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Difficulty: EasyNecessary vs. Sufficient Condition Flaws

In certain species of frogs, exposure to high levels of dietary phosphorus inevitably results in bright orange skin pigmentation. A newly discovered population of these frogs in the eastern swamps exhibits bright orange skin pigmentation. Thus, this population must have been exposed to high levels of dietary phosphorus.

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

  1. A
    It assumes a causal relationship between dietary phosphorus and skin pigmentation based merely on an observed correlation.
  2. It treats a condition that is sufficient to produce a given physical trait as a condition that is necessary to produce that trait.Answer
  3. C
    It uses the term "pigmentation" in two distinct and incompatible senses over the course of the argument.
  4. D
    It infers that a characteristic observed in a newly discovered population must apply to the species as a whole.
  5. E
    It challenges the truth of an established scientific fact in order to support its conclusion.

Answer

The correct answer is the option stating that the argument treats a condition that is sufficient to produce a given physical trait as a condition that is necessary to produce that trait.
The argument's premise states that high dietary phosphorus is sufficient to cause bright orange skin. However, by concluding that the orange-skinned frogs MUST have been exposed to phosphorus, the argument assumes there is no other possible cause for the orange skin. In doing so, it incorrectly treats a sufficient condition (phosphorus exposure) as a necessary condition.

Step-by-Step Solution

1
Identify the core premise and the main conclusion of the argument.
Premise: Exposure to high phosphorus results in orange skin. Conclusion: Because the new population has orange skin, they must have been exposed to high phosphorus.
Deconstructing the argument's structure is the necessary first step to evaluating its logical validity.
2
Translate the argument into formal conditional logic.
The premise is 'Phosphorus Exposure -> Orange Skin'. The argument observes 'Orange Skin' and concludes 'Phosphorus Exposure'.
Formalizing the logic makes it easier to spot structural errors like illegal reversals or illegal negations.
3
Evaluate the logical validity of the conclusion based on the premise.
The argument commits an illegal reversal. It assumes that because a sufficient condition (phosphorus) causes the result, nothing else can cause it, thereby mistakenly treating the sufficient condition as necessary.
Comparing the argument's structure to known logical fallacies identifies the exact description of the flaw.

Key Concept

Logical Flaws: Necessary vs. Sufficient Conditions (Illegal Reversal)
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Topics

Necessary vs. Sufficient Condition Flaws

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