Perhaps true beliefs come from your internal senses. Internal senses include an awareness of oneself, such as believing you have a stomach and a heartbeat, without seeing them. An example of this internal sense is seen when Descartes says “There is the fact that I am here, seated by the fire, having the paper in my hands...and how can I deny that these hands and this body is mine.” (Descartes, p.75, par.4) His internal sense makes him aware of his hands and body. Though, Descartes must explore all doubts involving this internal sense if he wants to use it as his foundation for knowledge.
Descartes brings up the possibility that perhaps at this point, right now, he is dreaming. A person who is dreaming may have difficulty differentiating between the dream and reality. Descartes says “How often has it happened to me that in the night I dreamt that I found myself in this particular place, that I was dressed and seated near the fire, whilst in reality I was lying undressed in bed!” (Descartes, p.76, par.1) According to this idea, I may believe, even now, I am dreaming, this not my body, and I am not writing this paper for philosophy but I am really lying in bed somewhere sleeping. This dream hypothesis would invalidate the beliefs that are based on internal sense; for if you are dreaming then what you believe to be your awareness of self is truly false. You may say that everyday life exhibits a smoothness and understanding, which dreams do not. Dreams have little rhyme or reason; while life experience is orderly and controlled. However, this scale of measuring the differences of coherence between dreams and reality is unreliable. Sometimes dreams are incoherent and sometimes they appear to be real. Beliefs derived from internal senses cannot be true due to the possibility that you may be dreaming.

Descartes goes on to say that maybe the only true beliefs come from intellect, or clear and distinct ideas. Despite the falsity of internal senses based on dreams, dreams are based on reality. Whether this hand is real or dreamed, it is my hand, and it exists somewhere. In addition, certain things are true in any context, such as “simple Arithmetic, Geometry and other sciences of that kind which only treat of things that are very simple and very general....For whether I am awake or asleep, two and three together always form five, and the square can never have more than four sides.” (Descartes, p.76, par.4) The philosopher, John Locke argues this point by stating that mathematical truths are learned from experience and are not innate ideas. Locke, an empiricist, believes that all people begin with a clean slate and knowledge is added by experience. Descartes argues that simple mathematical ideas are an Apriori form of knowledge. Two plus three can never equal fifty-three. Simple mathematics is a clear and distinct idea, it is self-evident and needs no experience. Yet, Descartes takes another twist on this concept. He proposes that perhaps even these clear and distinct ideas cannot provide a foundation for knowledge. Descartes asks what if he is being deceived. What if all he believes to be true, is being planted by some sort of intelligence force, such as an evil demon? Perhaps he is being deceived in these fundamental beliefs. “How do I know I am not deceived every time that I add two and three, or count the sides of a square?” (Descartes, p.76, par. 4) This thought experiment leads Descartes to another method in doubt.

Descartes then goes on to assume that there is a God, who is all powerful, and created this world; yet he asks, “How do I know that He has not brought it to pass that there is no earth, no heaven, no extended body, no magnitude, no place, and that nevertheless they seem to me to exist just exactly as I know see them?” (Descartes, p.76, par.5) Without a guarantee of reality, maybe all of his previous beliefs are false. Descartes doubts the supreme goodness of a God that would let him be deceived even occasionally. Moreover, if a perfect God does not exist then it becomes probable that Descartes himself is increasingly imperfect and therefore is constantly being misled. “If, however, it is contrary to His goodness to have made me such that I constantly deceive myself, it would also appear contrary to His goodness to permit me to be sometimes deceived, and nevertheless I cannot doubt that he does permit this.” (Descartes, p.76, par.5) Descartes assumes the scenario that God is really an “evil demon“.

I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me. I shall think 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. (Blackburn, 19) (Descartes, p.77, par.3)

Descartes explores the ultimate source of all his beliefs by bringing up the evil demon. Now that Descartes has obliterated all the foundations of his previous beliefs, he can develop a starting point or origin, which all other true knowledge can be built upon.

Here is a science fiction possibility, which clarifies the evil demon thought experiment. Imagine that a human being has been subjected to an operation by an evil scientist. The person’s brain has been removed from the body and placed in a vat of nutrients, which keeps the brain alive. The nerve endings have been connected to a super scientific computer, which causes the person whose brain it is to have the illusion that everything is perfectly normal. The computer is so clever that, for example, if the person tries to raise his hand, the feedback from the computer will cause the person to ’see’ and ’feel’ the hand being raised. The evil scientist can input any information into the computer and cause the person to experience anything he wishes. This scenario explains Descartes’ idea of the evil demon. That there is something, which controls all the thoughts, beliefs, and experiences a person, may have. Descartes continues with this possibility in order to find a strong foundation for knowledge. As Descartes says, “I shall remain obstinately attached to this idea...and with firm purpose avoid giving credence to any false thing, or being imposed upon by this arch deceiver...but the task is a laborious one.” (Descartes, p.77, par.3) And so ends the First Meditation.

One philosopher, Hume, argues against Descartes’ conclusions. Hume, like Descartes accepts the belief that knowledge needs a foundation. Yet, Hume argues that knowledge cannot have the type of foundation, which Descartes wanted. Hume argues that the legitimacy of our senses and reasonings is part of the foundation. All humans, including philosophers, grew up trusting their senses. For example, people became adept at recognizing danger and trust in these instincts. This is called natural foundationalism. Unlike Descartes, Hume believes in trusting your senses. While trying to find a foundation for his knowledge in the sciences and methodological purposes, Descartes insists on using his strict method of doubt. Rather than using natural foundationalism’s basis of knowledge through sense experience Descartes develops his foundation through reason. Descartes’ hyperbolic doubt is unrealistic.

Hume knows Descartes argument is doomed to failure. As Hume says,

“There is a species of skepticism, antecedent to all study and philosophy, which is much inculcated by Descartes...as a sovereign preservative against error and precipitate judgment....It recommends a universal doubt...of our very faculties...therefore, were it ever possible to be attained by any human creature (as it plainly is not) it would be entirely incurable; and no reasoning could ever bring us to a state of assurance and conviction upon any subject.” (Blackburn, 40)
Descartes idea of ‘all or nothing’ is unnecessary. According to Hume, the evil demon notion does not matter when it comes to thoughts. The harmony between our minds and the world is due to the fact that the world influences our minds. The senses, which we utilize in this world, aid us in the correct way. If they were untrue we would be unable to survive. Hume does not believe in the need for the evil demon hypothesis. He believes you can find a foundation of knowledge through the senses.
Furthermore, there is another response that rids of Descartes’ necessity of an absolute foundation and Hume’s naturalism. This approach emphasizes the notion of coherent structures; in which a system of beliefs “hangs together” rather than having one strict base. This can be portrayed with a ship or web, which are made up from a tissue of interconnecting parts, and derive their strength from those interconnections. Each part supports the other part without needing one base, or foundation to support it. If a belief should be challenged than all the other parts can support it. As compared to a spider web, if one strand in the spider web is broken the rest of the web will still remain strong. Any element can be changed or replaced due to the strength of the connections between all the elements. However, this approach may backfire if all the elements have strong connections, and they are all wrong. The argument will stand strong, but incorrectly. In that case, Descartes’ evil demon hypothesis seems necessary.

One final problem with Descartes’ evil demon hypothesis is that it causes Descartes to contradict himself. How can Descartes know the Evil Demon is not implanting him with the thoughts for his argument in the first place? Since the evil demon may be deceiving Descartes even about logic and mathematics, perhaps he is being deceived about his own argument. For instance, might Descartes be deceived into thinking that the conclusion of the argument “I cannot be certain about any of my beliefs” follows logically from the premises of the first meditation. How could Descartes argue for scepticism at all since the evil demon may be deceiving him about the validity of his arguments? Even if Descartes were arguing validly, he could never know it.