The vigour with which he reasons from known to unknown facts and causes may be judged most fairly by his arguments on the progress of society, where he is more on an equality with modern speculation. He discards, altogether, as might be expected, the fancies concerning a heroic or a golden age, and assumes as his data the facts of human nature as observed in his own day. The grounds from which he starts, his method of reasoning, and the nature of his conclusions remind a reader of the positive tendencies of Thucydides, as they are displayed in the introduction to his history. The importance of personal qualities, such as beauty, strength, and power of mind, in the earliest stage of civil society, the influence of accumulated wealth at a later period, the causes of the establishment and overthrow of tyrannies and of the rise of commonwealths in their room, are all set forth with a degree of strong sense and historical sagacity, such as no other Roman writer has shown in similar investigations. The inferiority even of Tacitus in his occasional digressions into the philosophy of history is very marked. On such topics, where the data were accessible to the natural faculties of observation and inference, and where conclusions were sought which, without aiming at definite certainty, should yet be true in the main, the reader of Lucretius has no sense of that wasted ingenuity which he often feels in following the investigations into some of the primary conditions of the atoms, the component elements of the soul, the process by which the world was formed, or the causes of electric or volcanic phenomena. Lucretius makes a copious, and often a very happy use, of analogies, both in the illustration of his philosophy, and in passages of the highest poetical power. Some of the most striking of the former kind have already been noticed as sources of error, or at least of disguising ignorance, in his reasoning, viz., those founded on the supposed parallel between the world and the human body; others again are employed with force and ingenuity in support of various positions in his argument. Among these may be mentioned his comparison of the effect of various combinations of the same letters in forming different words, with that of the various combinations of similar atoms in forming different objects in nature. So too the ceaseless motion of the atoms is brought visibly before the imagination by the analogy of the motes dancing in the sunbeam. There is something striking in the comparison of the human body immediately after death to wine 'cum Bacchi flos evanuit,' and again, in that of the relation of body and soul to the relation of frankincense and its odour-- E thuris glaebis evellere odorem Haud facile est quin intereat natura quoque eius.[404]