In his Principles of Philosophy (1644), Descartes claims that the total quantity of motion remains constant in all natural change, where the quantity of motion is measured by the product of the volume of a moving body and the speed of its motion (and where the latter is a scalar quantity that does not include directionality). Even though the quantities of motion in particular parts of matter can change because of collision, according to his principle of the conservation of motion the sum total of all of these quantities remains constant. Descartes’ argument for this principle appeals to the fact that God, in acting as “the universal and primary cause of motion,” and in virtue of his immutability, conserves the material world “in the same way and by the same plan [ratione] that he first created it” (PP II.36, AT VIIIA 62, CSM I 240). The immutability of the divine conservation of the material world is supposed to ensure the constancy of the total quantity of motion in all changes brought about by collision (see cause and concurrence versus conservation, divine).

There is the influential claim in Pierre Duhem that the modern emphasis on the conservation of motion has its roots in the late Scholastic theory of impetus. According to this theory, the continuing motion of a body derives from an “impetus” that the mover imparts to the body. Duhem (1955, 34–53) takes the conclusion in Descartes and other seventeenth-century mechanists that motion is conserved in all natural change to be anticipated in the claim in the Scholastic master John Buridan that this impetus in bodies is “something permanent in nature.” However, Anneliese Maier (1982, 89–91) has noted in response that Buridan proposed the permanence of the impetus only in the case of celestial motion and allowed that, in the case of terrestrial motion, impetus is opposed by contrary tendencies in bodies. Indeed, Francisco Suárez (1967, 1:801), a sixteenth-century Scholastic proponent of impetus theory, emphasized that when an impetus is impressed on a body, “there is no requirement that it be conserved there permanently,” inasmuch as, “because in other respects the subject of the quality is always resisting it and its action, the nature of such a quality requires that it should stop being conserved little by little.” It seems, then, that Descartes’ conservation principle is a marked departure even from the sort of impetus theory that Duhem highlights.