[Sidenote: Arrangement and contents of the poem.] The arrangement of the poem is as follows: Book i sets forth the atomic theory, invented by Democritus and held by Epicurus, that the world consists of atoms—infinitely small particles of matter—and void, i. e., empty space. The theories of other Greek philosophers, such as Heraclitus, Empedocles, and Anaxagoras, are refuted. In Book ii it is explained how the atoms combine to form the various things in the world, because as they fall through space they depart from a straight line and come in contact with each other. It is also shown that the atoms, although infinite in number, are limited in variety. In Book iii the mind and the soul, or principle of life, are shown to be material and to die when the body dies. Religion and the fear of death, which Lucretius regards as a result of religion, are attacked. Since the soul dies with the body, there is no reason to fear death, because after death we shall feel no lack of anything, shall have no troubles, but shall be as if we had not been born, or as if we lay wrapped in dreamless sleep: So death to us is naught, concerns us not, When the soul’s nature is as mortal known.[22] Book iv shows how the impressions made upon our senses are caused by minute images detached from the objects about us. We see, for instance, because minute images of the object seen strike our eyes. Dreams and love are also treated in this book. In Book v the origin of the earth, sun, moon, and stars is described, the beginning of life is explained, and the progress of civilization, from the time when men were savages, is depicted. Some passages in this book anticipate in a measure the modern doctrine of the survival of the fittest. Since our world was not created, but came into being naturally by the combinations of atoms, it will also come to an end at some time by the separation of the atoms. In Book vi various striking phenomena are treated, such as thunder, lightning, earthquakes, tempests, and volcanoes. The book ends with a description of the plague at Athens, derived from the account of Thucydides. [Sidenote: Ethical doctrine.] Since the main purpose of the poem is to free men from religion and the fear of death by showing that all things, including the soul, came into being and are to pass away without any action of the gods, ethical doctrines are not systematically treated. Lucretius accepts, however, the Epicurean dogma that pleasure is the chief good, “the guide of life,”[23] but the pleasure he has in mind is not the common physical pleasure, but the calm repose of the philosopher: Oh wretched minds of men, oh blinded hearts! Within what shades of life and dangers great Is passed whate’er of age we have! Dost thou Not see that nature makes demand for naught Save this, that pain be absent from our frame, That she, removed from care at once and fear, May have her pleasure in the joys of mind?[24]