Aristotle's criticism of Democritus and Leucippus in De caelo is widely neglected. Yet, this confrontation plays a central role in Aristotle's strategy. Against the Atomists, Aristotle argues that a consistent theory of elements has to meet two epistemological requirements: 1) taking into account the interdependency between motion, nature of things, theory of natural places and natural motion; 2) endorsing Aristotle's own conception of principles (matter, form and privation), which entails that the substance of things cannot be reduced to material and quantitative components.