In this book St. Ambrose purposes to treat of the training of virgins, using examples rather than precepts, and explains why he does so in writing rather than by word of mouth.
1. In the former book I wished (though I was not able) to set forth how great is the gift of virginity, that the grace of the heavenly gift might of itself invite the reader. In the second book it is fitting that the virgin should be instructed and, as it were, be educated by the teaching of suitable precepts.
2. But, inasmuch as I am feeble in advising and unequal to teaching (for he who teaches ought to excel him who is taught), lest I should seem to have abandoned the task I have undertaken, or to have taken too much upon myself, I thought it better to instruct by examples than by precepts; for more progress may be made by means of an example, inasmuch as that which has been already done is considered to be not difficult, and that which has been tried to be expedient, and that which has been transmitted in succession to us by a kind of hereditary practice of ancestral virtue to be binding in religion.
3. But if any one rebukes me for presumption, let him rather rebuke me for zeal, because I thought that I ought not to refuse even this to the virgins who asked it of me. For I preferred rather to run the risk of perilling my own modesty, than not to fulfil the wish of those whose pursuits even our God favours with kindly approbation.
4. Nor can the mark of presumption be set on my task, since, when they had those from whom they could learn, they sought my good-will rather than my teaching, and my zeal may be excused, since when they had the guidance of a martyr for the observance of discipline, I did not think it superfluous if I could turn the persuasion of my discourse into an allurement to profession. He who teaches with facility restrains fault with severity; I, who cannot teach, entice.
5. And because many who were absent desired to have the use of my discourse, I compiled this book, in order that holding in their hands the substance of what my voice had uttered to them, they might not think that he whom they were holding failed them. But let us go on with our plan.
Source: Concerning Virgins (New Advent)