Descartes' Philosophy: Reason, Doubt, and Reality

René Descartes: Foundations of Modern Philosophy

Descartes' Theory of Knowledge

René Descartes, often regarded as the founder of modern philosophy, posited that reason is the sole source of true knowledge. His philosophical journey began with an analysis of how to acquire knowledge reliably. He distinguished between two primary types of knowledge:

• Intuition: The immediate, clear, and distinct apprehension of simple ideas that emerge directly from reason.
• Deduction: The process of arriving at complex truths and judgments through a succession of insights and logical connections made between simple ideas by reason.

To systematically pursue truth, Descartes proposed a rigorous method, which implicitly involves several steps. While the original text mentions "Analysis" and "Briefs," Descartes' method typically includes four rules:

• Analysis: Dividing complex ideas into their simplest, most fundamental components that can be clearly understood.
• Synthesis/Enumeration: (Interpreting "Briefs") Systematically reconstructing complex truths from these simple ideas, ensuring no steps are omitted and the reasoning is complete.

To establish a foundation of certain knowledge, Descartes employed methodical doubt, questioning everything that could possibly be doubted. This process unfolded in three stages:

1. Doubt of Sensory Knowledge: Questioning knowledge derived from the senses, as they can often deceive us (e.g., optical illusions).
2. Doubt of External Reality: Doubting the existence of an extra-mental reality, as it is impossible to definitively distinguish between waking life and dreams.
3. Doubt of Rational Knowledge: Questioning knowledge derived from reason itself, by hypothesizing the existence of an "evil demon" or deceiver who might systematically mislead us even in our most certain mathematical or logical deductions.

However, Descartes found one undeniable truth: "I doubt, therefore I exist" (Cogito, ergo sum). Even if deceived, the act of doubting itself proves the existence of a thinking self. Thus, we exist as a thinking substance.

Descartes further categorized ideas into three types:

• Adventitious Ideas: Those that appear to come from external experience (e.g., the idea of a tree).
• Factitious Ideas: Those constructed by the mind from other ideas (e.g., the idea of a unicorn, formed from horse and horn).
• Innate Ideas: Those that reason possesses within itself from birth, independent of experience (e.g., the idea of God, the idea of perfection, mathematical axioms).

Descartes' Theory of God

Within the mind, Descartes discovered the innate idea of infinity, which he readily identified with the idea of God. He argued that this idea cannot be adventitious (from experience) or factitious (constructed by the mind), because a finite being (like a human) cannot be the cause of an infinite idea. Therefore, this innate idea of infinity must have been caused by an infinite being itself, thereby demonstrating that God exists as an infinite, omniscient, and perfect being.

Furthermore, Descartes used God's existence to validate the reality of the external world. He reasoned that because God is supremely good and perfect, God would not deceive us. Therefore, our clear and distinct ideas about the external world must correspond to an actual extra-mental reality.

A Note on Hume's Causality

In contrast to Descartes' rationalist approach, later philosophers like David Hume offered different perspectives on knowledge. For Hume, the belief that what has happened in the past will be repeated in the future (the principle of causality) is not a necessary truth derived from reason, but merely an assumption or belief based on habit. As such, for Hume, the truth of every law of nature is only probable, not absolutely certain.