And the last was to make everywhere enumerations so complete, and surveys so wide, that I should be sure of omitting nothing. Exact observation of these precepts gave me such facility in unravelling the questions comprehended in geometrical analysis and in algebra, that in two or three months not only did I find my way through many which I had formerly accounted too hard for me, but, towards the end, I seemed to be able to determine, in those which were new to me, by what means and to what extent it was possible to resolve them. And so I promised myself that I would apply my system with equal success to the difficulties of other sciences; but since their principles must all be borrowed from philosophy, in which I found no certain principles of its own, I thought that before all else I must try to establish some therein. By way of preparation (for I was then but twenty-three years old) I must root up from my mind my previous bad opinion of it, and must practise my method in order that I might be confirmed in it more and more. _III.--A RULE OF LIFE_ Meanwhile I must have a rule of life as a shelter while my new house was in building, and this consisted of three or four maxims. The first was to conform myself to the laws and customs of my country, and to hold to the religion in which, by God's grace, I had been brought up; guiding myself, for the rest, by the least extreme opinions of the most intelligent. Among extremes I counted all promises by which a man curtails anything of his liberty; for I should have deemed it a grave fault against good sense if, because I approved something in a given moment, I had bound myself to accept it as good for ever after. My second maxim was to follow resolutely even doubtful opinions when sure opinions were not available, just as the traveller, lost in some forest, had better walk straight forward, though in a chance direction; for thus he will arrive, if not precisely at the place where he desires to be, at least probably at a better place than the middle of a forest. My third maxim was to endeavour always to conquer myself rather than fortune, and to change my desires rather than the order of the world, and in general to bring myself to believe that there is nothing wholly in our power except our thoughts. And I believe that herein lay the secret of those philosophers who, in the days of old, could withdraw from the domination of fortune, and, despite pain and poverty, challenge the felicity of their gods. Finally, after looking out upon the divers occupations of men, I pondered that I could do no better than persevere in that which I had chosen--so deep was my content in discovering every day by its means truths which seemed to me important, yet were unknown to the world.