In 1644, the French philosopher, scientist, and mathematician René Descartes formulated several “laws of nature,” which helped lay the groundwork for classical physics and the Scientific Revolution. Descartes’ laws, published in his Principles of Philosophy, included progenitors of both the concept of inertia (Isaac Newton’s famous first law of motion) and the conservation of momentum — another key principle of classical mechanics.
Descartes’ conservation law refers to “motion” rather than “momentum” and holds for what physicists today would call “speed,” rather than “velocity” (speed plus direction). Later scientists, including Newton, would overcome these deficiencies and articulate the modern concept of momentum. But Descartes’ formulation nevertheless marked an important step in the development of modern physics.
How did Descartes hit upon this conservation law? Not simply by observing nature. “In fact,” he points out, “it often happens that experience may appear to conflict with the rules I have just explained.”