All bodies are either in motion or at rest.
Bodies are distinguished from one another in respect of motion and rest, quickness and slowness, and not in respect of substance. All bodies agree in certain respects. A body in motion or at rest must have been determined to motion or rest by another body, which likewise has been determined to motion or rest by another body, and that body by another, and so ad infinitum. If from a body, or an individual thing composed of a number of bodies, certain bodies are separated, and at the same time a like number of bodies of the same nature take their place, the individual thing will retain its nature as before, without any change in its form. Furthermore, the individual thing so composed retains its own nature, whether as a whole it is moving or at rest, and in whatever direction it moves, provided that each constituent part retains its own motion and continues to communicate this motion to the other parts.