In Boolean logic, a proposition equivalent to a conditional P → Q and defined by the conditional ¬Q →¬P.


In logic, the contraposition is a type of reasoning that consists of affirming the implication “if not B then not A” from the implication “if A then B”. The implication “if not B then not A” is called the contraposition of “if A then B”.

For example, the contraposition to the proposition “if it rains, then the ground will be wet” is “if the ground is not wet, then it is not raining“.

For example, the contraposition of the proposition “if n is a multiple of 9, then n is not a prime number” is “if n is a prime number, then n is not a multiple of 9“.

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