The discussion of these subjects leads on to a disquisition on the portents that may be drawn from different kinds of thunder and various forms of lightning. Seneca infers from the effects produced by it that lightning possesses an inherent divine power. Among these effects he enumerates some in which he seems to have thoroughly believed, such, for instance, as the smashing of a wine jar already quoted, and the freezing of the wine for the space of three days thereafter. He is thus disposed to attach credit to the opinion that future events are foretold by both lightning and thunder. Yet he cannot change his Stoic faith that fate, that is, the necessity for the happening of all things and all actions, can be set aside by no force, can be altered by no portents, nor averted by any prayer or sacrifice. Though he admits that vows and supplications may be useful to the worshippers, he knows that even these also are included in the decrees of fate. These reflections lead the philosopher to a characteristic peroration on the moral lessons to be derived from the subjects he has been discussing. From the dangers incident to thunderstorms he passes to the enforcement of the Stoic doctrine that death must be despised, and everything which leads to death will then cease to have any terror. BOOK III The subjects comprised in this section of the treatise have reference chiefly to the springs and rivers which appear on the surface of the earth or flow underneath it. The Book begins with a preface, which may have been originally designed to stand at the beginning of the volume. It bears internal evidence of having probably been written at the time of the author’s resolve to take up the discussion of physical problems, as it speaks of old age pressing upon him and leaving him but a short while to cover the immense field which he wished to survey. The years lost among vain pursuits must be repaired by diligence in the task now undertaken; night must be added to day, and every social or business care which can possibly be set aside must be abandoned. The contemplation of the work before him then leads the philosopher into his moralising mood, wherein he inquires what should be the principal object of human life, concluding with the reflection that the best thing a man can set before himself, among the ups and downs of this world, is courage to accept them calmly and to be ready to meet death boldly whenever summoned. To the acquisition of such a courage a contemplation of nature will greatly conduce.