The Greek works and treatises on astrology that have come down to us reveal this essential feature only very imperfectly. The Byzantines stripped this pseudo-science, always regarded suspiciously by the church, of everything that savored of paganism. Their process of purification can, in some instances, be traced from manuscript to manuscript.[20] If they retained the name of some god or hero of mythology, the only way they dared to write it was by cryptography. They have especially preserved purely didactic treatises, the most perfect type of which is Ptolemy's Tetrabiblos which has been constantly quoted and commented upon; and they have reproduced almost exclusively expurgated texts, in which the principles of various doctrines are drily summarized. During the classic age works of a different character were commonly read. Many "Chaldeans" interspersed their cosmological calculations and theories with moral considerations and mystical speculations. In the first part of a work that he names "Vision," ([Greek: Horasis]) Critodemus, in prophetic language, represents the truths he reveals {171} as a secure harbor of refuge from the storms of this world, and he promises his readers to raise them to the rank of immortals.[21] Vettius Valens, a contemporary of Marcus Aurelius, implored them in solemn terms, not to divulge to the ignorant and impious the arcana he was about to acquaint them with.[22] The astrologers liked to assume the appearance of incorruptible and holy priests and to consider their calling a sacerdotal one.[23] In fact, the two ministries sometimes combined: A dignitary of the Mithraic clergy called himself _studiosus astrologiae_[24] in his epitaph, and a member of a prominent family of Phrygian prelates celebrated in verse the science of divination which enabled him to issue a number of infallible predictions.[25] The sacred character of astrology revealed itself in some passages that escaped the orthodox censure and in the tone some of its followers assumed, but we must go further and show that astrology was religious in its principles as well as in its conclusions, the debt it owed to mathematics and observation notwithstanding.