Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XV. How a thing that is good in its own nature ought not to be done for the sake of some lesser good.

Chapter XV.

How a thing that is good in its own nature ought not to be done for the sake of some lesser good.

For we ought not to practise pity, patience and love, and the precepts of the virtues mentioned above, wherein there is what is good in its own nature, for the sake of fasting, but rather fasting for the sake of them. For our endeavour must be that those virtues which are really good may be gained by fasting, not that the practice of those virtues may lead to fasting as its end. For this then the affliction of the flesh is useful, for this the remedy of abstinence must be employed; viz., that by it we may succeed in attaining to love, wherein there is what is good without change, p. 510 and continually with no exception of time. For medicines, and the goldsmith’s art, and the systems of other arts which there are in this world are not employed for the sake of the instruments which belong to the particular work; but rather the implements are prepared for the practice of the art. And as they are useful for those who understand them, so they are useless to those who are ignorant of the system of the art in question; and as they are a great help to those who rely on their aid for doing their work, so they cannot be of the smallest use to those who do not know for what purpose they were made, and are contented simply with the possession of them; because they make all their value consist in the mere having of them, and not in the performance of work. That then is in its own nature the best thing, for the sake of which things indifferent are done, but the very chiefest good is done not for the sake of anything else but because of its own intrinsic goodness.