The guide for the perplexed · Moses Maimonides
Proposition VI.
The motion of a thing is either essential or accidental; or it is due to an external force, or to the participation of the thing in the motion of another thing. This latter kind of motion is similar to the accidental one. An instance of essential motion may be found in the translation of a thing from one place to another. The accident of a thing, as, e.g., its black colour, is said to move when the thing itself changes its place. The upward motion of a stone, owing to a force applied to it in that direction, is an instance of a motion due to an external force. The motion of a nail in a boat may serve to illustrate motion due to the participation of a thing in the motion of another thing; for when the boat moves, the nail is said to move likewise. The same is the case with everything composed of several parts: when the thing itself moves, every part of it is likewise said to move. Things which are changeable are, at the same time, divisible. Hence everything that moves is divisible, and consequently corporeal; but that which is indivisible cannot move, and cannot therefore be corporeal. A thing that moves accidentally must come to rest, because it does not move of its own accord; hence accidental motion cannot continue for ever.
A corporeal thing that sets another corporeal thing in motion can only effect this by setting itself in motion at the time it causes the other thing to move. 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. A force which occupies all parts of a corporeal object is finite, that object itself being finite. None of the several kinds of change can be continuous, except motion from place to place, provided it be circular. 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.
Text from Project Gutenberg, public domain in the United States.