This remark applies not only to those who are devotees of a mystical theology more or less erroneous, but also to certain philosophical writers with their disciples. Ontologism is a kind of mystical philosophy; for its fundamental doctrine ascribes to man a mode of knowledge which is proper only to the purely intellectual being, and even a direct, immediate intuition of God which is above the natural power not only of men but of angels. There are two fundamental errors underlying all these false systems of mystical theology――or more properly theosophy――and philosophy. One is distinctively anti-theistic, the other distinctively anti-Christian; but we may class both under one logical species with the common _differentia_ of denial of the real essence and personality, and the real operation _ad extra_, of the Incarnate Word. The first error denies his divine nature and creative act, the second his human nature and theandric operation. By the first error identity of substance in respect to the divine nature and all nature is asserted; by the second, identity of the human nature and its operation with that nature which is purely spiritual. The first error manifests itself as a perversion of the revealed and Catholic doctrine of the deification of the creature in and through the Word, by teaching that it becomes one with God in its mode of being by absorption into the essence whose emanation it is, in substantial unity. The second manifests itself by teaching that the instrumentality and the process of this unification are purely spiritual. The first denies the substantiality of the soul and the proper activity which proceeds from it and constitutes its life. The second denies the difference of the human essence as a composite of spirit and body, which separates it from purely spiritual essences and marks it as a distinct species. The first error is pantheism; for the second we cannot think of any designating term more specific than idealism. Both these errors, however disguised or modified may be the forms they assume, conduct logically to the explicit denial of the Catholic faith, and even of any form of positive doctrinal Christianity. Their extreme developments are to be found outside of the boundaries of all that is denominated Christian theology. Within these boundaries they have developed themselves more or less imperfectly into gross heresies, and into shapes of erroneous doctrine which approach to or recede from direct and palpable heresy in proportion to the degree of their evolution. Our purpose is not directly concerned with any of the openly anti-Christian forms of these errors, but only with such as have really infected or have been imputed to the doctrines and writings of mystical authors who were Catholics by profession, and have flourished within the last four centuries. There is a certain more or less general and sweeping charge made by some Catholic authors of reputation, and a prejudice or