21.  For these reasons I allege that our office as physicians far exceeds in toilsomeness, and consequently in worth, that which is confined to the body; and further, because the latter is mainly concerned with the surface, and only in a slight degree investigates the causes which are deeply hidden.  But the whole of our treatment and exertion is concerned with the hidden man of the heart, 1 Pet. iii. 4. and our warfare is directed against that adversary and foe within us, who uses ourselves as his weapons against ourselves, and, most fearful of all, hands us over to the death of sin.  In opposition then, to these foes we are in need of great and perfect faith, and of still greater co-operation on the part of God, and, as I am persuaded, of no slight countermanoeuvring on our own part, which must manifest itself both in word and deed, if ourselves, the most precious possession we have, are to be duly tended and cleansed and made as deserving as possible.