M. Desargues puts me under obligations on account of the pains that it has pleased him to have in me, in that he shows that he is sorry that I do not… - René Descartes

I suppose therefore that all things I see are illusions; I believe that nothing has ever existed of everything my lying memory tells me. I think I have no senses. I believe that body, shape, extension, motion, location are functions. What is there then that can be taken as true? Perhaps only this one thing, that nothing at all is certain ... But I cannot forget that, at other times I have been deceived in sleep by similar illusions; and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished; and in amazement I almost persuade myself that I am now dreaming ... I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake ... There is nothing more ancient than the truth.

Staying as I am, one foot in one country and the other in another, I find my condition very happy, in that it is free.

I was thus led to infer that the ground of our opinions is far more custom and example than any certain knowledge.