appeal to the objectification of will, instead of giving a physical explanation, as we have to appeal to the creative power of God. For physics demands causes, and the will is never a cause. Its whole relation to the phenomenon is not in accordance with the principle of sufficient reason. But that which in itself is the will exists in another aspect as idea; that is to say, is phenomenon. As such, it obeys the laws which constitute the form of the phenomenon. Every movement, for example, although it is always a manifestation of will, must yet have a cause from which it is to be explained in relation to a particular time and space; that is, not in general in its inner nature, but as a _particular_ phenomenon. In the case of the stone, this is a mechanical cause; in that of the movement of a man, it is a motive; but in no case can it be wanting. On the other hand, the universal common nature of all phenomena of one particular kind, that which must be presupposed if the explanation from causes is to have any sense and meaning, is the general force of nature, which, in physics, must remain a _qualitas occulta_, because with it the etiological explanation ends and the metaphysical begins. But the chain of causes and effects is never broken by an original force to which it has been necessary to appeal. It does not run back to such a force as if it were its first link, but the nearest link, as well as the remotest, presupposes the original force, and could otherwise explain nothing. A series of causes and effects may be the manifestation of the most different kinds of forces, whose successive visible appearances are conducted through it, as I have illustrated above by the example of a metal machine. But the difference of these original forces, which cannot be referred to each other, by no means breaks the unity of that chain of causes, and the connection between all its links. The etiology and the philosophy of nature never do violence to each other, but go hand in hand, regarding the same object from different points of view. Etiology gives an account of the causes which necessarily produce the particular phenomenon to be explained. It exhibits, as the foundation of all its explanations, the universal forces which are active in all these causes and effects. It accurately defines, enumerates, and distinguishes these forces, and then indicates all the different effects in which each force appears, regulated by the difference of the circumstances, always in accordance with its own peculiar character, which it discloses in obedience to an invariable rule, called _a law of nature_. When all this has been thoroughly accomplished by physics in every particular, it will be complete, and its work will be done. There will then remain no unknown force in unorganised nature, nor any effect, which has not been proved to be the manifestation of one of these forces under definite circumstances, in accordance with a law of nature.