Modus Tollens

Mar 2, 2022
One man's modus ponens is another man's modus tollens.

Modus tollens (Latin for method of removing) is a method of propositional logic of the form (where P and Q are logical propositions)

P implies Q.
Q is false.
Therefore, P is false.

For example: If it is a car, it has wheels. It does not have wheels. Therefore, it is not a car.

It's important not to make the converse error, which would be: If it is a car, it has wheels. It has wheels. Therefore, it is a car. (not true, it could be a bus).

There's also modus ponens (method of putting), which is a more intuitive method of interface of the form:

P implies Q.
P is true.
Therefore, Q is true.