The word entelecheia was invented by Aristotle, but never defined by him. It is at the heart not only of his definition of motion, but of all his thought. Its meaning is the most knowable in itself of all possible objects of the intellect. There is no starting point from which we can descend to put together the cements of its meaning. We can come to an understanding of entelecheia only by an ascent from what is intrinsically less knowable than it, indeed knowable only through it, but more known because more familiar to us. We have a number of resources by which to begin such an ascent, drawing upon the linguistic elements out of which Aristotle constructed the word, and upon the fact that he uses the wordenergeia as a synonym, or all but a synonym, for entelecheia.

The root of energeia is ergonó deed, work, or actó from which comes the adjective energon used in ordinary speech to mean active, busy, or at work. Energeia is formed by the addition of a noun ending to the adjective energon; we might construct the word is-at-work-ness from Anglo-Saxon roots to translate energeia into English, or use the more euphonious periphrastic expression, being-at-work. If we are careful to remember how we got there, we could alternatively use Latin roots to make the word "actuality" to translate energeia. The problem with this alternative is that the word "actuality" already belongs to the English language, and has a life of its own which seems to be at variance with the simple sense of being active. By the actuality of a thing, we mean not its being-in-action but its being what it is. For example, there is a fish with an effective means of camouflage: it looks like a rock but it is actually a fish. When an actuality is attributed to that fish, completely at rest at the bottom of the ocean, we don’t seem to be talking about any activity. But according to Aristotle, to be something always means to be at work in a certain way. In the case of the fish at rest, its actuality is the activity of metabolism, the work by which it is constantly transforming material from its environment into parts of itself and losing material from itself into its environment, the activity by which the fish maintains itself as a fish and as just the fish it is, and which ceases only when the fish ceases to be. Any static state which has any determinate character can only exist as the outcome of a continuous expenditure of effort, maintaining the state as it is. Thus even the rock, at rest next to the fish, is in activity: to be a rock is to strain to be at the center of the universe, and thus to be in motion unless constrained otherwise, as the rock in our example is constrained by the large quantity of earth already gathered around the center of the universe. A rock at rest at the center is at work maintaining its place, against the counter-tendency of all the earth to displace it. The center of the universe is determined only by the common innate activity of rocks and other kinds of earth. Nothing is which is not somehow in action, maintaining itself either as the whole it is, or as a part of some whole. A rock is inorganic only when regarded in isolation from the universe as a whole which is an organized whole just as blood considered by itself could not be called alive yet is only blood insofar as it contributes to the maintenance of some organized body. No existing rock can fail to contribute to the hierarchical organization of the universe; we can therefore call any existing rock an actual rock.

Energeia, then, always means the being-at-work of some definite, specific something; the rock cannot undergo metabolism, and once the fish does no more than fall to earth and remain there it is no longer a fish. The material and organization of a thing determine a specific capacity or potentiality for activity with respect to which the corresponding activity has the character of an end (telos). Aristotle says “the act is an end and the being-at-work is the act and since energeia is named from the ergon it also extends to the being-at-an-end (entelecheia)” (Metaphysics 1050a 21-23). The word entelecheia has a structure parallel to that of energeia. From the root word telos, meaning end, comes the adjective enteles, used in ordinary speech to mean complete, perfect, or full-grown. But while energeia, being-at-work, is made from the adjective meaning at work and a noun ending, entelecheia is made from the adjective meaning complete and the verb exein. Thus if we translate entelecheia as “completeness” or “perfection,” the contribution the meaning of exein makes to the term is not evident. Aristotle probably uses exein for two reasons which lead to the same conclusion: First, one of the common meanings of exein is “to be” in the sense of to remain, to stay, or to keep in some condition specified by a preceding adverb as in the idioms kalos exei, “things are going well,” or kakos exei, “things are going badly.” It means “to be” in the sense of to continue to be. This is only one of several possible meanings of exein, but there is a second fact which makes it likely that it is the meaning which would strike the ear of a Greek-speaking person of Aristotle’s time. There was then in ordinary use the word endelecheia, differing from Aristotle’s word entelecheia only by a delta in place of the tau. Endelecheia means continuity or persistence. As one would expect, there was a good deal of confusion in ancient times between the invented and undefined term entelecheia and the familiar word endelecheia. The use of the pun for the serious philosophic purpose of saying at once two things for whose union the language has no word was a frequent literary device of Aristotle’s teacher Plato. In this striking instance, Aristotle seems to have imitated the playful style of his teacher in constructing the most important term in his technical vocabulary. The addition of exein to enteles, through the joint action of the meaning of the suffix and the sound of the whole, superimposes upon the sense of “completeness” that of continuity. Entelecheia means continuing in a state of completeness, or being at an end which is of such a nature that it is only possible to be there by means of the continual expenditure of the effort required to stay there. Just as energeia extends to entelecheia because it is the activity which makes a thing what it is, entelecheia extends to energeia because it is the end or perfection which has being only in, through, and during activity. For the remainder of this entry, the word “actuality” translates both energeia and entelecheia, and “actuality” means just that area of overlap between being-at-work and being-at-an-end which expresses what it means to be something determinate. The words energeia and entelecheia have very different meanings, but function as synonyms because the world is such that things have identities, belong to species, act for ends, and form material into enduring organized wholes. The word actuality as thus used is very close in meaning to the word life, with the exception that it is broader in meaning, carrying no necessary implication of mortality.