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.
Because he believed that an object's speed is directly related to the force applied (or, in natural motion, its weight) and inversely related to the medium's density, he reasoned that objects moving through a vacuum would continue at infinite speed, causing all surrounding objects to instantly fill it. The vacuum, therefore, could never exist.