St. Thomas Aquinas on Idle Words and Intention

(35) The good man from his good treasure brings forth good. The statement that from the abundance of the heart the mouth speaks he explains in particular. The word which proceeds from a thought is as a gift from a treasure. Hence if the thought is good, the word is good; and conversely. The good treasure is knowledge of the truth and fear of the Lord: "Abundance of salvation, wisdom and knowledge; the fear of the Lord is his treasure" (Is 33:6). Likewise, and evil treasure is evil thinking; and from this treasure only evil proceeds: "Treasures of wickedness do not profit" (Pr 10:2). Note: what is said there of words is understood also of deeds. For as the thought is the wellspring of speech, so intention is of action. Therefore, if the intention is good, the action is good. Hence a Gloss: "You do according as you intend." 

This statement seems open to objection in regard to good. Suppose that someone wants to steal in order to give an alms: the act is evil and the intention good. Therefore... I answer that intending and willing are sometimes distinguished, namely, when in one and the same act the willing and the intention differ. Willing bears on the object wanted, but intention on the end. Willing, for example, is when I will to go to a window to see the people passing by; the latter is the intention. Hence it is necessary that willing and intending be one. Hence we can consider intention and willing in a wide sense, as in the assertion that if the willing is evil, the act is evil. Yet if it be excluded and taken in the proper sense, it is not true. 

But granted that intention and the act of willing are one, what then? I answer that the root of merit lies in charity, but consequently in the merit of other virtues. For merit looks at the essential reward, within which charity is considered. Thus, any work whatsoever that is performed with greater charity has more merit. But charity alone has God for object and end. Hence the merit of charity corresponds to the substantial reward, the merit of the other virtues to the accidental reward. Therefore, because charity permeates the intention, to the extent that one does something from greater charity to that extent he achieves; but in regard to accidental reward, this is not so.

(36) I tell you... The Lord rebuked them for the gravity of their sin and their malice; here he warns them about the future judgment, which we hold by faith: "Be afraid of the sword, for anger brings the punishment of the sword; for know that there will be a judgment" (Jb 19:29); "He will make room for every act of mercy; everyone will receive in accordance with his deeds" (Sir 16:14). Again 2 Corinthians (5:10): For we must all appear before the judgment seat of Christ, so that each one may receive good or evil, according to what he has done in the body." Hence there will be an investigation, because each one will render an account of his deeds. 

Therefore, he also adds something about words: I say to you, on the day of judgment men will render account for every careless word they utter. This is said in Wisdom (1:9): "He that speaks unrighteous things will not escape notice" and that the sound of murmuring will not go unheard. Why does he say for every carless [idle] word? A word is called idle in two ways: in one way every evil word is called idle; because that is called idle which does not attain its purpose, as when a person hunts for a man and does not find him, the search is said to have been idle. But if a word is given to instruct, when it succeeds, it is not idle: "Let no evil talk come out of your minds, but only such as is good for edifying, that it may impart grace to those who hear" (Eph 4:29). And according to Chrysostom, it refers to the fact that they said, "by Beelzebub". That word was most pernicious and also idle. According to Jerome the former is a word that inflicts harm[; an] idle one is that which does not afford any benefit. According to Gregory any word lightly spoken is called idle, unless it has a pious use or a pious need. But it is clear that they spoke a pernicious word. Why does he mention only idle? Because he wishes to argue from the lesser; because if one must render an account for an idle word, then more so for a pernicious one.

Source: St. Thomas Aquinas, Commentary on Saint Matthew's Gospel (Super Evangelium S. Matthaei lectura), trans. R.F. Larcher, chapter 12, lecture 2, verses 35-36; http://dhspriory.org/thomas/SSMatthew.htm.