  The sequence is one of ascending scale: the first pair of books deals with the microscopic world of atoms, the second with human beings, the third with the cosmos as a whole. Within each pair of books, the first explains the basic nature of the entity or entities in question, the second goes on to examine a range of individual phenomena associated with them. A further symmetry lies in the theme of mortality, treated by the odd-numbered books. Book I stresses from the outset the indestructibility of the basic elements, while books III and V in pointed contrast give matching prominence to the perishability and transience of, respectively, the soul and the cosmos.