Problems with Descartes' Philosophy
It is from the views of Descartes that most of the metaphysical systems of the last three centuries begin, trying to improve upon them, or to overcome what they regard as difficulties in the Cartesian system.
René Descartes is responsible for the predominance of the problem of human knowledge in modern philosophy. Many of the systems of philosophy and theories of knowledge which have arisen in the last three centuries can trace their lineage directly to the influence of the questions Descartes raised and the method he employed in answering them. He promulgated the principle of "science without presuppositions" and thereby introduced a new epoch in science and philosophy. It will, therefore, not be amiss to analyze his fundamental ideas and evaluate his method.
As his starting point Descartes begins with the contention that we rely entirely too much on traditional doctrines and spontaneous convictions, so that our supposed knowledge of truth rest mostly on unproved presuppositions. This makes it difficult for us to distinguish between truth and error, since we do not know what is true knowledge and what is unwarranted belief. Hence, he would tear down the whole edifice of knowledge and rebuilt it from the foundation, and he would not begin to build until he had reached the one and ultimate truth which the bedrock of human knowledge.
Being a mathematician, Descartes felt convinced that he could deduce all truth from a single fundamental principle. As the instrument of his search for truth he used a universal methodic doubt. His own words will best reveal his line of thought.
I. In order to seek truth, it is necessary once in the course of our life, to doubt, as far as possible, of all things.
As we were at one time children, and as we formed various judgments regarding the objects presented to our senses, when as yet we had not the entire use of our reason, numerous prejudices stand in the way of our arriving at the knowledge of truth; and of these it seems impossible for us to rid ourselves, unless we undertake, once in our lifetime, to doubt of all those things in which we may discover even the smallest suspicion of uncertainty.
II. We ought also to consider as false all that is doubtful.
Moreover, it will be useful likewise to esteem as false the things of which we shall be able to doubt, that we may with greater clearness discover what possesses most certainty and is easiest to know.
III. We ought not meanwhile to make use of doubt in the conduct of life...
IV. Why we may doubt of sensible things.
Accordingly, since we now only design to apply ourselves to the investigation of truth, we will doubt, first, whether of all the things that have ever fallen under our senses, or which we have ever imagined, any one really exists; in the first place, because we know by experience that the senses sometimes err, and it would be imprudent to trust too much to what has even once...