Throughout the Middle Ages, motion and change were seen as the fundamental and immediate expressions of the innate natures of physical things. The genius of Aristotle's definition of motion is that it appeals only to prior and more general metaphysical principles, actuality and potentiality. The ancient atomists Democritus and Leucippus had posited the existence of an empty space or void to allow their atoms to move rather than be fixed motionless against each other. In the tour de force of medieval natural philosophy, Thomas Bradwardine devised a simple rule to govern the relationship between moving and resisting powers and speeds that was both a brilliant application of mathematics to motion and also a tolerable interpretation of Aristotle's text. Two controversial problems in the medieval science of motion concerned the acceleration of falling bodies and the continued motion of projectiles.