Descartes is philosophizing in his study when he sets out to remove all assumptions from what he knows. Descartes strips away all knowledge that is empirical first. He notices that his table looks different in different lights, that the ball of wax he is able to manipulate is at one time soft, and another liquid, and another firm. Descartes’ question is when is the wax actually itself? How can the wax exist in one condition and then exist in another condition? Descartes then looks outside and observes people walking on his street. He catches himself assuming that there are people underneath the hats and jackets, but it is possible that he is deceived by clothes-wearing robots, or, that the people cease to animate until he actively observes. It is from this possibility that empirical observations cannot be trusted forms of knowledge.

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We have no innate connection or knowledge of the existence of everything that we sense, and Descartes will eventually get to the argument that we do not know anything about the sensory world, aside from knowing that we sense it. This is Descartes’ first building block toward cogito ergo sum. This building block is one that Descartes concludes that the only thing he knows for sure about empirical reality is that he is an active sensor of these empiricisms. He has not arrived at cogito ergo sum at this point.
The next building block is Descartes’ deconstruction of the reliability of the senses. Descartes posits that it is possible that everything he senses is not as it seems to him, and that there is a possibility that he mistaken in his perceptions. For instance, Descartes puts forth the realistic experience of dreams. There are many times when dreams seem real, and one is involved in their dream as though it is reality. Emotions and experiences happen in dreams as they do while awake. Descartes wonders if this world is a dream-world without any reliable sensory information.

The dream argument is broken into three premises: 1. Descartes states that he often has perceptions that are similar to the sensation of dreaming, or that while dreaming the sensation is similar to being awake; 2. There are not any definitive differences that differentiate a dream from reality; 3. Descartes puts forth that it is possible that he is dreaming as he writes the Meditations, and that his waking state is actually a dream state.
If we are dreaming, then sensory perceptions cannot be trusted because although they seem real, they are not. If this is possible, then the entire sensory world is doubted by Descartes, at least in relation to the perceiver. Descartes then expands the dream argument into the evil demon argument.
Firstly, Descartes posits the possible existence of a deceiving God and then transitions this premise to the possible existence of an evil demon. His argument goes as such: 1. We believe in an all-powerful God, and that this God created us; 2. Since God is all-powerful he is powerful enough to deceive us about anything and everything-including mathematical and scientific knowledge; 3. Since God can deceive it is possible that we are deceived about reality. This argument seems to hold water for Descartes until he realizes that God would not be a deceiver, because he is “good”. Descartes modifies the deceiving God argument and replaces God with an evil demon. An evil demon would be performing his evilness when we are deceived; whereas God would not deceive.
The evil demon argument posits the existence of an evil demon who has the purpose of deceiving us, and is powerful enough to do so. If this demon exists, then it is possible that all sensory knowledge can be doubted, for all sensory knowledge can be manipulated by the evil demon.
At this point, Descartes has undermined the senses as reliable tools for discerning truth and reality. Since our senses are able to be deceived, and there is no certainty about any sensory knowledge, then it makes sense that knowledge is not known through the senses. Descartes stripped away any reliability of the senses to accurately “know” anything; therefore, the senses are not our source for truth or knowledge. The senses cannot be trusted, therefore, it is impossible that the senses can be a source for truth.
Descartes has now removed the foundation for knowledge that is sensible and theoretical (such as math). It is the next obstacle to figure out what, if anything, can be known for certain. Descartes ponders whether there is any knowledge that he can be certain of, and suddenly the evil demon who destroyed the possibility to have sensory knowledge, is the source for the next building block of cogito ergo sum.
Descartes comes to the realization that regardless of how deceiving this demon is, regardless of whether any sensory or theoretical knowledge is able to be known, Descartes asserts that there is a “he” which is deceived. What Descartes means is that the evil demon may give unreliable knowledge by deceiving him, but the evil demon cannot undermine that there is a “he” who the demon deceives.
The actual deception, at this point, is not a concern for Descartes, but rather that there is a subject being deceived. Descartes concludes that the evil demon can deceive all he wants, but the evil demon cannot counter the certitude that there is someone who is deceived, who senses the world (through deception), and that there is someone who thinks these thoughts about his deception.
Descartes uses the demon to prove that he (Descartes) exists. The demon also proves that sensory knowledge may not exist. Now that Descartes can conclude that sensory knowledge is not trustworthy, and that nothing can be known for certain in the sensory world, he must establish the only certitude that he has: cogito ergo sum. Descartes arrives at “I think, therefore, I am” because he concludes that a demon could deceive him about what he is thinking, but could not deceive him that he is, in fact, thinking.
I think that Descartes’ argument is strong: the one constant through all the doubt is that there is a doubter. AS for the dream argument, Descartes argument seems true in so much as there are realistic dreams, and sometimes it is difficult to tell the dream state from a waking state. However, there are some definitive differences between dreaming and awake. These differences are not known while dreaming, only after waking. Descartes evil demon argument is an improvement over the deceiving God argument because God does not deceive; and in the extension of his argument this is how God’s existence and the existence certainty is possible at the end of Descartes’ arguments. Because God does not deceive, and it is clear that there is a thinking soul, then the soul does not perceive deceptions but truths, because God would not allow deception. I think that the evil demon argument sets up a solid foundation for the conclusions of Descartes’ Meditations: Descartes wants to prove certitude and God. Descartes evil demon argument is solid because regardless of the nature of deception, there is someone, some mind, whom the demon deceives.

    References
  • Faculty, St. Anselm. Important Arguments from Descartes’ “Meditations”. Web anselm.edu/homepage/dbanach/dcarg, n.d. Web. 22 Feb. 2016.
  • Solomon, Robert, et al. Introducing Philosophy: A Text with Integrated Readings. Oxford: University Press, 2016.