Time is an accident that is related and joined to motion in such a manner that the one is never found without the other. Motion is only possible in time, and the idea of time cannot be conceived otherwise than in connexion with motion; things which do not move have no relation to time. Proposition XVI. Incorporeal bodies can only be numbered when they are forces situated in a body; the several forces must then be counted together with substances or objects in which they exist. Hence purely spiritual beings, which are neither corporeal nor forces situated in corporeal objects, cannot be counted, except when considered as causes and effects. Proposition XVII. When an object moves, there must be some agent that moves it, from without, as, e.g., in the case of a stone set in motion by the hand; or from within, e.g., when the body of a living being moves. Living beings include in themselves, at the same time, the moving agent and the thing moved; when, therefore, a living being dies, and the moving agent, the soul, has left the body, i.e., the thing moved, the body remains for some time in the same condition as before, and yet cannot move in the manner it has moved previously. The moving agent, when included in the thing moved, is hidden from, and imperceptible to, the senses. This circumstance gave rise to the belief that the body of an animal moves without the aid of a moving agent. When we therefore affirm, concerning a thing in motion, that it is its own moving agent, or, as is generally said, that it moves of its own accord, we mean to say that the force which really sets the body in motion exists in that body itself. Proposition XVIII. Everything that passes over from a state of potentiality to that of actuality, is caused to do so by some external agent; because if that agent existed in the thing itself, and no obstacle prevented the transition, the thing would never be in a state of potentiality, but always in that of actuality. If, on the other hand, while the thing itself contained that agent, some obstacle existed, and at a certain time that obstacle was removed, the same cause which removed the obstacle would undoubtedly be described as the cause of the transition from potentiality to actuality, [and not the force situated within the body]. Note this. Proposition XIX. A thing which owes its existence to certain causes has in itself merely the possibility of existence; for only if these causes exist, the thing likewise exists. It does not exist if the causes do not exist at all, or if they have ceased to exist, or if there has been a change in the relation which implies the existence of that thing as a necessary consequence of those causes. Proposition XX. A thing which has in itself the necessity of existence cannot have for its existence any cause whatever. Proposition XXI.