There follows an account of sleep, and of the condition of the mind during that state; and the book concludes with a physical account of the passion of love, which is dependent on the action of the simulacra on the mind. Love is shown also to arise from natural causes, and not to be engendered by divine influence. The fatal consequences of yielding to the passion are then enforced with much poetical and satirical power. The object of the fifth book is to explain the formation of our system--of earth, sea, sky, sun, and moon,--the origin of life upon the earth, and the advance of human nature from a savage state to the arts and usages of civilisation. The purpose of these discussions is to show that all our system was produced and is maintained by natural agency, that it is neither itself divine nor created by divine power, and that, as it has come into existence, so it must one day perish. As the parts of our system,--earth, water, air, and heat,--are perishable, and constantly passing through processes of decay and renovation, the system must have had a beginning, and will have an end. There must at last be an end of the long war between the contending elements. The world came into existence as the result not of design, but of every variety of combination in the elemental atoms throughout infinite time. Originally all were confused together. Gradually those that had mutual affinities combined and separated themselves from the rest. The earthy particles sank to the centre. The elemental particles of the empyrean (aether ignifer) formed the 'moenia mundi.' The sun and moon were formed out of the particles that were neither heavy enough to combine with the earth, nor light enough to ascend to the highest heaven. Finally, the liquid particles separated from the earth and formed the sea. Highest above all is the empyrean, entirely separated from the storms of the lower air, and moving round with its stars by its own impetus. The earth is at rest in the centre of our system, supported by the air, as our body is by the vital principle. The movements of the stars and of the sun and moon through the heavens are next explained; then the origin of vegetable and animal life on the earth, and the beginning and progress of human society. First plants and trees, afterwards men and animals, were produced from the earth in the early and vigorous prime of the world. Many of the animals originally produced afterwards became extinct. Those only were capable of continuation which had either some faculty of self-preservation against others, or were useful to man, and so shared his protection. The existence of monsters such as Scylla, the Centaurs, the Chimaera, is shown to be impossible according to the natural laws of production.