For more than 1500 years, Aristotelism dominates answers to the question of what causes motion. Every motion requires a force to produce it.
Scientists of the early modern times are more and more in conflict with this doctrine. Newton, with his law of gravitation and his principle of inertia, finally seals the fate of Aristotelism. From now on Newton's mechanics is believed to be the firm basis of science.
According to Newton's axioms, all material bodies are inertial. They maintain their state of rest or uniform, straight motion unless acted upon by an external force.