peas of matter floating in a thin soup of ether. The Cartesians saw this vision as freeing them from the “fruitless speculations” of the medieval scientists, who held that all things by their nature seek to come to rest, those with gravity seeking their rest downward and those with levity seeking it upward. With this vision before them the Cartesians set out on a research program designed to discover the detailed mechanistic laws governing the motion of matter. Then in 1687 Newton published his Principia,