Before that, Aristotle believed that objects stop moving when the mover stops moving them. Implying that movement is unnatural, and the stopping is due to some natural inclination of every object towards remaining stationary.
This goes against modern physics, Newton said that only an opposing force can make an object stop moving, or change direction. It's not an object's innate property to move or change direction, but every movement is caused by a force.
This is just like taking gravity for granted. Just because we're used to objects falling "down" doesn't mean that it's the object's natural inclination to go "down", it means there must be a force that is pushing every object in it's range downward.
