Proposition XVII.

A thing which is said to be contained in a corporeal object must satisfy either of the two following conditions: it either exists through that object, as is the case with accidents, or it is the cause of the existence of that object; such is, e.g., its essential property. In both cases it is a force existing in a corporeal object.

Among the things which exist through a material object, there are some which participate in the division of that object, and are therefore accidentally divisible, as, e.g., its colour, and all other qualities that spread throughout its parts. On the other hand, among the things which form the essential elements of an object, there are some which cannot be divided in any way, as, e.g., the soul and the intellect.

Locomotion is in the natural order of the several kinds of motion the first and foremost. For genesis and corruption are preceded by transformation, which, in its turn, is preceded by the approach of the transforming agent to the object which is to be transformed. Also, increase and decrease are impossible without previous genesis and corruption.

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.

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.

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.