 A theme that is equally present in Seneca’s natural philosophy and in his therapeutic practice is time. Book 3 of the Natural Questions is entitled On the waters of the earth and begins with reflections on the enormous time which the task of natural philosophy may consume; on time that has been wasted with worldly concerns; and the claim that it can be regained if we make diligent use of the present. The fact that human life is finite is thus present from the very first lines of the book. Seneca then turns to the way in which the world’s life-cycle is as finite as that of a human being. Just as a human foetus already contains the seed of its death, the beginnings of the world contain its end (3.28.2–3). It is precisely for this reason that things are easy for nature. Its death does not, as it were, come as a surprise—nature is well-prepared. Nature does what it initially determined; nothing in nature’s doings is ad hoc (3.30.1). Seneca points to examples: Look at the way the waves roll onto the beaches; the oceans are trained in how to flood the earth (3.30.2). The world’s preparedness for its death seems to be the perfect analogue of how, for Seneca, we ought to spend our lives. In Letter 12.6–8, Seneca says that everything, light and darkness, is contained in a single day. To use the present well is to be aware of this completeness. More days, and months, and years, will (or at least may) make up our lives. But we should not think of them as stretching out into the future; rather, they are concentric circles surrounding the day which, right now, is present. And since even this very day stretches out, from its beginning to its end, we can appreciate it as containing everything—there can be more such days, but they will be more of the same. Thus, on every such day, if it is lived well, we can be fully prepared to die.