I doubt, therefore I think, therefore I am. - René Descartes

And I have always had an especially great desire to learn to distinguish the true from the false, in order to see my way clearly in my actions, and to go forward with confidence in this life. It is true that, so long as I merely considered the customs of other men, I found hardly anything there about which to be confident, and that I noticed there was about as much diversity as I had previously found among the opinions of philosophers. Thus the greatest profit I derived from this was that, on seeing many things that, although they seem to us very extravagant and ridiculous, do not cease to be commonly accepted and approved among other great peoples, I learned not to believe anything too firmly of which I had been persuaded only by example and custom; and thus I little by little freed myself from many errors that can darken our natural light and render us less able to listen to reason. But after I had spent some years thus studying in the book of the world and in trying to gain some experience, I resolved one day to study within myself too and to spend all the powers of my mind in choosing the paths that I should follow.

When it is not in our power to determine what it true, we ought to follow what is most probable.

Είμαι, λοιπόν, εγώ ένα σκεπτόμενο πράγμα, δηλαδή ένα πράγμα που αμφιβάλλει, συμφώνει, αρνείται, αντιλαμβάνεται με το πνεύμα (αν και, προς το παρόν, πολύ λίγα), αγνοεί (σίγουρα ακόμα πολλά), θέλει, δεν θέλει, και ακόμα φαντάζεται και αντιλαμβάνεται (γιατί, όπως έχω ήδη παρατηρήσει, παρότι τα πράγματα που φαντάζομαι και αισθάνομαι ίσως να μην υπάρχουν έξω από εμένα, είμαι βέβαιος ότι αυτά τα είδη σκέψης που αποκαλώ φαντασιώσεις και αισθήματα, μόνο ως είδη σκέψης, βρίσκονται μέσα μου).