Struck with the contradictions which I encountered in endeavoring to disentangle the truth and falsehood of these opinions, I was led to make the following reflection: "The search after truth being the aim which I propose to myself, I ought in the first place to ascertain what are the bases of certitude." In the next place I recognized that certitude is the clear and complete knowledge of things, such knowledge as leaves no room for doubt nor possibility of error and conjecture, so that there remains no room in the mind for error to find an entrance. In such a case it is necessary that the mind, fortified against all possibility of going astray, should embrace such a strong conviction that, if, for example, any one possessing the power of changing a stone into gold, or a stick into a serpent, should seek to shake the bases of this certitude, it would remain firm and immovable. Suppose, for instance, a man should come and say to me, who am firmly convinced that ten is more than three, "No; on the contrary, three is more than ten, and, to prove it, I change this rod into a serpent," and supposing that he actually did so, I should remain none the less convinced of the falsity of his assertion, and although his miracle might arouse my astonishment, it would not instil any doubt into my belief. I then understood that all forms of knowledge which do not unite these conditions (imperviousness to doubt, etc.) do not deserve any confidence, because they are not beyond the reach of doubt, and what is not impregnable to doubt can not constitute certitude. _The Subterfuges of the Sophists_ I then examined what knowledge I possessed, and discovered that in none of it, with the exception of sense-perceptions and necessary principles, did I enjoy that degree of certitude which I have just described. I then sadly reflected as follows: "We can not hope to find truth except in matters which carry their evidence in themselves—that is to say, in sense-perceptions and necessary principles; we 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.