section_path: History of the concept/Quantity of motion/René Descartes
section_name: René Descartes
paragraph_text: René Descartes
  
  In Principles of Philosophy (Principia Philosophiae) from 1644, the French philosopher René Descartes defined "quantity of motion" (Latin: quantitas motus) as the product of size and speed, and claimed that the total quantity of motion in the universe is conserved.
  
  If x is twice the size of y, and is moving half as fast, then there's the same amount of motion in each.
  
  created matter, along with its motion... merely by letting things run their course, he preserves the same amount of motion... as he put there in the beginning.
  
  This should not be read as a statement of the modern law of conservation of momentum, since Descartes had no concept of mass as distinct from weight and size. (The concept of mass, as distinct from weight, was introduced by Newton in 1686.) More important, he believed that it is speed rather than velocity that is conserved. So for Descartes, if a moving object were to bounce off a surface, changing its direction but not its speed, there would be no change in its quantity of motion. Galileo, in his Two New Sciences (published in 1638), used the Italian word impeto to similarly describe Descartes's quantity of motion.