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.” Descartes believed that the methodology of physics resembles mathematics more than the “scientific method” we learn in school — beginning not with observations and experience but with certain “clear and distinct” principles, from which conclusions may be derived with logical rigor. 

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.” Descartes believed that the methodology of physics resembles mathematics more than the “scientific method” we learn in school — beginning not with observations and experience but with certain “clear and distinct” principles, from which conclusions may be derived with logical rigor. It is not that observation and experience were irrelevant to Descartes; rather, his idea was that truths about the natural world should hold with the same degree of certainty as those of mathematics.

One such principle to which Descartes appeals in his “proof” of the conservation of motion is “the immutability of the workings of God.” From this, he argues, it follows that “the motion which [God] preserves is not something permanently fixed in given pieces of matter, but something which is mutually transferred when collisions occur.” It is the finite motion that God imparts to the material world which gets conserved in physical interactions. Inversely, then, “the very fact that creation is in a continual state of change is…evidence of the immutability of God.”

This reasoning should shock the modern reader. It is, first of all, explicitly un-empirical; even more scandalously, it makes explicit appeal to God! How could Descartes — a key figure in the development of modern science — invoke a theological principle at a crucial point in his derivation of a physical law?

Descartes is often remembered today for criticizing the Aristotelian physics associated with Medieval scholasticism and formulating an alternative — modern — account of nature. But he clearly did not adhere to our own “modern” assumptions about science and religion. For Descartes, science is hardly incompatible with revealed religion, as today’s “metaphysical naturalists argue — he thought the existence of God could be demonstrated rationally. Nor, apparently, did he think that scientific reasoning is radically distinct from theological reasoning — as “methodical naturalists” maintain. In this, Descartes, the so-called father of modern philosophy, appears rather un-modern.

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