Aristotle’s account of motion can be found in the Physics. By motion, Aristotle (384-322 BCE) understands any kind of change. He defines motion as the actuality of a potentiality. Initially, Aristotle’s definition seems to involve a contradiction. However, commentators on the works of Aristotle, such as St. Thomas Aquinas, maintain that this is the only way to define motion. In order to adequately understand Aristotle’s definition of motion it is necessary to understand what he means by actuality and potentiality.
Aristotle uses the words energeia and entelechiainterchangeably to describe a kind of action. A linguistic analysis shows that, by actuality, Aristotle means both energeia, which means being-at-work, and entelechia, which means being-at-an-end. These two words, although they have different meanings, function as synonyms in Aristotle’s scheme. For Aristotle, to be a thing in the world is to be at work, to belong to a particular species, to act for an end and to form material into enduring organized wholes.
Actuality, for Aristotle, is therefore close in meaning to what it is to be alive, except it does not carry the implication of mortality. From the Middle Ages to modern times, commentators disagreed on the interpretation of Aristotle’s account of motion. An accurate rendering of Aristotle’s definition must include apparently inconsistent propositions: (a) that motion is rest, and (b) that a potentiality, which must be, if anything, a privation of actuality, is at the same time that actuality of which it is the lack. St.
Thomas Aquinas was prepared to take these propositions seriously. St. Thomas observes that to say that something is in motion is just to say that it is both what it is already and something else that it is not yet. Accordingly, motion is the mode in which the future belongs to the present, it is the present absence of just those particular absent things which are about to be. St. Thomas thus resolves the apparent contradiction between potentiality and actuality in Aristotle’s definition of motion by arguing that in every motion actuality and potentiality are mixed or blended. St.Thomas’ interpretation of Aristotle’s definition of motion, however, is not free of difficulties.
His interpretation seems to trivialize the meaning of entelechia. One implication of this interpretation is that whatever happens to be the case right now is an entelechia, as though something which is intrinsically unstable as the instantaneous position of an arrow in flight deserved to be described by the word which Aristotle everywhere else reserves for complex organized states which persist, which hold out in being against internal and external causes tending to destroy them.
In the Metaphysics, however, Aristotle draws a distinction between two kinds of potentiality. On the one hand, there are latent or inactive potentialities. On the other hand, there are active or at-work potentialities. Accordingly, every motion is a complex whole, an enduring unity which organizes distinct parts. Things have being to the extent that they are or are part of determinate wholes, so that to be means to be something, and change has being because it always is or is part of some determinate potentiality, at work and manifest in the world as change.