Descartes viewed that the whole of human knowledge was a tree, with each part relying on the others for the purposes of functioning - and, in a philosophical sense, validity. The tree's trunk was comparable to physics. The branches Descartes considered to be the applied sciences of morals, medicine, and mechanic. The roots of the tree provided support and nourishment to the whole of the system; these roots, Descartes believed, were metaphysics, which he defined as the study of the nature of God, the universe, and everything contained in it. Descartes intended the Principles to serve as a coherent picture of that tree. He hoped that the Principles would serve as a foundational guide to his thought - and all philosophical thought, in general.
Descartes was reacting to a philosophical worldview that was dominated by Aristotle and the teachings of the Scholastics. The Scholastics were concerned with natural philosophy and the study of change. Their main concepts were the dubious metaphysical notions of essence, matter, and form. Essence can be thought of as the trait that defines something. Matter is that thing that remains constant, even during periods of change. Form refers to the thing that changes when change takes place. In addition to these aspects of change, there were the four elements - earth, fire, water, and air. The Scholastics held that the most basic units of existence were substances composed of various mixtures of these four basic elements (Franklin).
Descartes held that the Scholastics' muddled notion of the universe, with all their excessive concepts, effectively obscured our knowledge of the universe. The new science was attempting to explain the universe in terms of the motion of matter; it thus demanded a philosophical justification for this explanation. Descartes thus set out to unite science and philosophy with a new worldview. In order to do this, he had to first simplify the Scholastic view of the universe. Descartes thus argued that only two types of substances existed in the world: mental substance and extension. The first was the essence of thinking, while the second was physical. As the entire observable could thus be reduced to a single type of substance - the corporal - then all natural phenomena could be explained by relying on a limited number of principles, based on the property of extension.
This mechanistic view of the universe gave rise to a new Cartesian epistemology. Whereas the Scholastics and Aristotelians had believed that all human knowledge is transmitted through the senses Descartes would combat this naive empiricism with a radical form of doubt in which there was nothing in the universe but thinking and extension, and the properties that arose from extension (i.e. motion, shape, size, etc.) Descartes effectively freed the human intellect from the senses altogether, arguing that certain concepts exist in the human mind from birth. Human beings are born with innate knowledge, such as an awareness of God and concepts of shape and form. By using these innate concepts and our faculty of reason, we can effectively unravel chains of logical connections in order to make sense of things.
Thus, Rene Descartes arrived at his famous saying, "I think, therefore I am," and spawned a debate in philosophy between rationalists and empiricists that would last up until the present day (Baird and Kaufmann 1997). Influenced by the advances in science of his era, as well as the philosophy of the past, Descartes was able to forge a novel way of perceiving the universe that continues to resonate. His philosophy would influence such philosophers as Gottfried Leibniz, John Locke, Immanuel Kant, George Berkeley, and Baruch Spinoza. Indeed, the problems that he raised through his rigorous forms of questioning and doubt have no easy answers. It is likely that Cartesian questions will continue to occupy philosophers, scientists, and theologians alike for centuries to come.
Baird, Forrest E. And Kaufmann, Walter. Modern Philosophy. Philosophic Classics, Vol. 3, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 1997.
Descartes, Rene. Principles of Philosophy. New York: Springer, 1984.
Descartes, Rene. The World. 1664. Retrieved 5 May 2008 at http://www.princeton.edu/~hos/mike/texts/descartes/world/worldfr.htm.
Franklin, James. "The Genius of the Scholastics, and the Orbit of Aristotle." N.D. Retrieved 5 May 2008 at http://web.maths.unsw.edu.au/~jim/scholastics.html.