must therefore establish these on a firm basis. Is my absolute confidence in sense-perceptions and on the infallibility of necessary principles analogous to the confidence which I formerly possessed in matters believed on the authority of others? Is it only analogous to the reliance most people place on their organs of vision, or is it rigorously true without admixture of illusion or doubt?"
I then set myself earnestly to examine the notions we derive from the evidence of the senses and from sight in order to see if they could be called in question. The result of a careful examination was that my confidence in them was shaken. Our sight, for instance, perhaps the best practised of all our senses, observes a shadow, and finding it apparently stationary pronounces it devoid of movement. Observation and experience, however, show subsequently that a shadow moves not suddenly, it is true, but gradually and imperceptibly, so that it is never really motionless.
Again, the eye sees a star and believes it as large as a piece of gold, but mathematical calculations prove, on the contrary, that it is larger than the earth. These notions, and all others which the senses declare true, are subsequently contradicted and convicted of falsity in an irrefragable manner by the verdict of reason.
Then I reflected in myself: "Since I can not trust to the evidence of my senses, I must rely only on intellectual notions based on fundamental principles, such as the following axioms: 'Ten is more than three. Affirmation and negation can not coexist together. A thing can not both be created and also existent from eternity, living and annihilated simultaneously, at once necessary and impossible.'" To this the notions I derived from my senses made the following objections: "Who can guarantee you that you can trust to the evidence of reason more than to that of the senses? You believed in our testimony till it was contradicted by the verdict of reason, otherwise you would have continued to believe it to this day. Well, perhaps, there is above reason another judge who, if he appeared, would convict reason of falsehood, just as reason has confuted us. And if such a third arbiter
