As every instrument and every bodily member subserves some partial end that is to say some special action so the whole body must be destined to minister to some Plenary sphere of action. Thus the saw is made for sawing for sawing is a function and not sawing for the saw. Similarly the body too must somehow or other be made for the soul and each part of it for some subordinate function to which it is adapted.
We have then first to describe the common functions common that is to the whole animal kingdom or to certain large groups or to the members of a species.