“… Moreover, no one can be sure, apart from faith, whether he is sleeping or waking, because when we are asleep we are just as firmly convinced that we are awake as we are now. As we often dream we are dreaming, piling up one dream on another, is it not possible that this half of our life is itself just a dream, on to which the others are grafted, and which we shall awake when we die? That while it lasts we are as little in possession of the principles of truth and goodness as during normal sleep? All this passage of time, of life, all these different bodies which we feel, the different thoughts which stir us, may be no more than illusions like the passage of time and vain phantoms of our dreams. We think we are seeing space, shape, movement, we feel time pass, we measure it, in fact we behave just as we do when we are awake. As a result, since half our life is spent in sleep, on our own admission and despite appearances we have no idea of the truth because all our intuitions are simple illusions during that time. Who knows whether the other half of our lives, when we think we are awake, is not another sleep slightly different from the first, on to which our dreams are grafted as our sleep appears, and from which we awake when we think we are sleeping? And who can doubt that, if we dreamed in the company of others and our dreams happened to agree, which is common enough, and if we were alone when awake, we should think things had been turned upside-down?…”

by Blaise Pascal.