Describe what is meant by a valid argument in propositional logic and show that the argument If the earth is flat, then you can sail off the edge of the earth," "You cannot sail off the edge of the earth," therefore. "The earth is not flat" is a valid argument.

Step 1:

In this problem, we have shown that the given argument is valid.

Step 2:

For this condition, we can use the combining rules of inference for Propositions and Quantified statement

Universal Modus Ponens:

)

P(a), where a is a specific element in the given domain

Therefore, Q(a) is true

Universal Modus Tollens:

)

Q(a), where a is a specific element in the given domain

Therefore, P(a) is true