How does a counterexample show that an argument form is invalid?

In formal logic, an argument is considered deductively valid only if the truth of its premises logically guarantees the truth of its conclusion based solely on the argument's structure.12

A counterexample demonstrates that an argument form is invalid by providing a specific instance where the premises are true but the conclusion is false. Because a valid argument form requires that the conclusion must be true whenever the premises are true, the existence of even one such counterexample proves that the logical structure does not necessitate the conclusion. Consequently, the argument fails to meet the standard of deductive validity, revealing a defect in the inference process.12