purpose that is possible in general; but also its parts are all organs reciprocally producing each other. This can never be the case with artificial instruments, but only with nature which supplies all the material for instruments (even for those of art). Only a product of such a kind can be called a natural purpose, and this because it is an organised and self-organising being.

In a watch one part is the instrument for moving the other parts, but the wheel is not the effective cause of the production of the others; no doubt one part is for the sake of the others, but it does not exist by their means. In this case the producing cause of the parts and of their form is not contained in the nature (of the material), but is external to it in a being which can produce effects