"But the greatest change I introduce is in the form itself of induction and the judgments made thereby. For the induction of which the logicians speak, which proceeds by simple enumeration, is a puerile thing; concluded at hazard, is always liable to be upset by a contradictory instance, takes into account only what is known and ordinary, and leads to no result. Now, what the sciences stand in need of is a form of induction which shall analyze experience and take it to pieces, and by a due process of exclusion and rejection lead to an inevitable conclusion." "Now, my method, though hard to practise, is easy to explain; and it is this: I propose to establish progressive stages of certainty. The evidence of sense, helped and guarded by a certain process of correction, I retain; but the mental operation which follows the act of sense I for the most part reject; and instead of it I open and lay out a new and certain path for the mind to proceed in, starting directly from the simple sensuous perception." The same dissatisfaction with mediæval philosophy expressed itself in Descartes. The incompetence of philosophers to solve the problems they occupied themselves with--the anarchy which reigned in the scientific world, where no two thinkers could agree upon fundamental points--the extravagance of the conclusions to which some accepted premises led, determined him to seek no more to slake his thirst at their fountains. "And that is why, as soon as my age permitted me to quit my preceptors," he says, "I entirely gave up the study of letters; and resolving to seek no other science than that which I could find in myself, or else in the great book of the world, I employed the remainder of my youth in travel, in seeing courts and camps, in frequenting people of diverse humors and conditions, in collecting various experiences, and above all in endeavoring to draw some profitable reflection from what I saw. For it seemed to me that I should meet with more truth in the reasonings which each man makes in his own affairs, and which, if wrong, would be speedily punished by failure, than in those reasonings which the philosopher makes in his study, upon speculations which produce no effect, and which are of no consequence to him, except perhaps that he will be more vain of them the more remote they are from common-sense, because he would then have been forced to employ more ingenuity and subtlety to render them plausible." For many years he led a roving, unsettled life; now serving in the army, now making a tour, now studying mathematics in solitude, now conversing with scientific men. One constant purpose gave unity to those various pursuits. He was elaborating his answers to the questions which perplexed him; he was preparing his method.