also sometimes used for things like musical instruments. Throughout his works, Aristotle clearly distinguishes things that are stable or persistent, with 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, shape or look 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 Actuality is often used to translate both (ενέργεια) and (ἐντελέχεια) (sometimes rendered in English as entelechy). Actuality comes from Latin and is a traditional translation, but its normal meaning in Latin is 'anything which is currently happening'. The two words and were coined by Aristotle, and he stated that their meanings were intended to converge. In practice, most commentators and translators consider the two words to be interchangeable. They both refer to something being in its own type of action or at work, as all things are when they are real in the fullest sense, and not just potentially real. For example, "to be a rock is to strain to be at the center of the universe, and thus to be in motion unless constrained otherwise". is a word based upon (), meaning 'work'. It is the source of the modern word energy but the term has evolved so much over the