In the process of proving absurdity, contradictory judgments are obtained. According to the law of contradiction, these contradictory judgments cannot be true at the same time, but one of them is bound to be false, and the known conditions, known axioms, theorems, rules or propositions that have been proved to be correct are all true, so the "negative conclusion" is bound to be false. According to "law of excluded middle", the contradictory and mutually negative judgments of conclusion and negative conclusion cannot be false at the same time, and there must be a truth, so we get that the original conclusion must be true. Therefore, reduction to absurdity is based on the basic laws and theories of logical thinking, and reduction to absurdity is credible.