"Some years ago I was struck by the large number of falsehoods which I had accepted as true in my childhood, and by the highly doubtful nature of the whole edifice that I had subsequently based on them. I realized that it was necessary, once in the course of my life, to demolish everything completely and start again right from the foundations if I wanted to establish anything at all ... that was stable and likely to last.

Reason now leads me to believe that I should hold back my assent from opinions which are not completely certain and indubitable just as carefully as I do from those which are patently false. So, for the purpose of rejecting my opinions, it will be enough if I find in each of them some reason for doubt."

"I can now give a large number of different proofs, each of which is a perfectly rigorous proof; and at many other times I have been in a position to give many others. I can prove now, for instance, that two human hands exist. How? By holding up my two hands and saying, as I make a certain gesture with the right hand, “Here is one hand,” and adding, as I make a certain gesture with the left, “and here is another.” And if, by doing this, I have proved the existence of external things, you will all see that I can also do it now in numbers of other ways; there is no need to multiply examples.

I certainly did at the moment know that which I expressed by the combination of certain gestures with saying the words ‘There is one hand and here is another.’ ... How absurd it would be to suggest that I did not know it, but only believed it, and that perhaps it was not the case! You might as well suggest that I do not know that I am now standing up and talking — that perhaps after all I’m not, and that it’s not quite certain that I am.”