Descartes

However he soon realises that there have been times when he has been asleep and dreaming of the same experiences he had when awake. Descartes argues: "How often asleep at night, am I convinced of just such familiar events-that I am here in my dressing-gown, sitting by the fire-when in fact I am lying undressed in bed!" (Cottingham 1984, p13). Descartes comes to realise that dreaming and reality are so blurred that he cannot know if anything occurring is true or false and that therefore the external world may be an illusion. Descartes thus asserts that all the knowledge derived from the senses cannot be regarded as absolutely certain.
             Descartes then considers the propositions of mathematics and the fact that a square has four sides and that two and three always makes five. This he perceives to be true whether he is asleep or awake and it appears that this cannot be doubted. On further consideration, however, Descartes considers the possibility that God may have brought it about that he just imagines these things are true, that God is making his mind incorrect when he adds two and three or counts the sides of a square. He further imagines that God is causing him to have sensations in his mind of the things he perceives around him. However when considering God as supremely good, Descartes finds it hard to believe that this supreme good being would deceive, therefore, Descartes invents the idea of an evil demon who is deceiving him, Descartes says of the evil demon: " I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgment" (Cottingham 1984, p13). Descartes introduces this idea of the evil demon in order to prevent the return of the former beliefs previously called into doubt. Therefore at the end of the first meditation, Descartes is now in the position where he has put aside all his beliefs as not being fully certain.