -> Elevated terrain AND Internal water supply. Premise 2: Dunhollow Keep has elevated terrain and an internal water supply. Conclusion: Dunhollow Keep withstood a siege.
Deconstructing the argument into formal logic is essential for identifying reasoning flaws.
2
Evaluate the relationship between the premises and the conclusion.
The argument establishes necessary conditions (prerequisites) for withstanding a siege, but then uses the presence of these conditions to conclude that the siege definitively occurred.
Identifying mismatched conditional indicators reveals the core flaw.
3
Match the identified flaw with the abstract descriptions provided in the answer choices.
The flaw is an illegal reversal, treating necessary conditions as sufficient. This matches the option stating the argument assumes meeting prerequisites guarantees the event actually happened.
Answer choices in flaw questions often use abstract language to describe conditional errors.

Key Concept

Illegal Reversal (Confusing Necessary and Sufficient Conditions)
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Topics

Necessary vs. Sufficient Condition Flaws

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