Descartes" role of the first meditation is to doubt many things. One of the things that he begins to disbelief is the concept of dreaming. He questions the concepts of how does one really know if he is dreaming or if he is awake. He asks, are there any definite signs that distinguish the dream world from reality.
Descartes discards the senses as a reliable source of truth and as an indicator of an object's existence. For example, one may dream similar experiences to what the senses communicate is reality when one is awake. When we are in a dream, we are not usually aware that we are in a dream, so our circumstances seem just as real as when we are awake, when in fact they are not. In actual fact, is there any way we could determine that we are not dreaming right now? The truths we appear to perceive while dreaming, however, are simple contingent truths that could change to falsehood upon waking. There are still some necessary truths which remain the same both dreaming and waking, and it is these that Descartes shall attempt to uncover. .
He is often persuaded that when he is dreaming he is sensing real objects. He feels certain that he is awake and sitting by the fire, but mirrors that often he has dreamed this very sort of thing and been completely convinced by it. There are no definite signs by which to distinguish being awake from being asleep. Despite the fact that his current feelings may be dream images, he proposes that even dream images are drawn from waking experience, much like paintings in that respect. Even when a painter creates an imaginary creature, like a mermaid, the amalgamated parts are drawn from real things, such as a women and a fish. And even when a painter creates something entirely new, at least the colors in the painting are drawn from real experience. Thus, Descartes concludes, though he can doubt complex things, he cannot doubt the simple and universal parts from which they are constructed like shape, quantity, size, time, etc.
