Discussing at the outset various theories of origins, and dismissing these, notably that which asserts that things came from nothing--"for if so, any kind might be born of anything, nothing would require seed," Lucretius proceeds to expound the teaching of Leucippus and other atomists as to the constitution of things by particles of matter ruled in their movements by unvarying laws. This theory he works all round, explaining the processes by which the atoms unite to carry on the birth, growth, and decay of things, the variety of which is due to variety of form of the atoms and to differences in modes of their combination; the combinations being determined by the affinities or properties of the atoms themselves, "since it is absolutely decreed what each thing can and what it cannot do by the conditions of Nature." Change is the law of the universe; what is, will perish, but only to reappear in another form. Death is "the only immortal"; and it is that and what may follow it which are the chief tormentors of men. "This terror of the soul, therefore, and this darkness, must be dispelled, not by the rays of the sun or the bright shafts of day, but by the outward aspect and harmonious plan of Nature." Lucretius explains that the soul, which he places in the centre of the breast, is also formed of very minute atoms of heat, wind, calm air, and a finer essence, the proportions of which determine the character of both men and animals. It dies with the body, in support of which statement Lucretius advances seventeen arguments, so determined is he to "deliver those who through fear of death are all their lifetime subject to bondage." These themes fill the first three books. In the fourth he grapples with the mental problems of sensation and conception, and explains the origin of belief in immortality as due to ghosts and apparitions which appear in dreams. "When sleep has prostrated the body, for no other reason does the mind's intelligence wake, except because the very same images provoke our minds which provoke them when we are awake, and to such a degree that we seem without a doubt to perceive him whom life has left, and death and earth gotten hold of. This Nature constrains to come to pass because all the senses of the body are then hampered and at rest throughout the limbs, and cannot refute the unreal by real things."