Descartes spends the beginning of Meditations on First Philosophy by discussing his skepticism of the senses. Though the entire dream sequence in Meditations was not more than a few pages, it is easily one of the most discussed topics of the book. The dream argument can be broken down into three parts. 1st is that while I am asleep and dreaming I often feel sensations and perceptions that I feel when I am awake. 2nd is that there are no definitive signs to tell me if I am awake or dreaming, and this brings forward the 3rd point which is that I could be dreaming right now and not know it. The main objection to the dream argument is that the only way we can form images in our dreams is from seeing things and experiencing things in real life which would feed our minds.
Descartes knows that his image of everything he knows is built on his sense experiences which make them unreliable so he must tear down everything he knows and start again with the foundations of true knowledge. Descartes realizes that he is often convinced when he is dreaming that the things he sees are real objects. An example could be that I KNOW that I am sitting at a computer writing this paper, but while I am dreaming I can dream the exact same thing and be completely convinced that is it real. After pondering this situation Descartes realizes that though the things we see in dreams are fake, they can be drawn from real objects. Even though we may dream of something completely new, Descartes concluded, we cannot doubt simple universal units and the parts they represent such as time, size, shapes, etc. This helps Descartes to discover that we cannot doubt the studies of simple things (such as geometry) but instead can doubt the studies of more collected ideas such as medicine and astronomy.
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