Now the will governs the faculty of our exterior motion as a serf or slave: for unless some external thing hinder, it never fails to obey. We open and shut our mouth, move our tongue, our hands, feet, eyes, and all the members to which the power of this movement refers without resistance, according to our wish and will.
But as for our senses and the faculties of nourishing, growing, and producing, we cannot with the same ease govern them, but we must employ industry and art. If a slave be called he comes, if he be told to stop, he stops; but we must not expect this obedience from a sparrowhawk or falcon: he that desires it should return to the hand must show it the lure; if he would keep it quiet he must hood it. We bid our servant turn to the right or left hand and he does it, but to make a horse so turn we must make use of the bridle. We must not, Theotimus, command our eyes not to see, our ears not to hear, our hands not to touch, our stomach not to digest, or our body not to grow, for these faculties not having intelligence are not capable of obedience. No one can add a cubit to his stature. We often eat without nourishing ourselves or growing; he that will prevail with these powers must use industry. A physician who has to do with a child in the cradle commands him nothing, but only gives orders to the nurse to do such and such things, or else perchance he prescribes for the nurse to eat this or that meat, to take such and such medicine. This infuses its qualities into the milk which enters the child's body, and the physician accomplishes his will in this little weakling who has not even the power to think of it. We must not give the orders of abstinence, sobriety or continency unto the palate or stomach, but the hands must be commanded only to furnish to the mouth meat and drink in such and such a measure, we take away from or give our faculties their object and subject, and the food which strengthens them, as reason requires. If we desire our eyes not to see we must turn them away, or cover them with their natural hood, and shut them, and by these means we may bring them to the point which the will desires. It would be folly to command a horse not to wax fat, not to grow, not to kick, -- to effect all this, stop his corn; you must not command him, you must simply make him do as you wish.
The will also exercises a certain power over the understanding and memory, for of many things which the understanding has power to understand and the memory has power to remember, the will determines those to which she would have her faculties apply themselves, or from which divert themselves. It is true she cannot manage or range them so absolutely as she does the hands, feet or tongue, on account of the sensitive faculties, especially the fancy, which do not obey the will with a prompt and infallible obedience, and which are necessarily required for the operations of the understanding and memory: but yet the will moves, employs and applies these faculties at her pleasure though not so firmly and constantly that the light and variable fancy does not often divert and distract them, so that as the Apostle cries out: I do not the good which I desire, but the evil which I hate.  So we are often forced to complain that we think not of the good which we love, but the evil which we hate.
 Romans 7:15.