Whereas there is so little to suggest a belief in astrology in most of Galen’s works, we find among them two devoted especially to astrological medicine, namely, a treatise on critical days in which the influence of the moon upon disease is assumed, and the _Prognostication of Disease by Astrology_. In the latter he states that the Stoics favored astrology, that Diodes Carystius represented the ancients as employing the course of the moon in prognostications, and that, if Hippocrates said that physicians should know physiognomy, they ought much more to learn astrology, of which physiognomy is but a part.[842] There follows a statement of the influence of the moon in each sign of the zodiac and in its relations to the other planets.[843] On this basis is foretold what diseases a man will have, what medical treatment to apply, whether the patient will die or not, and if so in how many days. This treatise is the same as that ascribed in many medieval manuscripts to Hippocrates and translated into Latin by both William of Moerbeke and Peter of Abano. [Sidenote: Critical days.] The treatise on critical days discusses them not by reason or dogma, lest sophists befog the plain facts, but solely, we are told, upon the basis of clear experience.[844] Having premised that “we receive the force of all the stars above,”[845] the author presents indications of the especially great influence of sun and moon. The latter he regards not as superior to the other planets in power, but as especially governing the earth because of its nearness.[846] He then discusses the moon’s phases, holding that it causes great changes in the air, rules conceptions and birth, and “all beginnings of actions.”[847] Its relations to the other planets and to the signs of the zodiac are also considered and much astrological technical detail is introduced.[848] But the Pythagorean theory that the numbers of the critical days are themselves the cause of their significance in medicine is ridiculed, as is the doctrine that odd numbers are masculine and even numbers feminine.[849] Later the author also ridicules those who talk of seven Pleiades and seven stars in either Bear and the seven gates of Thebes or seven mouths of the Nile.[850] Thus he will not accept the doctrine of perfect or magic numbers along with his astrological theory. Much of this rather long treatise is devoted to a discussion of the duration of a moon, and it is shown that one of the moon’s quarters is not exactly seven days in length and that the fractions affect the incidence of the critical days. [Sidenote: _On the history of philosophy._]