 Aristotle’s classification of motions into  those contrary to nature and those according to nature applies not  only to the motions of the moved objects, but transfers also to the  movers effecting motions. A mover can effect a motion which is  contrary to its own nature. Aristotle’s  example of such an unnatural mover is the lever, an object heavy by  nature, with which loads can  be lifted (Physics 8.4,  255a20–23).  Although such movers can effect motions in the  contrary direction to the motion at the head of the causal chain  (levers are operated by the downward push of something heavy at the  other end), the crucial consideration for Aristotle in this case is  that the original, initiating cause of the causal chain should  effect the motion according to its nature. Taken together, these  considerations imply that we have a complete account of the physical  domain once we have a thorough description of what is natural to the  entites in that domain, together with a specification of all the  circumstances in which they   operate.[41]