Potentiality and actuality

In philosophy, potentiality and actuality are a pair of closely connected principles which Aristotle used to analyze motion, causality, ethics, and physiology in his Physics, Metaphysics, Nicomachean Ethics, and On the Soul.

The concept of potentiality, in this context, generally refers to any "possibility" that a thing can be said to have. Aristotle did not consider all possibilities the same, and emphasized the importance of those that become real of their own accord when conditions are right and nothing stops them. Actuality, in contrast to potentiality, is the motion, change or activity that represents an exercise or fulfillment of a possibility, when a possibility becomes real in the fullest sense. Both these concepts therefore reflect Aristotle's belief that events in nature are not all natural in a true sense. As he saw it, many things happen accidentally, and therefore not according to the natural purposes of things.

These concepts, in modified forms, remained very important into the Middle Ages, influencing the development of medieval theology in several ways. In modern times the dichotomy has gradually lost importance, as understandings of nature and deity have changed. However the terminology has also been adapted to new uses, as is most obvious in words like energy and dynamic. These were words first used in modern physics by the German scientist and philosopher, Gottfried Wilhelm Leibniz. More controversially, Aristotle's concept of entelechy retains influence on occasional calls for the use of "entelechy" in biology.

"Potentiality" and "potency" are translations of the Ancient Greek word dunamis (δύναμις). They refer especially to the way the word is used by Aristotle, as a concept contrasting with "actuality". The Latin translation of dunamis is potentia, which is the root of the English word "potential"; it is also sometimes used in English-language philosophical texts. In early modern philosophy, English authors like Hobbes and Locke used the English word power as their translation of Latin potentia.

Dunamis is an ordinary Greek word for possibility or capability. Depending on the context, it could be translated as 'potency', 'potential', 'capacity', 'ability', 'power', 'capability', 'strength', 'possibility', or 'force' and is the root of modern English words dynamic, dynamite, and dynamo.

In his philosophy, Aristotle distinguished two meanings of the word dunamis. According to his understanding of nature there was both a weak sense of potential, meaning simply that something "might chance to happen or not to happen", and a stronger sense, to indicate how something could be done well. For example, "sometimes we say that those who can merely take a walk, or speak, without doing it as well as they intended, cannot speak or walk." This stronger sense is mainly said of the potentials of living things, although it is also sometimes used for things like musical instruments.

Throughout his works, Aristotle clearly distinguishes things that are stable or persistent, which hold their own strong natural tendency to a specific type of change, from things that appear to occur by chance. He treats these as having a different and more real existence. "Natures which persist" are said by him to be one of the causes of all things, while natures that do not persist, "might often be slandered as not being at all by one who fixes his thinking sternly upon it as upon a criminal." The potencies which persist in a particular material are one way of describing "the nature itself" of that material, an innate source of motion and rest within that material. In terms of Aristotle's theory of four causes, a material's non-accidental potential is the material cause of the things that can come to be from that material, and one part of how we can understand the substance (ousia, sometimes translated as "thinghood") of any separate thing. (As emphasized by Aristotle, this requires his distinction between accidental causes and natural causes.) According to Aristotle, when we refer to the nature of a thing, we are referring to the form or shape of a thing, which was already present as a potential, an innate tendency to change, in that material before it achieved that form. When things are most "fully at work" we can see more fully what kind of thing they really are.

Actuality is often used to translate both energeia (ἐνέργεια) and entelecheia (ἐντελέχεια) (sometimes rendered in English as entelechy). Actuality comes from Latin actualitas and is a traditional translation, but its normal meaning in Latin is 'anything which is currently happening.'