The earliest condition of man was one of savage vigour and power of endurance, but liable to danger and destruction from many causes. The first humanising influence is traced to domestic union and the affection inspired by children-- Et Venus inminuit viris puerique parentum Blanditiis facile ingenium fregere superbum.[389] The origin of language is next explained, then that of civil society, of religion, and of the arts,--the general conclusion being that all progress is the result of natural experience, not of divine guidance. The last source of superstition is our ignorance of the causes of natural phenomena-- Praesertim rebus in illis Quae supera caput aetheriis cernuntur in oris.[390] Hence the sixth book is devoted to the explanation of thunderstorms, tempests, volcanoes, earthquakes, and the like,--phenomena which are generally attributed to the direct agency of the gods. The whole work terminates with an account of the Plague at Athens, closely following that given by Thucydides. The first question which arises after a review of the whole argument is that suggested by the statement of Jerome, and brought into prominence since the publication of Lachmann's edition of Lucretius, viz. whether there is good reason for believing that the poem was left by the author in an unfinished state. In answering this question, it is to be observed, on the one hand, that there is no incompleteness in the fulfilment of the original plan of the work, unless from one or two hints[391] we conclude that the poet intended giving a fuller account of the blessed state of the Gods than that given at iii. 17-24. He announces at i. 54, etc., and again at i. 127, etc., the design of the poem as embracing the first principles of natural philosophy, and the application of these principles to certain special subjects, viz. the nature of soul and body, the origin of the belief in ghosts, the natural causes of creation, and the meaning of certain celestial phenomena. The practical purpose of the poem--the overthrow of superstition--limits the argument to these subjects of discussion. They are severally mentioned where the argument is resumed in Books iii, iv, v, and vi, as those matters which require a clear explanation from the poet. All the topics enunciated in the opening statement are discussed with the utmost fulness. The great strongholds of superstition are attacked and overthrown in regular succession. In the introduction to the sixth book, the lines (91-95) Tu mihi supremae praescribta ad candida calcis, etc. clearly show that the poet considered himself approaching the end of his task.