If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.

Bad books engender bad habits, but bad habits engender good books.

Good sense is, of all things among men, the most equally distributed; for every one thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

Here I beg you to observe in passing that the scruples that prevented ancient writers from using arithmetical terms in geometry, and which can only be a consequence of their inability to perceive clearly the relation between these two subjects, introduced much obscurity and confusion into their explanations.

The senses deceive from time to time, and it is prudent never to trust wholly those who have deceived us even once.

Before examining this more carefully and investigating its consequences, I want to dwell for a moment in the contemplation of God, to ponder His attributes in me, to see, admire, and adore the beauty of His boundless light, insofar as my clouded insight allows. Believing that the supreme happiness of the other life consists wholly of the contemplation of divine greatness, I now find that through less perfect contemplation of the same sort I can gain the greatest joy available in this life.

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.