Let us hear him speak for himself. He is relating his attempt to detach himself from the opinions which he had imbibed in his childhood: "I said to myself, 'My aim is simply to know the truth of things; consequently, it is indispensable for me to ascertain what is knowledge.' Now it was evident to me that certain knowledge must be that which explains the object to be known in such a manner that no doubt can remain, so that in future all error and conjecture respecting it must be impossible. [Sidenote: The certitude of knowledge.] Not only would the understanding then need no efforts to be convinced of certitude, but security against error is in such close connexion with knowledge, that, even were an apparent proof of falsehood to be brought forward, it would cause no doubt, because no suspicion of error would be possible. Thus, when I have acknowledged ten to be more than three, if any one were to say, 'On the contrary, three is more than ten, and to prove the truth of my assertion, I will change this rod into a serpent,' and if he were to change it, my conviction of his error would remain unshaken. His manoeuvre would only produce in me admiration for his ability. I should not doubt my own knowledge. "Then was I convinced that knowledge which I did not possess in this manner, and respecting which I had not this certainty, could inspire me with neither confidence nor assurance; and no knowledge without assurance deserves the name of knowledge. "Having examined the state of my own knowledge, I found it divested of all that could be said to have these qualities, unless perceptions of the senses and irrefragable principles were to be considered such. [Sidenote: Fallibility of the senses.] I then said to myself, 'Now, having fallen into this despair, the only hope of acquiring incontestable convictions is by the perceptions of the senses and by necessary truths.' Their evidence seemed to me to be indubitable. I began, however, to examine the objects of sensation and speculation, to see if they possibly could admit of doubt. Then doubts crowded upon me in such numbers that my incertitude became complete. Whence results the confidence I have in sensible things? The strongest of all our senses is sight; and yet, looking at a shadow, and perceiving it to be fixed and immovable, we judge it to be deprived of movement; nevertheless, experience teaches us that, when we return to the same place an hour after, the shadow is displaced, for it does not vanish suddenly, but gradually, little by little, so as never to be at rest. If we look at the stars, they seem to be as small as money-pieces; but mathematical proofs convince us that they are larger than the earth. These and other things are judged by the senses, but rejected by reason as false. I abandoned the senses, therefore, having seen all my confidence in their truth shaken.