 Now if we throw a body, then it is, in this framework, relatively unproblematic to account for the motion when it is in contact with our hand: what is difficult is to account for its continued motion thereafter.  Aristotle’s theory accounts for it by saying that, when it is moving, a temporary vacuum is caused behind it, and, in order to fill in this vacuum, air rushes around from the front, thus leaving a void in front of the projectile which is filled by the continued motion of the projectile. This explanation was vulnerable to a large number of objections – for example, it is clearly easier to throw a moderately heavy object, such as a stone, than a light object, such as a bean, whereas light objects ought to be more susceptible than others to motions of the air. And Aristotle’s theory, when confronted with the example of two stones thrown in opposite directions so as to pass near to one another, cannot consistently say how the air is supposed to move in the neighbourhood of their close encounter. These objections were made by numerous medieval authors, most significantly by John Buridan (De Caelo et Mundo III, qu. 22, pp. 227ff.) and Nicole Oresme (Du ciel et du monde II, ch. 25ff., pp. 525ff.).