Principles of PhilosophyRené Descartes

Descartes intended the Principles of Philosophy as a textbook to replace Aristotle. It lays out, in geometrical order, the full system that the Meditations and Discourse on the Method had only sketched: from the foundations of knowledge through the nature of material substance to the structure of the physical universe.

Part I recapitulates the epistemological program. Doubt clears the ground; the cogito provides the first certainty; God's existence guarantees that clear and distinct ideas correspond to reality. Parts II through IV build a complete physics on this foundation. Matter is nothing but extension. There is no void; space is filled with matter in motion. All natural change reduces to the rearrangement of particles governed by laws of motion that God sustains. Descartes derives from these principles a vortex cosmology in which planets are carried around the sun by swirling matter, an account of terrestrial phenomena from magnetism to tides, and a mechanical physiology of the human body.

Much of the specific physics was wrong, and Newton would dismantle the vortex theory within a generation. But the ambition of the work shaped modern science: the conviction that all natural phenomena admit of explanation through matter and motion alone, without appeal to substantial forms or occult qualities. Descartes gave the mechanical philosophy its most systematic early statement and forced every subsequent natural philosopher to reckon with its scope.