The idea of the great enterprise, in its essential outline, and with a full sense of its originality and importance, was early formed, and was even sketched on paper with Bacon's characteristic self-reliance when he was but twenty-five. Looking back, in a letter written in the last year of his life, on the ardour and constancy with which he had clung to his faith--"in that purpose my mind never waxed old; in that long interval of time it never cooled"--he remarks that it was then "forty years since he put together a youthful essay on these matters, which with vast confidence I called by the high-sounding title, The Greatest Birth of Time." "The Greatest Birth of Time," whatever it was, has perished, though the name, altered to "Partus Temporis _Masculus_" has survived, attached to some fragments of uncertain date and arrangement. But in very truth the child was born, and, as Bacon says, for forty years grew and developed, with many changes yet the same. Bacon was most tenacious, not only of ideas, but even of the phrases, images, and turns of speech in which they had once flashed on him and taken shape in his mind. The features of his undertaking remained the same from first to last, only expanded and enlarged as time went on and experience widened; his conviction that the knowledge of nature, and with it the power to command and to employ nature, were within the capacity of mankind and might be restored to them; the certainty that of this knowledge men had as yet acquired but the most insignificant part, and that all existing claims to philosophical truth were as idle and precarious as the guesses and traditions of the vulgar; his belief that no greater object could be aimed at than to sweep away once and for ever all this sham knowledge and all that supported it, and to lay an entirely new and clear foundation to build on for the future; his assurance that, as it was easy to point out with fatal and luminous certainty the rottenness and hollowness of all existing knowledge and philosophy, so it was equally easy to devise and practically apply new and natural methods of investigation and construction, which should replace it by knowledge of infallible truth and boundless fruitfulness. His object--to gain the key to the interpretation of nature; his method--to gain it, not by the means common to all previous schools of philosophy, by untested reasonings and imposing and high-sounding generalisations, but by a series and scale of rigorously verified inductions, starting from the lowest facts of experience to discoveries which should prove and realise themselves by leading deductively to practical results--these, in one form or another, were the theme of his philosophical writings from the earliest sight of them that we gain.