Introductory Logic formally teaches two methods for determining the validity of a syllogism: rules of validity, and counterexamples. The rule that tells us that any AAO-4 syllogism is invalid is this: “A valid syllogism cannot have two affirmative premises and a negative conclusion.” But can we show the invalidity of AAO-4 with a counterexample? Here is the schema:
All P is M
All M is S
∴ Some S is not P
I contend that there is only one way to write a counterexample for a syllogism of that form. I challenge you to write a counterexample to AAO-4. Remember that a counterexample must be the same form, and have true premises and a false conclusion.