It is on thunderbolts as portents of the future that Seneca dwells longest, however. “They give,” he declares, “not signs of this or that event merely, but often announce a whole series of events destined to occur, and that by manifest decrees and ones far clearer than if they were set down in writing.”[181] He will not, however, accept the theory that lightning has such great power that its intervention nullifies any previous and contradictory portents. He insists that divination by other methods is of equal truth, though perhaps of minor importance and significance. Next he attempts to explain how dangers of which we are warned by divination may be averted by prayer, expiation or sacrifice, and yet the chain of events wrought by destiny not be broken. He maintains that just as we employ the services of doctors to preserve our health, despite any belief we may have in fate, so it is useful to consult a _haruspex_. Then he goes on to speak of various classifications of thunderbolts according to the nature of the warnings or encouragements which they bring.[182] IV. _Ptolemy’s Tetrabiblos and astrology._—Astrology was more than a popular belief which extended to men high in social rank and public life; it was held by scientists as well, though naturally in a less naïve and more scientific form. Nevertheless, the astrology of the scientist might be of an extreme enough type and of a more clearly magical variety than we were able to gather from Pliny, who, moreover, does not seem to have been acquainted with any systematic doctrine of the influence of the stars. Such a systematized treatment Claudius Ptolemaeus set forth in the little volume known as the _Tetrabiblos_, or _Quadripartitum_. It would seem as if we ought to be able to regard a book by that noted geographer and astronomer as an example of the best science of his time, the middle of the second century. His works quickly became classics, and in the third century Porphyry commented on the _Tetrabiblos_. The Arabs eagerly accepted his writings, and it is generally held that in the Middle Ages his word was law in all the subjects of which he treated. The _Tetrabiblos_, therefore, would seem a landmark in the entire history of astrology as well as a crucial instance of how that branch of magic formed a part of science in the Roman Empire. True, Ptolemy does not cover the whole field of sidereal influence. He limits himself to the effects of the stars on man and does not attempt to trace out how they affect all varieties of matter and of life upon our globe. However, to make the stars control each individual man is the climax of astrology and implies that the heavenly bodies govern everything else here on earth. So the _Tetrabiblos_ is a very satisfactory instance of belief in astrology by a scientist and its contents may well be briefly considered.[183]