Aristotle’s account of motion can be found in the physics. by motion, aristotle (384-322 b.c.e.) 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 entelechia interchangeably 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.