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. Throughout all distinctions between the potential and the actual there runs the conception of _the actual as something more perfect than the potential_. There is in the actual something positive and real over and above what is in the potential. This is an ultimate fact in our analysis; and its importance will be realized when we come to apply the notions we have been explaining to the study of change.