Sensory experiences are nor reliable for making any statements, since people often mistake one thing for another. (Descartes talks about mirages). Knowledge based on reasoning is not always trustworthy, because people often make mistakes. (adding numbers is a classical example). Finally, knowledge is deemed by Descartes to be illusory, since it may come from dreams or insanity or from demons able to deceive men by making them believe that they are experiencing the real world, when are they are in fact not doing so. (the metaphysical approach in Descartes work is can be easily recognized here).
Following this analysis of existent forms of knowledge, Descartes concludes that certainty can be found in his intuition that, even if deceived, if he thinks he must exist: "Cogito ergo sum." The thought ("cogito") is a self-evident truth that gives certain knowledge of a particular thing's existence, i.e. one's self, but only the existence of the person who thinks it is accepted as certain.
Finally, considering that all our thoughts which we have when we are awake can also come to us when we are sleeping without a single one of them being true, I resolved to pretend that everything I had ever thought was no more true that the illusions in my dreams. But I immediately realized that, though I wanted to think that everything was false, it was necessary that the "me" who was doing the thinking was something; and noticing that this truth -- I think, therefore I am -- was so certain and sure that all the wildest suppositions of skeptics could not shake it, I judged that I could unhesitatingly accept it as the first principle of the philosophy for which I was seeking."
If all one would know for certain was that one exists and if one acquiesced to Descartess method of doubting all things uncertain, the consequence would be that one would be reduced to solipsism, which is the view that except for one's individual self and thoughts, nothing else exists. To avoid this trap, Descartes claims that all ideas that are as clear and as distinct as the cogito concept must be true. If they were not, than the cogito, which is also a member of the class of clear and distinct ideas would also be plagued by doubts. Since the statement "I think therefore I am" cannot be doubted, all clear and distinct ideas must also be true.
It is somehow ironic that the "Cogito ergo sum" is based on another famous statement formulated by Descartes: "Dubito ergo cogito." (I doubt, therefore I think"). Since cogito is a self-evident truth, a clear and distinct idea on which is concept of existence is founded, a similar reasoning may be applied to the "Dubito ergo cogito" statement. The whole idea of thought is based on constantly doubting the realities of the world. Descartes actually begins his work by analyzing the problems of various types of knowledge (see above).
Starting from the assumption that he is working with clear and distinct innate ideas, Descartes claims that each mind is a spiritual substance, while each body is a material substance. Unlike bodies, which can be extended and broken into parts, the mind or soul are immortal, since they are indivisible. Descartes also provided some proof for the existence of God. (the constant threat from the Church made such a section necessary. The French philosopher's ideas were already at the limit of heresy, so further threats to the authority of the Church had to be avoided).
Descartes states that God is perfect, since his is an innate idea. He argues that God must exist, because, if He is not, He wouldn't be perfect.. This ontological proof of God's existence is one of the pillars of Descartes' rationalism, since it sets a pattern of reasoning about an existing thing solely on the conclusions derived from innate ideas, with no contribution whatsoever from sensory experience. Descartes also argues that God cannot deceive people, since he is perfect, so the world is not a fiction invented by the human mind, and therefore must exist. Descartes has therefore provided the metaphysical foundations for the existence of his own mind, of God and of the World.
One objection to Descartes' reasoning procedure was presented by Arnauld in his Cartesian Circle, which exposes the circularity in Descartes' ideas. In order to state the existence of God, one must have confidence in the clear and distinct idea of God. But in order to be certain that clear and distinct ideas are true, one must know that God exists, that He is perfect and that He never deceives man. Although, based on his rationalist "Cogito ergo sum" statement, Descartes rejected magic, he was not able to observe that the ontological proofs he presented were mere word-magic, founded on the superstition that things can be determined by ideas and thoughts. Empiricists, on the other hand, support the idea that the description of things must come after and not before one knows by using experience that they exist.
Descartes' influence on philosophy
Descartes influence on the development of philosophy was immense. The maxims of Leonardo (constituting the Renaissance world view) may be encountered in Descartes' work. The Discourse on Method is an evidence of the Empiricism in physiological researches. However, the influence of mathematics is predominant in Descartes' work. The Discourse on Method, which provides a synoptic view of Cartesian philosophy, presents it to be not a metaphysics based upon physics (such as in the case of Aristotle), but rather a physics based upon metaphysics (which was characteristic to the 17th century).
Descartes' mathematical bias was manifested in his determination to base natural sciences not in sensation and probability, as Bacon has, but rather on a principle of absolute certainty. Therefore, he bases his metaphysics on three concepts:
1) Employment of the procedure of complete and systematic doubt in order to eliminate every belief that does not pass the test of indubitability (skepticism, "Dubito ergo cogito);
2) Accepting no idea, except the clear, distinct and free of contradiction ones, to be true (mathematicism)
3) Founding all knowledge on the absolute certainty of self-consciousness, so that "Cogito ergo sum" (I think, therefore I am) remains the only idea undisturbed by doubt. (subjectivism).
These principles have allowed Descartes to establish the existence of God, its perfect nature, and the reality of the corporal world, which has been implanted into man by this perfect being. The achievement of certainty was guaranteed by the perfection of God and by the clear and distinct ideas that he possesses.
The Cartesian metaphysics is considered to be the fountainhead of Rationalism in modern philosophy, for it claims that the mathematical ideas of clarity, distinctness and absence of contradiction are the ultimate and only test of meaningfulness and truth. This places Descartes in a clearly unempirical position. Bacon had argued that "reasoners resemble spiders who make cobwebs, out of their own substance," and that statement might have very well been applied on Descartes, since the Cartesian self is just a substance from which the idea of God originates and with which all deductive reasoning is initiated. Descartes regards understanding as superior to the senses, and considers reason the ultimate decision making factor of what constitutes truth.
Descartes' influence on Enlightenment
Descartes' work influenced in a very powerful way the advancement of the Enlightenment. The role of science and mathematics was especially important. The poet John Donne wrote, "The new philosophy puts all in doubt." Poetry and drama from the 17th century abounded in manifestations of confusion and dismay about the world, God and man.
Therefore, it is not surprising that the method presented in Descartes' Discourse on Method is one of doubt: all was considered to be uncertain until it was established by reasoning from self evident propositions, based on principle analogous to those applied in algebra in geometry. This philosophy could be applied in all sciences and a mechanistic model was available for all living things.
The intellectual life of continental Europe was dominated by Cartesian ideas until the end of the 17th century. This type of philosophy appealed to learned gentlemen and highborn ladies, and it was taught in the Universities as an alternative to the obsolete Scholasticism. For this particular reason, it continued to be a severe threat to the authority of the Roman Catholic Church, who placed Descartes' works on the 1663 Index of Forbidden Books. The University of Oxford forbade the teaching of his doctrines. The only places were Cartesianism was studied was the Netherlands, where the liberal Universities of Groeningen and Utrecht permitted and encouraged it.
1. Brians P., Gallwey M., Hughes D., Hussain, a., Law R., Myers M., Neville M., Schlesinger R., Spitzer a, Swan S. "Reading About the World," Volume 2, published by Harcourt Brace Custom Books. - excerpts from Descartes' works w www.wsu.edu:8080/~wldciv/world_civ_reader/world_civ_reader_2/descartes.html
2. "Descartes," "History of Philosophy," "European History and Culture" articles, Encyclopaedia Britannica - 1997 Edition