“I resolved one day to . . . use all the powers of my mind in choosing the paths I should follow” (Discourse Part I: AT VI 10: CSM I 116). Thus Descartes introduces his account of his celebrated first solitary retreat during the winter of 1619-20. But he goes on to note that he decided to postpone actually embarking on his life's work until he had reached “a more mature age than twenty-three, as [he] then was ” (Part II: AT VI 22: CSM 1122). Toward the end of the winter of 1619-20, then, he began to travel, and this occupied “the next nine years”; only after these “nine years ” did he finally work out his philosophy, which was to be “more certain than the commonly accepted one” (Part III: AT VI 28, 30: CSM I 125-6). This period of Descartes' early life is obscured by the errors of his chief biographers, which have been repeated down the centuries. There is a marked tendency to bring forward his interest in science and the search for its foundations - an interest which in fact developed gradually and relatively late.