It has already been indicated that the great work performed by Descartes was, like that of Bacon, the promulgation of a new method. This was rendered necessary by their separation from the ancient philosophy and their exclusion of authority. If inquiry is to be independent, if reason is to walk alone, in what direction must she walk? Having relinquished the aid of the Church, there were but two courses open: the one to tread once more in the path of the ancients, and to endeavor by the ancient methods to attain the truth; or else to open a new path, to invent a new method. The former was barely possible. The spirit of the age was deeply imbued with a feeling of opposition against the ancient methods; and Descartes himself had been painfully perplexed by the universal anarchy and uncertainty which prevailed. The second course was therefore chosen. Uncertainty was the disease of the epoch. Scepticism was widespread, and even the most confident dogmatism could offer no criterion of certitude. This want of criterion we saw leading, in Greece, to scepticism, Epicureanism, Stoicism, the New Academy, and finally leading the Alexandrians into the province of faith, to escape from the dilemma. The question of a criterion had long been the vital question of philosophy. Descartes could get no answer to it from the doctors of his day. Unable to find firm ground on any of the prevalent systems, distracted by doubts, mistrusting the conclusions of his own understanding, mistrusting the evidences of his senses, he determined to make a _tabula rasa_, and reconstruct his knowledge. He resolved to examine the premises of every conclusion, and to believe nothing but upon the clearest evidence of reason; evidence so convincing that he could not by any effort refuse to assent to it. He has given us the detailed history of his doubts. He has told us how he found that he could plausibly enough doubt of everything except of his own existence. He pushed his scepticism to the verge of self-annihilation. There he stopped; there in self, in his consciousness, he found at last an irresistible fact, an irreversible certainty. Firm ground was discovered. He could doubt the existence of the external world, and treat it as a phantasm; he could doubt the existence of a God, and treat the belief as a superstition; but of the existing of his thinking, doubting mind no sort of doubt was possible. He, the doubter, existed if nothing else existed. The existence that was revealed in his own consciousness was the primary fact, the first indubitable certainty. Hence his famous "_Cogito, ergo sum_" ("I think, therefore I am").