The last two books take up the prediction of events in the lives of individuals from the stars, in other words the science of nativities or genethlialogy. The third book discusses conception and birth, how to take the horoscope—Ptolemy insists that the astrolabe is the only reliable instrument for determining the exact time; sun-dials or water-clocks will not do—and how to predict concerning parents, brothers and sisters, sex, twins, monstrous births, length of life, the physical constitution of the child born and what accidents and diseases may befall it, and finally concerning mental traits and defects. The fourth book deals less with the nature of the individual and more with the prediction of external events which befall the individual: honors, office, marriage, offspring, slaves, travel, and the sort of death that he will die. Ptolemy in opening the fourth book makes the distinction that, while in the third book he treated of matters antecedent to birth or immediately related to birth or which concern the temperament of the individual, now he will deal with those external to the body and which happen to the individual from without. But of course it is difficult to maintain such a distinction with entire consistency. [Sidenote: Future influence of the _Tetrabiblos_.] The great influence of the _Tetrabiblos_ is shown not only in medieval Arabic commentaries and Latin translations, but more immediately in the astrological writings of the declining Roman Empire, when such astrologers as Hephaestion of Thebes,[516] Paul of Alexandria, and Julius Firmicus Maternus cite it as a leading authoritative work. Only the opponents of astrology appear to have remained ignorant of the _Tetrabiblos_, continuing to make criticisms of the art which do not apply to Ptolemy’s presentation of it or which had been specifically answered by him. Thus Sextus Empiricus, attacking astrology about 200 A. D., does not mention the _Tetrabiblos_ and some of the Christian critics of astrology apparently had not read it. Whether the Neo-Platonists, Porphyry and Proclus, wrote an introduction to and commentary upon it is disputed. CHAPTER IV GALEN I. _The Man and His Times_