species, and which we signify by the definition which states what the thing is”.(85) Thus understood, the essence is abstract, and gives the specific or generic type to which the individual thing belongs; but we may also mean by essence, the concrete essence, the individual person or thing (_persona_, _suppositum_, _res individua_). The relations between the objects of those two concepts of essence will be examined later. Since the specific essence is conceived as the most fundamental reality in the thing, and as the seat and source of all the properties and activities of the thing, it is sometimes defined or described, in accordance with this notion of it, as the primary constitutive of the thing and the source of all the properties of the thing. Conceived as the foundation of all the properties of the thing it is sometimes called _substance_ (οὐσία, _substantia_). Regarded as the source of the thing’s activities, and the principle of its growth or development, it is called the _nature_ of the thing (φύσις, _natura_, from φύω, _nascor_).(86) Since what makes a thing that which it is, by the same fact differentiates this thing from every other thing, the essence is rightly conceived as that which gives the thing its characteristic being, thereby marking it off from all other being. In reality, of course, each individual being is distinct by all that it is from every other. But since we get our intellectual knowledge of things by abstracting, comparing, generalizing, and classifying partial aspects of them, we apprehend part of the imperfectly grasped abstract essence of each individual as common to other classes (generic), and part as peculiar to that class itself (differential); and thus we differentiate classes of things by what is only part of their essence, by what we call the _differentia_ of each class, _distinguishing mentally_ between it and the generic element: which two are really _one_, really identical, in every individual of the species thus defined and classified.