derived from the animal itself: there are other natural motions in animals, which they do not experience through their own instrumentality, eg increase, decrease, and respiration: these are experienced by every animal while it is at rest and not in motion in respect of the motion set up by its own agency; here the motion is caused by the environment and by many things that enter into the animal: thus in some cases the cause is nourishment when it is being digested animals sleep, and when it is being distributed they awake and move themselves, the first principle of this motion being thus originally derived from outside.