Orbits in Space: Aristotle believed that the speed of the Sun, Moon, and planets in their orbits around the
Earth was proportional to the ratio of force to resistance. If resistance were zero, then speed would be
infinite, which is obviously impossible. Therefore, he thought that there must be some kind of rarified
substance that filled the cosmos. In contrast, Philoponus suggested that the relevant relationship was the
difference between force and resistance. From that point of view, motion through a vacuum was perfectly
reasonable. Speed would simply be proportional to the force that started it in motion. Infinite speeds were
not involved at all, so it was possible to think about how something might actually move in a vacuum,
which later became important for considering how the Sun, Moon, and planets might move.
indefinitely unless it encountered resistance.