Descartes’ Method of Doubt and Proof of Self-existence

In his Discourse on Method and Meditations on First Philosophy, Descartes looks back on his life and realizes that a great number of his opinions have been false. Subsequently, the knowledge he has built up over his life must also be markedly false, seeing as it is based upon such incorrect opinions. He resolves to restart his quest for knowledge from the bottom, building a philosophy to be the foundation upon which all further intellectual knowledge can be built. Descartes proposes that there is a distinct difference between truth and belief, and that all he can know to be true is the existence of his own mind.

Descartes plans to remove all uncertain beliefs, making sure that the beliefs constituting his philosophy are absolutely true. He calls this the Method of Doubt, and says that to go through his opinions and reject them one by one would take too long and be futile. Therefore, he dismisses all of his opinions that have something about them of which he is not one hundred percent sure (can be doubted). More specifically, Descartes suggests the rejection of the doubtable foundations and basic principles upon which all his other opinions are founded.

In the examination of his beliefs, Descartes first argues that one cannot trust the senses. Everyone has at some point been deceived by his/her senses into thinking that something seemed good looking from a distance, but was actually very ugly when seen up close, or food that looked very tasty turned out to be disgusting. Descartes doubts his senses because he knows that they have tricked him before and gives the example of something looking small from a distance but large when up close.

In addition to doubting his senses, Descartes also doubts whether anything he is experiencing is real. He claims that he might be dreaming everything, because sometimes dreams seem as real as our “waking perceptions.” Descartes realizes that while dreaming, he is often convinced that he is sensing real objects. He feels certain that he is awake and sitting beside the fire, but contemplates that he has often dreamed this very sort of thing and been fully convinced by it. If there is no way to determine whether he is dreaming or not, then he has no way to determine if anything he is experiencing is real.

The third and final argument in the Method of Doubt is that of the evil, deceiving demon. Assuming that he may be dreaming, Descartes first suggests that even dream images are drawn from waking experiences, just as paintings are representations of real things. He thus concludes that we can doubt complex things like paintings, physics or medicine but cannot doubt the simple things from which they are constructed like shape, geometry, quantity, size, arithmetic and time. Upon further thought however, Descartes concludes that even simple things can be doubted, because there could be a malicious God controlling his perceptions and making him hallucinate or dream that he is experiencing reality.

Based upon this, supposed “truths” of reason, such as mathematics (e.g. 3+7=10) may be wrong because of an evil God trying to trick us all. Thus, Descartes cannot be certain that any of his beliefs are true because there is always the possibility that an evil demon is constructing such illusions to fool him. If we suppose there is no God, then there is even greater likelihood of being deceived, since our imperfect senses would not have been created by a faultless being. By doubting everything, he can at least be sure not to be misled into falsehood by this demon.

Following his arguments in Meditation I, Descartes starts Meditation II with the assumption that he has no senses, and that body, movement and place are all non-existent. He then declares that he has found a belief that holds up to the Method of Doubt. This belief is that he exists; “Cogito, ergo sum” – I think, therefore I am. Though Descartes is assuming that his physical body does not exist, his mind does exist because it is thinking. Also, based upon the idea that a God is putting the thoughts in his head, Descartes says that while he may indeed be misled by an evil demon, there must first exist something