"Starting with no preconceptions at all, what is the most reasonable model of the world that we can build up? Only two things seem really certain: one exists, and one has perceptions. I may be a meat machine, a soul, an eye of God, a collection of ideas, or who knows what — but I am certain that I exist. I am the thing that writes these words. Of course, you may doubt whether I am real — perhaps you are only dreaming that you read this book — but you do know for certain that you yourself exist. The fact that one has experiences is equally certain; to put it more neutrally, one cannot doubt that perceptions occur."