On reflection and passion, and on observing propriety of speech, both in ordinary conversation and in holding discussions.
98. There are two kinds of mental motions — those of reflection and of passion. The one has to do with reflection, the other with passion. There is no confusion one with the other, for they are markedly different and unlike. Reflection has to search and as it were to grind out the truth. Passion prompts and stimulates us to do something. Thus by its very nature reflection diffuses tranquillity and calm; and passion sends forth the impulse to act. Let us then be ready to allow reflection on good things to enter into our mind, and to make passion submit to reason (if indeed we wish to direct our minds to guard what is seemly), lest desire for anything should shut out reason. Rather let reason test and see what befits virtue.
99. And since we have said that we must aim at the observance of what is seemly, so as to know what is the due measure in our words and deeds, and as order in speech rather than in action comes first; speech is divided into two kinds: first, as it is used in friendly conversation, and then in the treatment and discussion of matters of faith and justice. In either case we must take care that there is no irritation. Our language should be mild and quiet, and full of kindness and courtesy and free from insult. Let there be no obstinate disputes in our familiar conversations, for they are wont only to bring up useless subjects, rather than to supply anything useful. Let there be discussion without wrath, urbanity without bitterness, warning without sharpness, advice without giving offense. And as in every action of our life we ought to take heed to this, in order that no overpowering impulse of our mind may ever shut out reason (let us always keep a place for counsel), so, too, ought we to observe that rule in our language, so that neither wrath nor hatred may be aroused, and that we may not show any signs of our greed or sloth.
100. Let our language be of this sort, more especially when we are speaking of the holy Scriptures. For of what ought we to speak more often than of the best subject of conversation, of its exhortation to watchfulness, its care for good instruction? Let us have a reason for beginning, and let our end be within due limits. For a speech that is wearisome only stirs up anger. But surely it is most unseemly that when every kind of conversation generally gives additional pleasure, this should give cause of offense!
101. The treatment also of such subjects as the teaching of faith, instruction on self-restraint, discussion on justice, exhortation to activity, must not be taken up by us and fully gone into all at one time, but must be carried on in course, so far as we can do it, and as the subject-matter of the passage allows. Our discourse must not be too lengthy, nor too soon cut short, for fear the former should leave behind it a feeling of aversion, and the latter produce carelessness and neglect. The address should be plain and simple, clear and evident, full of dignity and weight; it should not be studied or too refined, nor yet, on the other hand, be unpleasing and rough in style.
Source: On the Duties of the Clergy (New Advent)