But a doctor, it might be objected, is not a fair parallel to knowledge or a knower. A doctor, though an instance of a relative term, is only an instance of one kind of relative term, that in which the elements related are capable of existing apart from the relation, the relation being one in which they can come to stand and cease to stand. But there is another kind of relative term, in which the elements related presuppose the relation, and any thought of these elements involves the thought of the relation. A universal, e. g. whiteness, is always the universal of certain individuals, viz. individual whites; an individual, e. g. this white, is always an individual of a universal, viz. whiteness. A genus is the genus of a species, and vice versa. A surface is the surface of a volume, and a volume implies a surface. A point is the end of a line, and a line is bounded by points. In such cases the very being of the elements related involves the relation, and, apart from the relation, disappears. The difference between the two kinds of relative terms can be seen from the fact that only in the case of the former kind can two elements be found of which we can say significantly that their relation is of the kind in question. We can say of two men that they are related as doctor and patient, or as father and son, for we can apprehend two beings as men without being aware of them as so related. But of no two elements is it possible to say that their relation is that of universal and individual, or of genus and species, or of surface and volume; for to apprehend elements which are so related we must apprehend them so related.[7] To apprehend a surface is to apprehend a surface of a volume. To apprehend a volume is to apprehend a volume bounded by a surface. To apprehend a universal is to apprehend it as the universal of an individual, and vice versa.[8] In the case of relations of this kind, the being of either element which stands in the relation is relative to that of the other; neither can be real without the other, as we see if we try to think of one without the other. And it is at least possible that knowledge and reality or, speaking more strictly, a knower and reality, are related in this way.