Descartes began his philosophical career by trying to set forth the basic principles of the new scientific method that Galileo had introduced and which had proved so successful. At the same time he wished to show that this new scientific methodology was consistent with Christianity and provided no threat to it. Thus, Descartes had two main aims in the Meditations:
1. To provide a sound basis for scientific method. He aimed to show that the real source of scientific knowledge lay in the mind and not in the senses.
2. To show how science and religion could be compatible. He will do this by splitting the world up into two different types of substances: mind and body. Science will be completely true of body, extended matter; religious truths will deal with the soul or mind.
I. The Arguments for Universal Doubt:
In order to show that science rested on firm foundations and that these foundations lay in the mind and not the senses, Descartes began by bringing into doubt all the beliefs that come to us from the senses. His aim in these arguments is not really to prove that nothing exists or that it is impossible for us to know if anything exists (he will prove that we can know external objects later), but to show that all our knowledge of these things through the senses is open to doubt. If our scientific knowledge came to us through the senses, we could not even be sure that anything outside of us existed. The obvious implication is that, since we do know that external objects exist, this knowledge cannot come to us through the senses, but through the mind.
Descartes uses three very similar arguments to open all our knowledge to doubt: The dream argument, the deceiving God argument, and the evil demon argument. The basis idea in each of these is that we never perceive external objects directly, but only through the contents of our own mind, the images the external objects produce in us. Since sense experience never puts us in contact with the objects themselves, but only with mental images, sense perception provides no certainty that there is anything in the external world that corresponds to the images we have in our mind. Descartes introduces dreams, a deceiving God, and an evil demon as ways of motivating this doubt in the veracity of our sense experience.
A. The dream argument:
1. I often have perceptions very much like the ones I usually have in sensation while I am dreaming.
2. There are no definite signs to distinguish dream experience from waking experience.
3. It is possible that I am dreaming right now and that all of my perceptions are false
Descartes realizes that someone may not accept that all of the elements of our dreams may be illusory, so he introduces another mechanism to increase the scope of our doubt.
B. The deceiving God argument:
1. We believe that there is an all powerful God who has created us and who is all powerful.
2. He has it in his power to make us be deceived even about matters of mathematical knowledge which we seem to see clearly.
3. It is possible that we are deceived even in our mathematical knowledge of the basic structure of the world.
For those who would hold (as Descartes himself will later) that God would not deceive us, Descartes introduces an evil demon instead.
C. The evil demon argument:
1. Instead of assuming that God is the source of our deceptions, we will assume that there exists an evil demon, who is capable of deceiving us in the same way we supposed God to be able.
Therefore, I have reason to doubt the totality of what my senses tell me as well as the mathematical knowledge that it seems I have.
Since the source of our knowledge cannot lie in the sense, Descartes must find a way to rebuild the edifice of knowledge upon material he can find within the contents of his own mind. The first thing he can be sure of on the basis of this alone is his own existence.
II. The argument for his existence (The "Cogito" argument)
1. Even if we assume that there is a deceiver, from the very fact that I am deceived it follows that I exist.
2. In general it will follow from any state of thinking (e.g., imagining, sensing, feeling, reasoning) that I exist. While I can be deceived about the objective content of any thought, I cannot be deceived about the fact that I exist and that I seem to perceive objects with certain characteristics. (The famous statement of this from D.'s Discourse on Method is "Cogito ergo sum." or "I think, therefore I am.")
3. Since I only can be certain of the existence of myself insofar as I am thinking, I have knowledge of my existence only as a thinking thing (res cogitans).
This shows that the contents of the mind are more easily known than the body:
The Argument that the Mind is More Certainly known than the Body:
It is possible that all knowledge of external objects, including my body, could be false as the result of the actions of an evil demon. It is not, however, possible that I could be deceived about my existence or my nature as a thinking thing.
Therefore, our mind is much more clearly and distinctly known to us than our body.
Descartes still has no knowledge of anything outside of his mind. He still has to make the crucial leap to the existence of an object outside of his mind. He must do this, however, strictly on the basis of the contents of his own mind. It is the idea of God that he finds in his mind that allows him to make this leap, and which forms the basis for his knowledge of all other external objects.
III. The argument for the existence of God from the fact that I have an idea of Him. (simplified version)
1. I have an idea of God, a perfect being.
2. There must be as much reality or perfection in the cause of any thing as in the effect.
a. This applies not only to the existence of ideas, but also to the reality of what they represent. Not only must the existence of the idea be explained, but also what it represents.
3. The idea of God represents something so perfect that I could not have been the cause of this idea.
Therefore, God must exist as the only possible cause of the perfection found in my idea of Him.
With the knowledge that God exists and that he is not a deceiver, Descartes can move on to explain how we know material objects to exist.
IV. The argument that material objects exist.
1. God is no deceiver.
2. He created me and gave me reason which tells me that my ideas come from external corporeal things.
3. If they do not come from external objects, then God must be a deceiver. But this is an absurdity.
4. Material objects exist.
Having put our scientific knowledge on a firm foundation and having shown that it comes from our mind, not our senses, Descartes needs to show how this new type of knowledge is compatible with religion. He has done this, partially by showing how it leads to knowledge of the existence of God. He still has to reconcile the seeming incompatibility in the objective and subjective views we can take of ourselves. He does this by spitting us up into two distinct substances: mind and body.
V. The argument for the distinction of mind and body.
Each of these arguments depends on Leibniz's law, which says:
Leibniz's law: If two things are the same thing, they must share all the same properties.
Descartes shows two ways in which mind and body seem to have different properties, and how, hence, they must be different things.
A. The argument from knowledge.
1. I can be certain that my mind exists.
2. I cannot be sure that my body exists.
Mind and body are not the same thing.
B. The argument from extension.
1. My mind is unextended.
2. My body is extended.
Mind and body are not the same thing.
Problems with Descartes radical split between the mind and the body:
If mind and body are radically different types of stuff, it is hard to see how they can interact with each other. In particular, it is hard to see how an unextended substance can interact with an extended one.
Yet mind and body do seem to interact in both directions:
1. The mind affects the body: This seems to happen whenever we act. The mind decides to do something and the body does it.
2. The body affects the mind:
a. In sense perception, our sense organs seem to affect and produce images in our mind.
b. Damage to our brain or the influence of drugs on our body often affects our mind.