Descartes: World, Substance, and Mechanism

The World and Principles of Material Things

When you have demonstrated the existence of God, as an assurance criterion of truth, we have the opportunity to demonstrate the existence of corporeal things, the world. Since God exists, he cannot deceive me into believing that the world exists if it does not; therefore, we conclude that the world exists. Returning to the theory of the objective reality of ideas, this theory posits that the causes of ideas of sensible things are corporeal things, and these corporeal things do exist. God guarantees the truth, but error is not attributable to God; rather, it arises when we deceive ourselves by misinterpreting.

Theory of Substance

Descartes identified three domains of reality: God (or infinite substance), the ego (or thinking thing), and corporeal things (the world). To address each of these, he uses the term substance: an infinite substance, a thinking substance, and an extended substance. While the definition of substance is strictly applicable only to God (as that which needs nothing else to exist), Descartes believed it could also apply to beings we perceive clearly and distinctly that do not require anything else to exist *apart from God*. These are two: the self or soul (res cogitans) and material bodies (res extensa), which are independent of each other and do not need each other to exist.

To perceive a substance, it is necessary to perceive an attribute. Attributes are qualities of a substance that we can perceive. Each substance has its own essential attribute, which constitutes its essence or nature. Knowing this essential attribute is necessary and sufficient to understand the substance. The essential attribute of the self or soul is thought (or the mind), and in the case of material things, it is extension. Descartes thus distinguishes two principal substances (apart from God): the ego or thinking substance (res cogitans) and bodies or extended substance (res extensa).

Qualities of Extended Substance

Descartes distinguishes between primary and secondary qualities. Primary qualities are those we perceive clearly and distinctly and can be expressed mathematically; these include length or volume, movement, and shape. Secondary qualities (such as heat, cold, or smell) do not exist objectively in things but play a very important utilitarian role in everyday life. These feelings tell us what we need and what harms us, but they do not reveal the true nature of things.

Mechanistic Explanation of the World

Descartes proposed a mechanistic theory of nature that explains natural phenomena solely through the derivatives or combinations of movements of bodies in space. This theory conceives of nature as a machine whose movements are automatic results of other movements transmitted through reciprocal action. In a nutshell, Cartesian physics is based exclusively on physical quantity (extension) and spatial movement. The mechanistic interpretation encompasses the entire universe, including the organic world.