René Descartes and Lucretius each use images of sleep and dreaming to convey portions of their philosophical concepts. Descartes uses sleep as an example, while Lucretius sees sleep as a freeing of the mind and the emotions in certain important ways.
Descartes began with his method of doubt, undertaken because he had reached an age where he now believed that he would be able to remove all of his earlier beliefs and begin with a clean slate, as it were. He says that everything he knew or thought he knew in the past was based on sense perception, and the senses can lie. Because of this, Descartes has to begin from a position of doubt and must prove everything to himself through the application of reason. Descartes notes a number of reasons why we have to doubt our senses. For one thing, the senses sometimes deceive us with regard to minute objects or objects that are at a great distance from us. Our senses have limitations when it comes to things that are too small or too far away. There are other things which are self-evident and not to be doubted, and Descartes cites the fact that he is sitting in his chair by the fire in a dressing-gown with a paper in his hands--these things are not to be doubted.
Descartes uses as an example what happens when he is asleep. He says that what the insane represent to themselves when they are awake he may represent to himself when he is asleep because dreams are often improbable. This fact raises issues about the validity of the senses, for when Descartes is asleep, he may have dreamed precisely that he is sitting in front of this fire in just the same pose. When he is, he is beset by the illusion that what he senses is real, when in fact it is a dream. How is it possible to tell the difference? Descartes finds that he is uncertain now whether he is awake or dreaming, and so the experience of the senses has to be doubted because it is indistinguishable at the time from the dreaming ...