Common sense is the most widely shared commodity in the world, for every man is convinced that he is well supplied with it.

So far, I have been a spectator in this theater which is the world, but I am now about to mount the stage, and I come forward masked.

How can you be certain that your whole life is not a dream?

Some years ago I was struck by the large number of falsehoods that 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 in the sciences that was stable and likely to last.

Common sense is the most fairly distributed thing in the world, for each one thinks he is so well-endowed with it that even those who are hardest to satisfy in all other matters are not in the habit of desiring more of it than they already have.

Even if I were to suppose that I was dreaming and whatever I saw or imagined was false, yet I could not deny that ideas were truly in my mind.

The greatest minds are capable of the greatest vices as well as of the greatest virtues.