Descartes’ theory marks a turning point in early modern philosophy. This theory introduces the famous “cogito”, a principle that provides a firm basis both for a new conception of knowledge and for a new worldview. This worldview entails the cleansing of matter of any qualitative property, matter being essentially pure geometrical extension. But this conception of mater implies a sharp distinction between two substances, a dualism of mind and body: each one of them must be conceived separately, in its own realm that excludes the presence of the other. But those two substances (thinking substance and extended substance) in Descartes’ eyes have also to interact in certain cases. Such a case concerns not only the nature of the human being considered as a whole consisting of a mind and a body, but also the case of matter in motion.
According to Descartes matter is by its nature inert and must be put in motion by an external cause. This primordial cause of motion is God: God does not only bestow motion on matter but also recreates each body in such a way that it continues its motion according to a set of laws: the three laws of nature introduced by Descartes in his Le Monde and reiterated in the Principia Philosophiae. Two of these laws compose the first complete formulation of the principle of inertia. While analyzing this principle, Descartes uses the example of a rotating body: this example entails the existence of a rectilinear inertial tendency. But Descartes’ analysis of this example shows that he conceives this tendency in terms of two other tendencies of the body, one of them being a tendency to move on a circular trajectory. The existence of such a tendency supports the idea that Cartesian matter by its nature obeys to rules that contradict the laws of nature, the laws that are based on God’s immutability.
The study of this case indicates that a closer look on Cartesian dualism and the problem of the interaction of two distinct substances is necessary. This nodal problem of Cartesian metaphysics can and must be approached through the history of science, especially the history of the problem of inertial motion.
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