Abstract

In De Caelo III 2, Aristotle observes that each element is determined by an intrinsic principle to move to its proper place: earth downward, fire upward, and water and air to their respective places in the middle. However, how are we to determine the cause of elemental motion? Aristotle admits that this ranks among the most difficult problems (μάλιστα δ’ ἀπορεῖται) as it is directly related to the argument of Physics VIII 4, which defends the view that whatever is in motion is moved by something. The question of how the elements move is not resolved until we are presented with his definition of inclination (ῥοπή) in De Caelo IV 1. Inclination is the nature of each element to move to its proper place. Aristotle concludes that the principle of inclination is a cause of motion continuously orienting the elements toward their proper places by activating that which is potentially heavy or potentially light. In what follows, I shall demonstrate the internal coherence of Aristotle’s natural philosophy by deploying inclination as a principle of motion and rest to define the nature of each element. This account of inclination as a principle of motion is consistent with Aristotle’s definition of nature in Physics II 1. As an active principle of motion, inclination is identified with the activity (ἐνέργεια) of the element enacting and maintaining its own end (ἐντελέχεια) by being at rest in its proper place. As a passive principle of motion, inclination is identified with the passivity of the element to be moved to its proper place by another element insofar as the respective element undergoes generation or substantial change.