A Source Book in Physics

Author: René Descartes

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Quantity of Motion

§ 36. Now that we have examined the nature of motion, we come to consider its cause, and since the question may be taken in two ways, we shah commence by the first and more universal way, which produces generally all the motions which are in the world; we shall consider afterwards the other cause, which makes each portion of matter acquire motion which it did not have before. As for the first cause, it seems to me evident that it is nothing other than God, Who by His Almighty power created matter with motion and rest in its parts, and Who thereafter conserves in the universe by His ordinary operations as much of motion and of rest as He put in it in the first creation. For while it is true that motion is only the behavior of matter which is moved, there is, for all that, a quantity of it which never increases nor diminishes, although there is sometimes more and sometimes less of it in some of its parts; it is for this reason that when a part of matter moves twice as rapidly as another part, and this other part is twice as great as the first part, we have a right to think that there is as much motion in the smaller body as in the larger, and that every time and by as much as the motion of one part diminishes that of some other part increases in proportion. We also know that it is one of God’s perfections not only to be immutable in His nature but also to act in a way which never changes: to such a degree that besides the changes that we see in the world, and those that we believe in because God has revealed them, and that we know have happened in nature without any change on the part of the Creator, we ought not to attribute to Him in His works any other changes for fear of attributing to Him inconstancy; from which it follows that, since He set in motion in many different ways the parts of matter when He created them and since He maintained them with the same behavior and with the same laws as He laid upon them in their creation, He conserves continually in this matter an equal quantity of motion.


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Chicago: René Descartes, "Descartes: Quantity of Motion," A Source Book in Physics in A Source Book in Physics, ed. William Frances Magie (Cambridge: Harvard University Press, 1935), 50–51. Original Sources, accessed November 29, 2022, http://originalsources.com/Document.aspx?DocID=JCVHG8BCHD6G7R7.

MLA: Descartes, René. "Descartes: Quantity of Motion." A Source Book in Physics, in A Source Book in Physics, edited by William Frances Magie, Cambridge, Harvard University Press, 1935, pp. 50–51. Original Sources. 29 Nov. 2022. http://originalsources.com/Document.aspx?DocID=JCVHG8BCHD6G7R7.

Harvard: Descartes, R, 'Descartes: Quantity of Motion' in A Source Book in Physics. cited in 1935, A Source Book in Physics, ed. , Harvard University Press, Cambridge, pp.50–51. Original Sources, retrieved 29 November 2022, from http://originalsources.com/Document.aspx?DocID=JCVHG8BCHD6G7R7.