Descartes' Philosophy: Methodical Doubt and the Three Substances

René Descartes' Core Philosophy

Descartes' philosophical system addresses several fundamental areas:

• Methodical Doubt
• The Cartesian Method
• The Interim Morality
• Metaphysics: The Three Substances (Thinking, Infinite, Extended)
• The Relationship of Substances
• Freedom and the Passions

Methodical Doubt

Through methodical doubt, Descartes seeks to find the simplest elements of knowledge—truths that can resist all skepticism. Once these foundational truths are established, he believed one could safely deduce the rest of human knowledge using the appropriate method.

To find these truths, Descartes questioned all accepted knowledge, employing three main arguments:

1. The deception of the senses.
2. The inability to distinguish waking from sleep (the Dream Argument).
3. The hypothesis of an evil genius (the Malicious Demon).

Even if everything perceived is false, there is one thing that cannot be doubted: the act of thinking itself. This leads to the foundational truth: "I think, therefore I am" (Cogito ergo sum).

The Cartesian Method

Reason, according to Descartes, operates through two primary mental operations: intuition and deduction.

The Method is governed by four rules:

1. Evidence: Accepting only what is presented clearly and distinctly to the mind.
2. Analysis: Dividing complex problems into simpler parts.
3. Synthesis: Reconstructing knowledge by moving from the simplest elements to the complex.
4. Enumeration: Reviewing the steps thoroughly to ensure nothing was omitted.

Metaphysics: The Three Substances

Thinking Substance (Res Cogitans)

Descartes builds his metaphysics starting from the one truth that cannot be doubted: the existence of the thinking substance (the self). He accepts as true only that which is presented to the mind clearly and distinctly.

He categorized ideas into three types:

• Adventitious: Ideas derived from sensory experience (e.g., hearing a noise).
• Fictitious: Ideas created by the imagination (e.g., a unicorn).
• Innate: Ideas born within the mind (e.g., God, perfection, infinity).

Infinite Substance (Res Infinita)

The idea of infinity cannot be derived from finite human experience. Since the concept of the finite presupposes the idea of the infinite, this idea must have been implanted in man by a perfect being—God.

Descartes uses two main proofs for the existence of God:

• The Argument for the Objectivity of Ideas (Causal Argument).
• The Ontological Argument.

Extended Substance (Res Extensa)

God guarantees that the primary qualities of the external world are objective, allowing for clear and distinct knowledge. Descartes essentially geometrizes bodies, reducing the physical world to matter and movement.

In contrast:

• Primary Qualities (e.g., extension, shape, motion) are objective.
• Secondary Qualities (e.g., color, taste, sound) are subjective.

Laws of Motion

Descartes established foundational principles of physics, including:

• The Principle of Inertia (First Law of Motion).
• The Law of Conservation of Motion (or quantity of motion).

The Relationship of Substances

Descartes defines Substance as that which exists in such a way that it does not need anything else to exist. In an absolute sense, only the Infinite Substance (God) truly corresponds to this definition.

The three substances are Thinking, Infinite, and Extended.

The connection between the Thinking Substance (soul/mind) and the Extended Substance (body) is famously located in the pineal gland.

Freedom and the Passions

The concepts of freedom and the passions are central to understanding the interaction between the mind and the body in Descartes' system.