That sometimes lighter vices are to be left alone, that more grievous ones may be removed.
But since, when the sickness of two vices attacks a man, one presses upon him more lightly, and the other perchance more heavily, it is undoubtedly right to haste to the succour of that through which there is the more rapid tendency to death. And, if the one cannot be restrained from causing the death which is imminent unless the other which is contrary to it increase, the preacher must be content by skilful management in his exhortation to suffer one to increase, to the end that he may keep the other back from causing the death which is imminent. When he does this, he does not aggravate the disease, but preserves the life of his sufferer to whom he administers the medicine, that he may find a fitting time for searching out means of recovery. For there is often one who, while he puts no restraint on his gluttony in food, is presently pressed hard by the stings of lechery, which is on the point of overcoming him, and who, when, terrified by the fear of this struggle, he strives to restrain himself through abstinence, is harassed by the temptation of vain-glory: in which case certainly one vice is by no means extinguished unless the other be fostered. Which plague then should be the more ardently attacked but that which presses on the man the more dangerously? For it is to be tolerated that through the virtue of abstinence arrogance should meanwhile grow against one that is alive, lest through gluttony lechery should cut him off from life entirely. Hence it is that Paul, when he considered that his weak hearer would either continue to do evil or rejoice in the reward of human praise for well-doing, said, Will you not be afraid of the power? Do that which is good, and you shall have praise of the same. For it is not that good things should be done in order that no human power may be feared, or that the glory of transitory praise may be thereby won; but, considering that the weak soul could not rise to so great strength as to shun at the same time both wickedness and praise, the excellent preacher in his admonition offered something and took away something. For by conceding mild ailments he drew off keener ones; that, since the mind could not rise all at once to the relinquishing of all its vices, it might, while left in familiarity with some one of them, be taken off without difficulty from another.
Source: Pastoral Rule (New Advent)