possibility of novelty, of invention, of radical deviation, but only that principle in virtue of which the acorn becomes the oak. Technically, it is the capacity for movement between opposites. Only the cold can become hot; only the dry can become wet; only the babe can become a man; the seed the full-grown wheat and so on. Potentiality instead of implying the emergence of anything novel means merely the facility with which a particular thing repeats the recurrent processes of its kind, and thus becomes a specific case of the eternal forms in and through which all things are constituted. In spite of the almost infinite numerical diversity of individuals, there are only a limited number of species, kinds or sorts. And the world is essentially a world which falls into sorts; it is pre-arranged into distinct classes. Moreover, just as we naturally arrange plants and animals into series, ranks and grades, from the lower to the highest, so with all things in the universe. The distinct classes to which things belong by their very nature form a hierarchical order. There are castes in nature. The universe is constituted on an aristocratic, one can truly say a feudal, plan. Species, classes do not mix or overlap--except in cases of accident, and to the result of chaos. Otherwise, everything belongs in advance to a certain class, and the class has its own fixed place in the hierarchy of Being. The universe is indeed a tidy spot whose purity is interfered with only by those irregular changes in individuals which are due to the presence of an obdurate matter that refuses to yield itself wholly to rule and form. Otherwise it is a universe with a fixed place for everything and where everything knows its place, its station and class, and keeps it. Hence what are known technically as final and formal causes are supreme, and efficient causes are relegated to an inferior place. The so-called final cause is just a name for the fact that there is some fixed form characteristic of a class or sort of things which governs the changes going on, so that they tend toward it as their end and goal, the fulfilment of their true nature. The supralunar region is the end or final cause of the proper movements of air and fire; the earth of the motions of crass, heavy things; the oak of the acorn; the mature form in general of the germinal.