Question

Difficulty: MediumForming Contrapositives

Consider the following principle from a city's architectural preservation guidelines:

A building is eligible for the heritage registry only if it retains its original facade and has not undergone major structural modifications.

The logically valid contrapositive of this principle is: If a building is not eligible for the heritage registry, then it must either not retain its original facade or have undergone major structural modifications.

Answer: Answer

Answer

False
The proposed statement is logically false because it is a Mistaken Negation of the original principle. The original conditional statement is 'If eligible, then retains facade AND no major modifications.' A valid contrapositive must both negate AND reverse the conditions, resulting in 'If does not retain facade OR has major modifications, then not eligible.' The statement provided correctly negates the terms using De Morgan's laws but fails to reverse them, incorrectly inferring the absence of the necessary conditions from the absence of the sufficient condition.

Step-by-Step Solution

1
Identify the sufficient and necessary conditions in the original principle.
Sufficient: A building is eligible for the heritage registry. Necessary: It retains its original facade AND has not undergone major structural modifications.
The phrase 'only if' introduces the necessary condition for a conditional statement.
2
Formalize the conditional statement.
Eligible → (Retains Facade AND NOT Major Modifications).
Translating the text into formal logic helps prevent structural errors when evaluating inferences.
3
Determine the logically valid contrapositive of the formal statement.
(NOT Retains Facade OR Major Modifications) → NOT Eligible.
A valid contrapositive is formed by reversing the sufficient and necessary conditions and negating both. According to De Morgan's laws, negating an 'AND' statement creates an 'OR' statement.
4
Evaluate the proposed statement against the valid contrapositive.
The proposed statement is: NOT Eligible → (NOT Retains Facade OR Major Modifications). This is a Mistaken Negation.
The statement negates the conditions but fails to reverse them, making it logically invalid.

Key Concept

Forming contrapositives using De Morgan's laws
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