He had disclosed what was in his mind in the letter to Lord Burghley, written when he was thirty-one (1590/91), in which he announced that he had "taken all knowledge for his province," to "purge it of 'frivolous disputations' and 'blind experiments,' and that whatever happened to him, he meant to be a 'true pioneer in the mine of truth.'" But the first public step in the opening of his great design was the publication in the autumn of 1605 of the _Advancement of Learning_, a careful and balanced report on the existing stock and deficiencies of human knowledge. His endeavours, as he says in the _Advancement_ itself, are "but as an image in a cross-way, that may point out the way, but cannot go it." But from this image of his purpose, his thoughts greatly widened as time went on. The _Advancement_, in part at least, was probably a hurried work. It shadowed out, but only shadowed out, the lines of his proposed reform of philosophical thought; it showed his dissatisfaction with much that was held to be sound and complete, and showed the direction of his ideas and hopes. But it was many years before he took a further step. Active life intervened. In 1620, at the height of his prosperity, on the eve of his fall, he published the long meditated _Novum Organum_, the avowed challenge to the old philosophies, the engine and instrument of thought and discovery which was to put to shame and supersede all others, containing, in part at least, the principles of that new method of the use of experience which was to be the key to the interpretation and command of nature, and, together with the method, an elaborate but incomplete exemplification of its leading processes. Here were summed up, and stated with the most solemn earnestness, the conclusions to which long study and continual familiarity with the matters in question had led him. And with the _Novum Organum_ was at length disclosed, though only in outline, the whole of the vast scheme in all its parts, object, method, materials, results, for the "Instauration" of human knowledge, the restoration of powers lost, disused, neglected, latent, but recoverable by honesty, patience, courage, and industry.