Bunch-Hellemans, Impetus and inertia, from John Philoponus to Newton

In Antiquity Greek scholars wondered what keeps the stars, which they believed are attached to a giant crystal sphere, in motion. Aristotle did not believe that motion without an agent of motion is possible and suggested that some agent keeps rotating the sphere that contains the fixed stars. Many of Aristotle’s ideas about motion were generally accepted until the 17th century.

However, in the sixth century CE, [Greek / Byzantine] philosopher Johannes Philoponus had already rejected Aristotle’s idea that a body would move only as long it is pushed. Philoponus believed that motion is a quality, like the Aristotelian qualities of warmth or moisture. Once a body is set in motion, it will keep moving in the absence of friction because a moving body contains a certain quantity of the quality of motion. The sphere of the stars can move because nothing opposes it. In the 13th century, Thomas Aquinas believed that it is God that keeps our sphere in rotation; the fact that the stars move was for him proof of the existence of God. This view was made official in the condemnation of 1277, a papal decree intended to suppress speculation that might contradict the teachings of the church. Nevertheless, the ideas of Philoponus were adopted by some in the 14th century.

William of Ockham and Jean Buridan called the quality of motion impetus. These scholars avoided the problem of the condemnation of 1277 by invoking God as the originator of impetus and by saying that God could change motions if He pleased. In the 17th century, Galileo added experimentation to medieval speculation and showed that something like the impetus idea must be true. Galileo came very close to the modern idea of inertia.

Finally, Newton replaced impetus with inertia. The main difference between impetus and inertia is that inertia applies both to bodies at rest and bodies in motion. Before Newton, none of the philosophers had thought that the same concept could describe stationary and moving bodies.