One of St. Thomas Aquinas's very last projects was a treatise on angels. With a more historical approach than that of either Summa on the subject, it addresses such topics the immateriality of angels, their origin, their knowledge, and the distinctions among them, including the distinction between the good ones and the bad ones. Along the way, it offers some of Thomas’s most sophisticated discussions of the metaphysics of creation, hylomorphism, and participation.