"I think, therefore I exist" -- Rene Descartes

Descartes was a geometrician. He found only in mathematics and geometry the certainty that he required. Therefore, he used the methods of geometry to think about the world. Now, in geometry, one begins with a search for axioms, simple undeniable truths – for example, the axiom that a straight line is the shortest distance between two points. On the foundations of such “self-evident” propositions, whole geometrical systems can be built.

Following his geometrical model, Descartes proceeds to doubt everything – de onmibus dubitandum. He will suspend belief in the knowledge he learned from childhood, all those things “which I allowed myself in youth to be persuaded without having inquired into their truth.” Doubt will be his method, a deliberate strategy for proceeding toward certainty. (Descartes is a doubter not by nature, but by necessity. What he really wants is secure understanding so he can stop doubting.)

Descartes finds that he has no trouble doubting the existence of real objects/events – our senses too easily deceive us. And we can doubt the existence of a supernatural realm of reality – figments and fantasies are too often conjured by our native imaginations. But now his geometrical model pays off: in trying to doubt everything, he discovers something that he can’t doubt. What he can’t doubt is that he is doubting. Obviously, I exist if I doubt that I exist. My doubt that I exist proves that I exist, for I have to exist to be able to doubt. Therefore I can’t doubt that I exist. Hence, there is at least one fact in the universe that is beyond doubt. “I am, I exist is necessarily true each time that I pronounce it, or that I mentally conceive it.

Descartes thus becomes the author of the most famous phrase in Western philosophy: Cognito ergo sum, or, in his original French, Je pense, donc je suis. – I think, therefore I exist. With roots in St. Augustine, this is certainly one of the catchiest ideas yet created by the human mind.

I haven't read that book. I think "I" is a useful word, but I just don't think it is the thing that does the thinking. Consciousness is, but by the time we start thinking about what is doing the thinking, we have let consciousness slip through our fingers and are only aware of the objects of consciousness, those things that consciousness is thinking about.

At least for Descartes, the quest for certitude requires initially dismissing the entire order of sense and bodily experience, even my experience of myself as a body. The putting into question of the entire physical order allows the pristine intelligibility of the order of mind to come to the fore. We know ourselves better than other things and we know out intellect better than our body. The intellectual self is known immediately and transparently. Following Aristotle, and in contrast to Descartes, Aquinas urges a methodological retreat in our pursuit of self-knowledge. There is no possibility of gaining immediate, introspective access to the intellect or the soul. The route to self-knowledge is indirect, oblique. To understand the essence of any species, we must begin with the objects naturally pursued by members of the species in question, then move back from these to examine the activities, powers, and, finally, the essence. The indirect route to self-knowledge follows from the fact that the intellect is a potency made actual only by knowing things. But a power is knowable in so far as it is in act. Thus, there is no possibility of knowing the intellect until it has been actualized by knowing something other than itself.
The indirect and mediated path to knowledge of the human soul does not diminish the importance of that knowledge. Indeed, the general investigation of soul culminates with an analysis of what is proper to human souls. Thus we find Aquinas explicating in great detail Aristotle's comparison of sensation and understanding and his argument that intellect so differs from sense that it must be an immaterial power, whose operation transcends every bodily organ. Like sense, the intellect is said to be passive with respect to sensible objects. It is a potency actualized by receiving the forms of things. But there are different sense of passivity and clarification of them is crucial to a comparison of sense and intellect. Aquinas writes:

To be passive may be taken in three ways. First, in its most strict sense, when from a thing is taken something which belongs to it by virtue either of its nature, or of its proper inclination, as when ...a man becomes ill. Secondly, less strictly, a thing is said to be passive when something either suitable or unsuitable is taken away from it. And in this way not only he who is ill is said to be passive, but also he who is healed.... Thirdly, in a wide sense a thing is said to be passive, from the very fact that what is in potency to something receives that to which it was in potency without being deprived of anything. And accordingly whatever passes from potency to act may be said to be passive, even when it is perfected. And thus with us to understand is to be passive (ST, I, 79, 2).

'sum' means 'I am' (Latin)
'suis' means 'am' (French)
Both have root at 'being'.
The concept of existence, especially the Philosophic notion (seperate and distinct from the concept of 'being') wasn't really bandied about until the 19th century. In fact, it grew out of the very questions Descartes raised.
The implication is that Descartes settled the questions he created by saying "I think therefore I exist" when, in fact, he said "I think, therefore I am."
No, I won't go into all of the nitty, gritty details here. Suffice it to say that an essentialist could never even phrase things in terms an existentialist would consider a worthwhile format.
By raising the question of 'being', the next question became "What is the nature of 'being'?" Many people made the jump from being to existence, some did not. Called 'existentialists' the problem before them had more to do with the nature of this existence and how we come to know things (empiricism) and less to do with the empirical certainty Descartes asserted.
Don't muddle things by changing the definition of the word after the fact. That's very clinton-esque.

The Cartesian credo of the current rationalist/materialist religion is "I think, therefore I am."

But how can one demonstrate the fact that one thinks?

They cannot. It is impossible to "prove" that one exists, because it is impossible to "prove" that one thinks. We directly experience our own existence -- but it is impossible to demonstrate that existence scientifically. We all take our own existence on faith.

Therefore, even rationalism is based on unprovable faith.

B-chan

Leaving aside the merits of Cartesian philosophy, Descartes was a heartless bastard who infamously nailed the paws of his wife's dog to a table and vivisected the animal without any concern for its pain. In a letter to a friend Descartes wrote: "They [animals] merely went through the external motions which in man were symptomatic of pain, without experiencing any of it's mental sensation."

Accordingly, Descartes decided to open up these "beast machines" to see just how they worked. Apart from dissecting dead animals and attending public vivisections, Descartes enjoyed practising vivisection. Amongst other things, rabbits were utilised by Descartes. The purpose of this was to observe the operations of the heart and the movement of the blood in the arteries.

According to Rupke, Descartes 'Beast Machine' theory provided a twofold advantage. It protected scientists' mechanistic view of both animals and human beings against charges of heresy, and protected the reserved immortality for man, which was the favoured doctrine promoted by Catholicism.

Descartes theory, therefore, entered medical and certain ecclesiastical circles. Two clergymen, Father Nicholas Malebranche and the theologian Antoine Arnauld, emphasised their commitment to Cartesian ideology by practising severe cruelty to animals. Malebranche, who took dualism one step further than Descartes, claiming that such action as the movement of a limb was a direct result of God's intervention, is said to have demonstrated his commitment to automata theory by kicking a pregnant dog, and declaring when reproached: "Don't you know that it has no feelings at all"? Indeed, Nicolas Fontaine, a secretary of prominent Jansenist Fathers eventually reported the equally violent Arnauld, to the monastery Port Royal.

Descartes legacy, though, had beaten him to it. Fontaine reports: "The solitaries
beat their dogs with the utmost indifference, and laughed at people who still
maintained they could feel pain...They nailed the poor animals to boards by four
paws to dissect them while still alive, in order to watch the circulation of the
blood, which was a great subject of discussion." (Fontaine quoted in Rupke,
27:1990)

Anselm's ontological argument assumes that existence is one of God's perfections. If a supremely perfect being didn't exist, then it wouldn't be supremely perfect, which is a contradiction; ergo, it must exist.

A standard way of criticizing the ontological argument is to claim that the argument rests on a false assumption; that is, it rests on the assumption that existence is a perfection. Kant claimed that existence is not a property at all, the way colors or shapes are properties, but rather an utterly different concept.

Kant's view was accepted by the framers of standard symbolic logic. In standard symbolic logic existence doesn't appear as a property at all; rather, it's a so-called quantifier. So, if one were to try to articulate the ontological argument using standard 20th-century symbolic logic, the articulation would fail because that logic doesn't allow for existence as a property.

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I think, therefore I'm a FReeper !!!