René Descartes' Philosophy: Method, Doubt, and Substance Theory

René Descartes' Quest for Certain Knowledge

René Descartes sought certain knowledge, knowledge placed beyond reasonable doubt, guided by the rigorous procedure of mathematics.

The Cartesian Method: Four Rules

Just as in mathematics, Descartes required a guaranteed foundation from which to deduce the rest of the philosophical system. This foundation is reached by following four methodological rules:

1. Rule of Evidence (Clarity and Distinctness): Accept nothing as true unless it is presented to the mind so clearly and distinctly that there is no room for doubt.
2. Rule of Analysis: A complex problem must be divided into its simplest, most manageable parts.
3. Rule of Synthesis: Once broken down and analyzed, the parts must be reassembled systematically, moving from the simplest truths to the more complex ones.
4. Rule of Enumeration (The List): Review the entire process thoroughly to ensure nothing was omitted or overlooked.

Methodological Doubt and the Cogito

The objective of methodological doubt is to achieve sure and certain knowledge beyond any possibility of error. This is not a skeptical doubt, but rather a systematic method used to approach the truth.

Descartes argues that the senses deceive us, as our dreams are often not easily distinguishable from real waking life. Mathematics seems to escape this uncertainty, but Descartes still posits the possibility of an "evil genius" (*malin génie*) who might deceive us about even the simplest mathematical truths.

Through this process of radical doubt, Descartes arrives at the first and only undeniable truth: "I think, therefore I am" (Cogito ergo sum). This is grasped as a clear and distinct intuition, not the conclusion of a syllogism.

Descartes' Theory of Substance

Descartes differentiates between the Infinite Substance (God) and Finite Substances (mind and matter). Every substance possesses an essential attribute and various modes (modifications of the attribute).

• Res Cogitans (Thinking Thing): This is the mind or soul. Its essential attribute is Thought. Modes include willing, imagining, and sensing.
• Res Extensa (Extended Thing): This is the physical world we perceive. Its essential attribute is Extension (occupying space). Modes include figure, shape, and motion.

The res extensa is completely heterogeneous and separate from thought. Since we can only be certain of our own existence (a form of subjective solipsism), Descartes must prove that the outside world exists. The connection between the soul and the body is hypothesized to occur via the pineal gland.

The Proof of God's Necessary Existence

Within the res cogitans, Descartes identifies three types of ideas:

1. Innate Ideas: Ideas born within us (e.g., God, substance, thought).
2. Adventitious Ideas: Ideas coming from outside (e.g., sensory perceptions).
3. Factitious Ideas: Fictitious ideas created by the subject (e.g., a unicorn).

The idea of God, being infinite and absolutely perfect, cannot have been created by man (a finite being). Therefore, the idea of God must be innate and demonstrated a priori. Since God is defined as absolutely perfect, He cannot deceive us, thus guaranteeing the truth of clear and distinct ideas and countering the threat posed by the "evil genius."

Descartes' Laws of Motion

God is considered the first cause of motion in things, maintaining the total quantity of motion in the universe. Descartes proposed three fundamental laws governing physical bodies (*res extensa*):

Law of Inertia

Every body remains in its state (rest or uniform motion) unless acted upon by an external force that changes that state.

Law of Rectilinear Motion

Any body in motion tends to continue moving in a straight line.

Law of Collision (Shock Law)

When a body collides with another, it cannot push the second body unless it loses some of its own movement, nor can it be deprived of movement unless the second body gains movement in the same proportion.

Physical bodies maintain objectivity (primary qualities like extension and motion) and subjectivity (secondary qualities like color and taste).