Within this tight and pent in universe, changes take place of course. But they are only of a small number of fixed kinds; and they operate only within fixed limits. Each kind of stuff has its own appropriate motion. It is the nature of earthly things to be heavy, since they are gross, and hence to move downward. Fire and superior things are light and hence move upward to their proper place; air rises only to the plane of the planets, where it then takes its back and forth motion which naturally belongs to it, as is evident in the winds and in respiration. Ether being the highest of all physical things has a purely circular movement. The daily return of the fixed stars is the closest possible approximation to eternity, and to the self-involved revolution of mind upon its own ideal axis of reason. Upon the earth in virtue of its earthly nature--or rather its lack of virtue--is a scene of mere change. Mere flux, aimless and meaningless, starts at no definite point and arrives at nothing, amounts to nothing. Mere changes of quantity, all purely mechanical changes, are of this kind. They are like the shiftings of the sands by the sea. They may be sensed, but they cannot be "noted" or understood; they lack fixed limits which govern them. They are contemptible. They are casual, the sport of accident. Only changes which lead to some defined or fixed outcome of form are of any account and can have any account--any _logos_ or reason--made of them. The growth of plants and animals illustrates the highest kind of change which is possible in the sublunary or mundane sphere. They go from one definite fixed form to another. Oaks generate only oaks, oysters only oysters, man only man. The material factor of mechanical production enters in, but enters in as accident to prevent the full consummation of the type of the species, and to bring about the meaningless variations which diversify various oaks or oysters from one another; or in extreme cases to produce freaks, sports, monsters, three-handed or four-toed men. Aside from accidental and undesirable variations, each individual has a fixed career to pursue, a fixed path in which to travel. Terms which sound modern, words like potentiality and development abound in Aristotelian thought, and have misled some into reading into his thought modern meanings. But the significance of these words in classic and medieval thought is rigidly determined by their context. Development holds merely of the course of changes which takes place within a particular member of the species. It is only a name for the predetermined movement from the acorn to the oak tree. It takes place not in things generally but only in some one of the numerically insignificant members of the oak species. Development, evolution, never means, as in modern science, origin of new forms, a mutation from an old species, but only the monotonous traversing of a previously plotted cycle of change. So potentiality never means, as in modern life, the