Descartes’ Philosophy: Certainty, Ideas, and God

Posted on Mar 16, 2025 in Philosophy and ethics

Descartes’ Pursuit of Certain Knowledge

René Descartes sought to achieve absolutely certain knowledge. To accomplish this, he employed methodical doubt, a process of exaggerating doubt and considering any statement false if it held even the slightest suspicion. This was done to isolate truly certain judgments.

Information from the senses, Descartes argued, must be considered potentially false, as it is subject to uncertainty and error, such as optical illusions.

Even rational truths are not beyond doubt. Descartes posited that even when reasoning correctly, one might be mistaken due to a defective mind created by an “evil genius.”

The First Indisputable Truth: “I Think, Therefore I Exist”

Descartes arrived at his first indisputable truth: “I think, therefore I exist” (Cogito, ergo sum). He realized he could not doubt his own thinking. Because he was thinking, he concluded that the “I” – the thinking subject – must exist. From this foundational evidence, and because the “I” is a subject with ideas, he reconstructed other possible thoughts.

Types of Ideas According to Descartes

Descartes categorized ideas into three types:

  • Adventitious Ideas: Derived from the material world and sense experience.
  • Fictitious Ideas: Created by our own consciousness and imagination, often combining adventitious ideas.
  • Innate Ideas: Implanted in the mind by a superior authority (God), such as the very idea of God.

He further classified ideas based on clarity and distinctness:

  • Clear Ideas: Ideas that are fully understood.
  • Dark Ideas: Ideas that are not understood.
  • Distinct Ideas: Ideas that are clearly differentiated from others.
  • Confused Ideas: Ideas that are not clearly differentiated.

For Descartes, valid ideas are both clear and distinct.

Descartes’ Proofs for the Existence of God

Descartes aimed to prove God’s existence rationally, offering three distinct arguments:

  1. The First Proof (Argument from Infinity): God is infinite. This notion, Descartes argued, cannot originate from a finite being like man, as an effect cannot be greater than its cause. A person cannot conceive of something beyond their limits; therefore, the idea of God must be external and existing.
  2. The Second Proof (Argument from Creation): Based on the Christian concept of man as God’s creation. If man exists – the only undeniable truth – then he must have a source, and that source is God.
  3. The Third Proof (Ontological Argument): God, by definition, possesses all perfections. Existence is a perfection; therefore, God must exist. If God lacked existence, one could imagine a being with all of God’s attributes *plus* existence, which would be more perfect. This contradicts the definition of God as the most perfect being.

Descartes’ Method of Thought

Descartes proposed a four-rule method for clear thinking:

  1. Evidence: Accept as true only what is clear and distinct.
  2. Analysis: Divide each problem into smaller, manageable parts.
  3. Synthesis: Start with the simplest elements and reconstruct knowledge of the more complex.
  4. Enumeration: Make comprehensive reviews to ensure nothing is overlooked.

The Three Levels of Reality

Descartes identified three levels of reality, corresponding to three substances:

  1. Thinking ‘I’ (Res Cogitans): A created substance, whose essential attribute is thought.
  2. God (Res Infinita): An uncreated substance, whose attribute is infinity.
  3. World (Res Extensa): A created substance, whose attribute is extension (subdivisible into three dimensions).

These three substances are interconnected. The first truth, “I think, therefore I exist,” forms the foundation of his philosophy. He uses methodical doubt to prove the existence of the other two substances.

Once the existence of the “I” is established, Descartes examines the ideas within the mind, including the idea of God, which he argues must originate externally. Because God is good by definition, Descartes rejects the hypothesis of the evil genius. If the ideas of reason are correct, they can be applied to the external world, providing data for new ideas.