Fasting is a medicine; but a medicine, though it be never so profitable, becomes frequently useless owing to the unskillfulness of him who employs it. For it is necessary to know, moreover, the time when it should be applied, and the requisite quantity of it; and the nature of the country, and the season of the year; and the corresponding diet; as well as various other particulars; any of which, if one overlooks, he will mar all the rest that have been named. Now if, when the body needs healing, such exactness is required on our part, much more ought we, when our care is about the soul, and we seek to heal the distempers of the mind, to look, and to search into every particular with the utmost accuracy.