Problem 8 of Review Exercises or Chapter 1

An argument is valid in propositional logic if all of the premises are valid and if by following the rules of inference you come up with the conclusion.

Premises:

The earth is flat = p

You can sail off the edge of the earth = q p --> q -q Therefore -p

This argument proposes that if you cannot sail off the edge of the earth then the earth is not flat. This argument is valid by the Inference rule Modus Tollens.