When, furthermore, we proceed to examine the constitutive principles essential to any being in the concrete, we may be able to distinguish between principles which are determinable, passive and persistent throughout all essential change of that being, and others which are determining, specifying, differentiating principles. In water, for example, we may distinguish the passive underlying principle which persists throughout the decomposition of water into oxygen and hydrogen, from the active specifying principle which gives that substratum its specific nature as water. The former or material principle (ὕλη, _materia_) is _potential_, compared with the latter or formal principle (μορφή, εἶδος, ἐντελέχεια, _forma_, _species_, _actus_) as _actual_. The concept of _actus_ is thus applied to the essence itself: the _actus_ “_essentialis_” or “_formalis_” of a thing is that which we conceive to be the ultimate, completing and determining principle of the essence or nature of that thing. In reference to this as well as the other constitutive principles of the thing, the actual existence of the thing is a “second actuality,” an _actus secundus_.
In fact all the constitutive principles of the essence of any existing thing, and all the properties and attributes involved in the essence or necessarily connected with the essence, must all alike be conceived as logically antecedent to the existential _actus_ whereby they are constituted something in the actual order, and not mere possible objects of our thought. And from this point of view the existence of a thing is called the ultimate actualization of its essence. Hence the scholastic aphorism: “_Esse est ultimus actus rei_”.
The term _actus_ may designate that complement of reality by which potential being is made actual (_actus_ “_actuans_”), or this actual being itself (_actus_ “_simpliciter dictus_”). In the latter sense we have already distinguished the Being that is immutable, the Being of God, as the _Actus Purus_, from the being of all mutable things, which latter being is necessarily a mixture of potential and actual, an _actus mixtus_.
Now if the essences of corporeal things are composite, if they are constituted by the union of some determining, formative principle with a determinable, passive principle—of “form” with “matter,” in scholastic terminology—we may call these formative principles _actus_ “_informantes_”; and if these cannot actually exist except in union with a material principle they may be called actus “_non-subsistentes_”: _e.g._, the formative principle or “_forma substantialis_” of water, or the vital principle of a plant. If, on the other hand, there exist essences which, being simple, do not actualize any material, determinable principle, but subsist independently of any such, they are called _actus_ “_non-informantes_,” or _actus_ “_subsistentes_”. Such, for example, are God, and pure spirits whose existence is known from revelation. Finally, there may be a kind of actual essence which, though it naturally actualizes a material principle _de facto_, can nevertheless continue to subsist without this latter: such an actual being would be at once an _actus informans_ and an _actus subsistens_; and such, in fact, is the human soul.