Because he believed that the speed of an object's motion is proportional to the force being applied (or, in the case of natural motion, the object's weight) and inversely proportional to the density of the medium, he reasoned that objects moving in a void would move indefinitely fastand thus any and all objects surrounding the void would immediately fill it. The void, therefore, could never form.
He believed that an object's speed is directly proportional to the force applied (or its weight in natural motion) and inversely related to the medium's density. Thus, he reasoned that objects moving through a void would travel at infinite speed, filling all surrounding objects immediately. Consequently, a vacuum could never form.