Conservation principles tell us that some quantity, quality, or aspect remains constant through change. Such principles already appear in ancient and medieval natural philosophy. In one important strand of Greek cosmology, the rotation of the celestial orbs is eternal and immutable. In optics at least from the time of Euclid (fl. c. 300 BCE), when a ray of light is reflected, the angle of reflection is equal to the angle of incidence. According to some versions of the medieval impetus theory of motion, impetus permanently remains in a projected body (and the associated motion persists) unless the body is subject to outside interference. Such examples abound.

In the seventeenth century, conservation principles began to play a central role in scientific theories. Galileo Galilei, René Descartes, Christian Huygens, Gottfried Leibniz, and Isaac Newton founded their approaches to physics on the principle of inertia—the principle that a body...