In Descartes’ first Meditation he is searching for a way to determine whether something is true. After finding that numerous opinions that he had previously believed to be true were in fact false he seeks to discover a secure foundation for his beliefs. To do so, he must discover something indubitable. However, throughout Meditation I Descartes is unable to be fully certain of anything and thus is unable to find a secure foundation for his beliefs which is why I will argue that we have no knowledge of the external world. In order to prove that we have no knowledge of the external world one must first define the two.

I will use Descartes definition of knowledge as a conclusively justified, true belief. For the purpose of this essay I will define the external world as all a priori and a posteriori knowledge. As defined in Philosophical Problems by Laurence BonJour and Ann Baker, a priori are reasons independent from experience and a posteriori are reasons based on or derived from experience. So the two types of knowledge are that which you’ve experienced (a posteriori) and that which is independent of experience but you have prior knowledge of (a priori).

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Descartes examines both types of knowledge in Mediation I in order to determine whether either can be certain or if they have doubts. According to Descartes’ Dream Argument in Mediation 1 we cannot trust our beliefs based on sense experience. Since any experiences you can have while awake can also be dreamt, you can never know for sure whether they happened or not. You must know you are not dreaming in order to trust your experiences. However, it is impossible to know that you are not dreaming.

Because of the possibility that our sense experience actually occurred in a dream and thus cannot be justified by experience, it is impossible to trust any of our sense experience. Therefore, we cannot know anything about the external world on the basis of our sensory experience. As seen in Descartes’ Evil Genius Argument in Mediation 1, we cannot trust our beliefs that are independent of experience. All knowledge about simple and general things is independent of experience for example, mathematics. These beliefs are dubitable because of the possibility of an Evil Genius who has the ability to deceive our thoughts.

Although one has the idea that 2+2=4, the Evil Genius could instill in our minds that instead 2+2=5. Because we the possibility that there is an Evil Genius who can deceive our thoughts we can’t trust our beliefs to be true and therefore cannot trust our beliefs that are independent of experience. Furthermore, we can’t know anything about the external world on the basis of our beliefs independent of experience. Nevertheless, the Evil Genius Argument and the Dream Argument are disproved by Descartes in his third Meditation by stating that he can trust that his clear and distinct thoughts are true.

According to Descartes, if God exists then he can trust his clear and distinct ideas because God would not deceive him like the Evil Genius. Descartes has an idea of God and furthermore believes that anything with objective reality must ultimately be caused by something with formal reality. He also believes that any cause must have at least as much existence as the effect. In other words, if one has an idea of something, that idea must have come from something in existence and therefore God must exist. And so, if God exists, Descartes can therefore trust his clear and distinct ideas.

This argument completely overthrows the Dream and Evil Genius Arguments because if he can trust his clear and distinct thoughts then he can trust that his thoughts were not dreams but are real and that his thoughts are not being deceived by the Evil Genius because he can trust them. Therefore this objection concludes that we have knowledge of the external world because we can trust our beliefs based on experience and those independent of experience. However, Descartes’ third Meditation has flaws and therefore is unable to disprove his first Meditation. His objection to the first Mediation in Mediation III is based on the fact that God exists.

If God exists, Descartes can trust his clear and distinct thoughts. However God’s existence is proven by the premise that he can trust his clear and distinct thoughts but that is proven by the existence of God. Therefore, these beliefs are a fallacy and because the rest of his argument is based on the fact that God exists and that he can trust his clear and distinct thoughts, none of Descartes’ third meditation is true. Therefore, Descartes cannot trust his clear and distinct thoughts as well as cannot disprove the Evil Genius Argument or the Dream Argument in Mediation I.

So, because the Evil Genius Argument and the Dream Argument cannot be disproved then our beliefs based on experience as well as our beliefs independent of experience cannot be certain and therefore cannot be trusted. After this realization, Descartes finds that he has no other beliefs except those based on sense experience and those independent of experience and because neither of those beliefs can be trusted, he concludes that he should not trust any of his beliefs. Because none of our beliefs can be found true or be justified, we do not have knowledge of them.

Therefore we do not have knowledge of the external world. We have no knowledge of the external world and it is proven by Descartes’ first Mediation. Because we can never know whether or not we are dreaming, we cannot trust our beliefs based on experience. And because of the possibility that the Evil Genius is deceiving our mind, we can’t trust our beliefs independent of experience. Because knowledge is made up of only a priori and a posteriori knowledge, neither of which we can trust; and knowledge requires certainty, we have no knowledge of the external world.