René Descartes on God's Existence and Substance Dualism

Descartes' Metaphysics: God and Substance

The question of my own existence leads to the proof of God's existence. An infinite being must be eternal.

Proofs for the Existence of God

There are two primary proofs for the existence of God:

1. The Idea of Infinity

   The idea of infinity itself must be eternal and is therefore an innate idea. Every idea has a cause, which must be more perfect than the idea itself. The cause of the idea of infinity has three possible sources:

   • The Cogito (my own self): This is rejected because I am a fallible being.
   • Nature: This is rejected because our senses can be illogical or deceptive.
   • An external cause: This cause must be infinite, necessary, and is therefore God.

2. The Ontological Argument

   The second proof is the ontological argument, which posits that God's essence necessarily involves existence. It begins with the idea that God is a being greater than which nothing can be conceived. Therefore, to deny God's existence would be a contradiction. This argument is a reproduction of St. Anselm's ontological argument.

The Three Innate Ideas

Innate ideas, based on philosophical idealism, are self-evident truths that are clearly present in our understanding. There are three primary innate ideas:

1. Res Cogitans (Thinking Substance): This is the self, or the Cogito. Its primary attribute is thought.
2. God (Res Infinita or Infinite Substance): The idea of God is innate and guarantees the standard of evidence. God's essence provides the reason that allows us to know truth. As the cause of our existence, God's existence negates the possibility of an all-powerful deceiving being. When the human mind conceives a clear and distinct idea, it corresponds to something real. The primary attribute of Res Infinita is infinity.
3. Res Extensa (Extended Substance): This is the idea of extension as an attribute of physical substance. Nature is a single substance where every body has extension, mobility, and dimension. Extension refers to quantifiable magnitudes, which are the primary qualities of bodies (e.g., size, shape). In contrast, secondary qualities are subjective perceptions like color, smell, and sound. The primary attribute of Res Extensa is extension.

The Communication of Substances

A substance is defined as that which is not dependent on another to exist. Strictly speaking, only God is a true substance (Res Infinita). God creates two other types of substances: thinking substance (Res Cogitans) and extended substance (Res Extensa).

There is no direct causal relationship between the Res Cogitans (soul) and the Res Extensa (body). A human being is formed by the union of these two substances. Although they are independent, there is an apparent connection between them. Descartes explained this communication through the pineal gland (hypophysis), which he proposed as the intermediary between the body and soul—the physical and the spiritual. This communication occurs in both directions.