Abstract

According to Aristotelian physics, there was a fundamental distinction between natural and violent motion. When the cause of the motion was internal to the moving body, that motion was regarded as natural. Violent motion was supposed to have an external efficient cause. It should stop as soon as this external cause ceased its action. The fall of a body was believed to have an internal cause – the very nature of the heavy body – but the motion of a projectile was supposed to be accidental and a violent one, produced by an external agent and maintained by the continuous action of the air around it. Those distinctions disappeared from physics during the 17th century. Descartes and Newton do not address those problems. However, in Galileo’s thought those distinctions were still present, and they did play a relevant role in his physics. Some central arguments presented by Galileo Galilei in the Dialogues concerning the two chief world systems depended on the characterisation of circular motion as “natural”. Considerations concerning natural or violent motion, circular and straight motions, provided a trap that led Galileo astray to ideas that seem extremely odd, from the point of view of classical mechanics. This article shows how Galileo struggled with those Aristotelian concepts when he tried to understand the nature of the motion of falling bodies.