Question

Choose A or B, explain why. __(A) Every or (B) No ______________substitution instance of the following argument form is invalid:

Premise 1: If p then not-q

Premise 2: q

Conclusion: not-p

Answer 1

Premise 1: If p then not-q

Premise 2: q

Conclusion: not-p

Suppose:

P= prices are low

Q = quality is good

Premise 1: If prices are low (P) then quality isn’t good (not q)

Premise 2: Quality is good (q)

Conclusion: prices are not low (not p)

Counterexample:

If prices are not low (not p)

Means quality is good (q)

Hence, prices are low (p) because quality is not good (not q)

The counterexample method proves the validity of both the premises. Therefore, we would choose B (No substitution instance of the argument is invalid)