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Difficulty: MediumFlaw in the Reasoning

Archaeologist: Every major ancient civilization that collapsed due to a prolonged drought left behind distinct geological markers in the soil sediment of their central water reservoirs. Recent excavations at the central reservoir of the ancient city of Olaria have revealed these exact geological markers in the soil sediment. Therefore, the collapse of the Olarian civilization must have been caused by a prolonged drought.

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

  1. It treats a condition that is necessary for a certain event to have occurred as a condition that is sufficient to guarantee that the event occurred.Answer
  2. B
    It infers a causal relationship between the presence of geological markers and the collapse of a civilization from a mere correlation.
  3. C
    It assumes that what is true of the central water reservoir must be true of the ancient city of Olaria as a whole.
  4. D
    It relies on an ambiguous use of the term "major ancient civilization" throughout the course of the argument.
  5. E
    It fails to distinguish between an intermediate conclusion regarding the excavation results and the main conclusion of the argument.

Answer

The correct answer is the option stating that the argument treats a necessary condition as a sufficient condition.
The argument's first premise establishes a conditional relationship: if a civilization collapses due to drought, it must leave distinct geological markers. Leaving markers is therefore a necessary condition of a drought collapse. The author observes that Olaria met this necessary condition (the markers were found) and concludes that the sufficient condition (a drought collapse) must have occurred. Other events could potentially cause these exact same markers, so finding them does not guarantee a drought was the cause.

Step-by-Step Solution

1
Map the logical structure of the argument's premises and conclusion.
Premise 1: If an ancient civilization collapsed due to drought (A), then it left distinct geological markers (B). Premise 2: Olaria left distinct geological markers (B). Conclusion: Therefore, Olaria collapsed due to drought (A).
Deconstructing the argument into formal logic reveals its underlying mechanics and makes structural flaws apparent.
2
Identify the logical flaw committed in the argument's structure.
The argument commits the formal fallacy of affirming the consequent (A -> B, B, therefore A).
The author takes a condition that is guaranteed to follow from an event (a necessary condition) and assumes its presence guarantees the event happened (a sufficient condition).
3
Evaluate the answer choices to find the abstract description that matches this specific flaw.
The option describing the confusion between a necessary condition and a sufficient condition perfectly matches the structural error identified in Step 2.
LSAT flaw questions often describe formal conditional logic errors in terms of confusing necessary and sufficient conditions.

Key Concept

Identifying Conditional Logic Flaws (Illegal Reversal / Affirming the Consequent)
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Topics

Flaw in the Reasoning

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