25. We have then to speak of that usefulness which is full of what is virtuous, as the Apostle himself has laid it down in so many words, saying: “And this I speak for your own profit, not that I may cast a snare upon you, but for that which is comely.” 1 Cor. vii. 35. It is plain, then, that what is virtuous is useful, and what is useful is virtuous; also that what is useful is just, and what is just is useful. I can say this, for I am speaking, not to merchants who are covetous from a desire to make gain, but to my children. And I am speaking of the duties which I wish to impress upon and impart to you, whom I have chosen for the service of the Lord; so that those things which have been already implanted and fixed in your minds and characters by habit and training may now be further unfolded to you by explanation and instruction.