Demon was a term applied by the Greeks and Romans to certain genii or spirits who made themselves visible to men, with the intention of doing them either good or harm. The Jews and early Christians ascribed a malignant nature to demons, the former endeavouring to trace their origin to intercourse between man and supernatural beings, and the latter maintaining that they were the souls of departed human beings, permitted to visit the earth to assist those they favoured, and punish persons against whom they or their favourites had a grudge. Certain spirits were supposed to be celestial, others watery, some airy, and not a few of them fiery. Tertullian said: "Spirits flew through the air faster than any winged fowl. Unless commissioned to act, they remained passive, neither doing good nor evil; but the evil spirits went and came at the devil's command, and both classes of spirits were at man's service if he only knew how to summon them into his presence." The ancient Egyptians had a tradition, that at a far past period men rebelled against the gods, and drove them away. Upon this taking place, the gods fled into Egypt, where they concealed themselves under the form of different animals; and this was the first reason assigned for the worship of inferior creatures. A leading principle in the religion of the ancient Arabians was their belief in fairies or genii. They thought that these genii attended people through life; that every man had two of these waiting on him, the one good and the other evil; that all evil actions were committed at the instigation of the evil spirit in the absence of the good genii, who sometimes went with messages to the celestial regions. The Arabians further believed these genii were continually at war with each other, which, the people considered, accounted for the contending passions in their minds. Their principal genius was Hafedhah, to whom the people, on setting out on a plundering expedition, prayed he would send them a strong genius to assist them.