Argument 1

Meditation I: Argument for skepticism about the external world.

1. Knowledge = justified, true, belief.
2. A belief P is justified only if P is absolutely certain.
3. P is certain only if P is indubitable, P cannot be doubted.
4. If it is conceivable that P is false, P is not certain.
5. Every belief based on the senses (beliefs about the external world) is such that we could conceive it to be false.
6. Thus, no beliefs based on the senses are indubitable.
7. Thus, no beliefs based on the senses are certain.
8. Thus, no beliefs based on the senses are justified.
9. Thus, no beliefs based on the senses are knowledge
10. All beliefs about the external world are based on our senses.
11. Thus, we have no knowledge of the external world, not even that there is an external world.

In defense of (5) above: The Dream Argument

1. My belief that I am in a café in front of a computer is based on my current visual, auditory and tactile sensations which all make it appear that I am in a café in front of a computer.
2. However, I would have the exact same sensations if I were having a very vivid dream of being in a café in front of a computer.
3. Since all of my sensations would be the same in the dream, I cannot rule out the possibility that right now I am actually dreaming in bed, not in a café.
4. Thus, it is possible to doubt that I am in a café right now.
5. Thus, I cannot be certain that I am in a café right now, and cannot know it.
6. The possibility that I might be dreaming perceptual experiences is sufficient to cast doubt on all beliefs about the external world, because for every such belief I cannot rule out the possibility that it is just a dream.
7. Thus, every belief about the external would is dubitable and can be conceived to be false.

Meditation II: Skepticism Overcome
1. Whenever I am doubting, I am thinking.
2. If I am thinking, I must exist, since there must be something that thinks the thoughts I am thinking.
3. Thus, if I am doubting, I exist.
4. I cannot doubt that I am doubting – “ I am doubting” is indubitable.
5. Thus, “I exist” is indubitable. I know that I exist.

Building on the Foundation Of Certainty
What is ‘this I”?
1. The ‘I’ that must exist is the subject of the thinking that cannot be doubted, the thing that thinks, doubts, wills, etc.
2. I CAN doubt that I have a body, since I only know the “I” as the thing that thinks.
3. Thus, I am essentially a thing that thinks (res cogitans), not an extended physical thing (res extensa).
4. The I is a thing that thinks!

---------End Argument 2------------------------------------------


Descartes' Arguments that the mind is separate from the body. This is now called the Mind/Body Problem.

Descartes’ Divisibility Argument For Mind-Body Dualism
1. My body is divisible.
2. My mind is not divisible.
3. My mind is not my body. (1,2)


Descartes' Conceptual Distinction Argument
1. God is omnipotent.
2. If I can clearly and distinctly understand X as Y, then God can make X as Y (in some possible world). (1)
3. I clearly understand the essence of my mind as thing that thinks, not a thing extended in space.
5. I clearly understand the essence of the essence of my body is a thing that is exended in space, not a thing that thinks. i.
6. Therefore, I can clearly and distinctly imagine my mind and body as separate entities, the understanding of one never implying an understanding of the other.
7. Thus, God can make my mind and my body as separate entities in some possible world. (2,6)
8. If my mind and body can be separate in some possible world, they cannot be strictly the same thing in this world.
9. Thus, my mind and my body are different things. (7, 8)

Crucial Assumption
Both of Descartes’ arguments rely on Leibniz’s law: "... items are numerically identical just in case any property had by either one of them is also had by the other." (IP3 293b)In other words: A is B if and only if whatever is true of A is true of B and vice versa.
Example: A cloud is a visible mass of water particles suspended in the air if and only if whatever is true of a cloud is true of a visible mass of water particles suspended in the air, and whatever is true of a visible mass of water particles suspended in the air is true of a cloud. Descartes’ arguments above are supposed to show that we cannot reasonably assert that mind is body because such a claim violates Leibniz’s Law, given the radical differences between them.

A Devasting Objection To Cartesian Dualism?
The Problem Of Interaction Facing Descartes' Dualism: If res cogitans (mind) and res extensa (body) are so very different in nature, as Descartes argues, how could they causally interact with one another? If mind is to casually interaction body, mind must have some physical properties that effect the body, but mind, by definition, lacks all physical properties. Thus, there is no explanation for how mind and body could interact on Descartes’ theory, but it is obvious that to do interact. Descartes is unable to explain this obvious data.