In Book 1 he establishes the general principles of the atomic system, refutes the views of rival physicists, and proves the infinity of the universe and of its two ultimate constituents, matter and void. In Book 2 he explains atomic movement, the variety of atomic shapes, and argues that the atoms lack colour, sensation, and other secondary qualities. In Book 3 he expounds the nature and composition of mind and spirit, proves their mortality, and argues that there is nothing to fear in death. Book 4 explains the nature of sensation and thought, and ends with an impressive account of sexual love. Book 5 describes the nature and formation of our world, astronomical phenomena, the beginnings of life on earth, and the development of civilization. In Book 6 the poet explains various atmospheric and terrestrial phenomena, including thunder, lightning, earthquakes, volcanoes, the magnet, and plagues.