But we may trust with even more confidence to the indications of his inner than of his outward life. The spirit and purpose which impelled Lucretius to expound his philosophy can be understood without any collateral knowledge of his history. The dominant impulse of his being is the ardent desire to emancipate human life from the fears and passions by which it is marred and degraded. He has more of the zeal of a religious reformer than any other ancient thinker, except one who in all his ways of life was most unlike him, the Athenian Socrates. The speculative enthusiasm which bears him along through his argument is altogether subsidiary to the furtherance of his practical purpose. Even the poetical power to which the work owes its immortality was valued chiefly as a pleasing means of instilling the unpalatable medicine of his philosophy[23] into the minds and hearts of unwilling hearers. It is the constant presence of this practical purpose, and the profound sense which he has of the actual misery and degradation of human life, and of the peace and dignity which are attainable by man, that impart to his words the peculiar tone of impassioned earnestness to which there is no parallel in ancient literature. Among his personal characteristics none is more prominent than his consciousness both of the greatness of the work on which he was engaged, and of his own power to cope with it. The passage in which his high self-confidence is most powerfully proclaimed (i. 920, etc.) has been imitated both by Virgil and Milton. The sense of novelty, adventure, and high aspiration expressed in the lines-- Avia Pieridum peragro loca nullius ante Trita solo-- moved Virgil less powerfully in speaking of his humbler theme-- Sed me Parnassi deserta per ardua dulcis Raptat amor; and inspired the English poet in his great invocation:-- 'I thence Invoke thy aid to my adventurous song, That with no middle flight intends to soar Above the Aonian mount, while it pursues Things unattempted yet in prose or rhyme. The sense of difficulty and the joy of overcoming it meet us with a keen bracing effect in many passages of the poem. He speaks disdainfully of those enquirers who fall into error by shrinking from the more adventurous paths that lead to truth-- Ardua dum metuunt amittunt vera viai. Without disowning the passion for fame,--'laudis spes magna,' so powerful an incentive to the Roman temperament,--he is more inspired and supported in his arduous task by 'the sweet love of the Muses.' The delight in the exercise of his art and the joyful energy sustained through the long processes of gathering and arranging his materials appear in such passages as iii. 419-20:-- Conquisita diu dulcique reperta labore Digna tua pergam disponere carmina cura: and again at ii. 730-- Nunc age dicta meo dulci quaesita labore Percipe.