Albertus Magnus and Thomas Aquinas wrote that demons and the hierarchy of angels are created by God with the same non–material substance. Although they have no body (either human or animal), they have no sexual identity and can not generate human beings or other angels. The incorporeality is related to their nature, eternal and unchangeable across the centuries. In the Book of Tobit, the Archangel Raphael shows himself in the outward look of a human body, which is not its own.