He was ancient greek philosopher lived in Sinop/Turkey. He kinda had "weird" philosophy about society and as we know he walked naked, lived in a barrel and masturbated in bazaar. He also mocked Plato and his school. However, as it is narratted he was the teacher of Alexander the Great. There is a famous story about them: When Alexander encountered with him he was shocked when he saw Diogenes. After talking with him, Alexander impressed by his speech and asked Diogenes what he wants. Then Diogenes simply replied, "Don't cast shadow, don't want any other favors". Then they became teacher-pupil. So it seems he had quite odd understanding of life but Alexander recognized him, that means he was not lunatic, retard or enemy.