If you would be a real seeker after truth, you must at least once in your life doubt, as far as possible, all things.
René Descartes, Discours de la Methode.
So blind is the curiosity by which mortals are possessed, that they often conduct their minds along unexplored routes, having no reason to hope for success, but merely being willing to risk the experiment of finding whether the truth they seek lies there.René Descartes
"Thinking is another attribute of the soul; and here I discover what properly belongs to myself. This alone is inseparable from me. I am--I exist: this is certain"René Descartes
"Cogito, ergo sum." (trans.: "I think, therefore I am.")René Descartes
The whole time of my life may be divided into an infinity of parts, each of which is in no way dependent on any other; and, accordingly, because I was in existence a short time ago, it does not follow that I must now exist, unless in this moment some cause create me anew as it were, that is, conserve me.René Descartes