According to Proposition XXV., a moving agent must exist which has moved the substance of all existing transient things and enabled it to receive Form. The cause of the motion of that agent is found in the existence of another motor of the same or of a different class, the term “motion,” in a general sense, being common to four categories (Prop. IV.). This series of motions is not infinite (Prop. III.); we find that it can only be continued till the motion of the fifth element is arrived at, and then it ends. The motion of the fifth element is the source of every force that moves and prepares any substance on earth for its combination with a certain form, and is connected with that force by a chain of intermediate motions. The celestial sphere [or the fifth element] performs the act of locomotion which is the first of the several kinds of motion (Prop. XIV.), and all locomotion is found to be the indirect effect of the motion of this sphere; e.g., a stone is set in motion by a stick, the stick by a man’s hand, the hand by the sinews, the sinews by the muscles, the muscles by the nerves, the nerves by the natural heat of the body, and the heat of the body by its form. This is undoubtedly the immediate motive cause, but the action of this immediate cause is due to a certain design, e.g., to bring a stone into a hole by striking against it with a stick in order to prevent the draught from coming through the crevice. The motion of the air that causes the draught is the effect of the motion of the celestial sphere. Similarly it may be shown that the ultimate cause of all genesis and destruction can be traced to the motion of the sphere. But the motion of the sphere must likewise have been effected by an agent (Prop. XVII.) residing either without the sphere or within it; a third case being impossible. In the first case, if the motor is without the sphere, it must either be corporeal or incorporeal; if incorporeal, it cannot be said that the agent is without the sphere; it can only be described as separate from it; because an incorporeal object can only be said metaphorically to reside without a certain corporeal object. In the second case, if the agent resides within the sphere, it must be either a force distributed throughout the whole sphere so that each part of the sphere includes a part of the force, as is the case with the heat of fire; or it is an indivisible force, e.g., the soul and the intellect (Props. X. and XI.). The agent which sets the sphere in motion must consequently be one of the following four things: a corporeal object without the sphere; an incorporeal object separate from it; a force spread throughout the whole of the sphere; or an indivisible force [within the sphere].