Book: Principles of Philosophy

Overview
Published in Latin in 1644, René Descartes' Principles of Philosophy lays out a unified system that begins with metaphysics and extends to physics and cosmology. Dedicated to Princess Elisabeth of Bohemia, it aims to replace scholastic Aristotelianism with a method grounded in clear and distinct ideas, mathematical reasoning, and mechanical explanations of nature. The book is organized into four parts: the nature and grounds of human knowledge, the principles of material things, the structure of the visible universe, and the physics of the earth.

Metaphysical Foundations
Part I establishes the epistemic starting point through methodic doubt. While doubting sensible appearances and inherited doctrines, the mind cannot doubt its own act of thinking; hence the certainty of the self as a thinking thing. From this secure base, Descartes argues for the existence of God as a perfect, non-deceptive being, whose veracity guarantees the truth of what is perceived clearly and distinctly. He introduces the real distinction between mind and body: the essence of mind is thought; the essence of body is extension. This dualism underwrites his program to found physics on metaphysical clarity, freeing it from final causes and occult qualities. He also defends simple, innate notions, such as substance, duration, number, as the mind’s primitive resources, clarified by analysis and evident intuition.

Matter, Motion, and Laws of Nature
Part II defines matter as nothing but extended substance, rejecting substantial forms and qualities. Because the essence of body is extension, matter is indefinitely divisible, and there is no vacuum; space and body are coextensive, yielding a plenum where every location is full. Motion is the change of relative position among contiguous bodies, and all motion is relative to frames of surrounding bodies. From divine immutability Descartes derives the laws of nature: each part of matter tends to persevere in its state; motion naturally proceeds in straight lines; and interactions conserve a fixed quantity of motion in the universe. He offers rules of collision grounded in these principles, treating physical processes as mechanical rearrangements of parts driven by contact, pressure, and impact. Light, heat, magnetism, and similar phenomena are to be explained without hidden forms, by the size, shape, and motion of particles and the pressure of subtle matter.

The Cosmos and Vortices
Part III applies these principles to a new cosmology. God initially imparts motion to an undifferentiated plenum, which spontaneously organizes into vortices of subtle matter. The sun and stars occupy the centers of vast whirlpools that carry planets and comets in their courses. Light is transmitted instantaneously as a tendency or pressure through the plenum, and celestial motions are explained by the circulatory mechanics of surrounding media rather than by action at a distance. Comets traverse multiple vortices, revealing the fluid continuity of the heavens and the relativity of local frames of motion. The system preserves conservation of motion and uniform laws across the universe while dispensing with Aristotelian spheres.

The Earth and Terrestrial Physics
Part IV turns to the earth’s structure and phenomena: gravity as the effect of surrounding subtle matter pressing bodies toward the center, the formation of mountains and seas by mechanical causes, and explanations of magnetism, fire, and meteorological events by particle motion and pressure. The same mechanical method governs both celestial and terrestrial domains, unifying natural philosophy under a single, mathematically expressible physics.

Method and Legacy
Principles of Philosophy offers a self-contained schema that begins with the certainty of thought and culminates in a contact-mechanics of nature. Its identification of matter with extension, denial of the vacuum, and vortex cosmology shaped seventeenth-century debates. Even where later physics revised its specific laws, the work’s insistence on clear foundations, mechanistic explanation, and the continuity of laws from earth to heavens became enduring hallmarks of modern science.