On the Principle of Contradiction in Aristotle

The principle of contradiction has, to be sure, no logical worth, since it is valid only as an assumption; but as a consequence it carries a practical-ethical value, which is all the more important. The principle of contradiction is the sole weapon against error and falsehood. Were we not to recognize this principle and hold joint assertion and denial to be possible, then we could not defend other propositions against false or deceitful propositions. One falsely accused of murder could find no means to prove his innocence before the court. At most, he could only manage to prove that he had committed no murder; this negative truth cannot, however, remove its contradictory positive from the world, if the principle of contradiction fails. If just one witness is found who (not shirking from committing perjury) implicates the accused, his false assertion can in no what be contradicted and the defendant is irretrievably lost.