Res Extensa

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René Descartes used the res extensa and its opposite res cogitans in his philosophical system called the Cartesian system. Res extensa was used to denote the physical world and res cogitans was used to denote the thinking being, the being that perceives its own beingness.

The corollary of this dualism is that the "I", the subject, is of another "substance" than the world and, hence, does not depend on this world to truly exist. Actually, Descartes here presents a subjectivist point of view : it is the subject that determines the world around him, and not the other way around.

subject ("res cogitans") -> world ("res extensa")

For example, a solid piece of wax has certain sensorial qualities. However, when the wax is melted, it loses every single apparent quality it had in its solid form. Still, Descartes recognizes in the melted substance the idea of wax. Thus, the thoughts of the subject determine the object. In other words, "res extensa" exists in virtue of "res cogitans". As a modern example of the impact of Cartesian philosophy, much controversy has arisen over the status of Pluto as a planet. Scientists changed the definition of the idea of "planet", and, subsequently, the appellation/idea of Pluto has changed.

This is a radical evolution from an Aristotelean philosophy, where the polis, i.e. the world, was anterior to the individual. With his new conception of the relation between the world and the subject, Descartes lays the basis of modern individualism ; man does not need the world to prove that he exists.

When speaking of the Theory of Knowledge, one may assert that Descartes represents the evolution from "Epistemological realism" (object -> subject) to "epistemological idealism" (subject -> object).