Objection 1. It would seem that thoughtlessness is not a special sin included in imprudence. For the Divine law does not incite us to any sin, according to Psalm 18:8, "The law of the Lord is unspotted"; and yet it incites us to be thoughtless, according to Matthew 10:19, "Take no thought how or what to speak." Therefore thoughtlessness is not a sin.
Objection 2. Further, whoever takes counsel must needs give thought to many things. Now precipitation is due to a defect of counsel and therefore to a defect of thought. Therefore precipitation is contained under thoughtlessness: and consequently thoughtlessness is not a special sin.
Objection 3. Further, prudence consists in acts of the practical reason, viz. "counsel," "judgment" about what has been counselled, and "command" [Cf. 47, 8]. Now thought precedes all these acts, since it belongs also to the speculative intellect. Therefore thoughtlessness is not a special sin contained under imprudence.
On the contrary, It is written (Proverbs 4:25): "Let thy eyes look straight on, and let thine eye-lids go before thy steps." Now this pertains to prudence, while the contrary pertains to thoughtlessness. Therefore thoughtlessness is a special sin contained under imprudence.
I answer that, Thought signifies the act of the intellect in considering the truth about. something. Now just as research belongs to the reason, so judgment belongs to the intellect. Wherefore in speculative matters a demonstrative science is said to exercise judgment, in so far as it judges the truth of the results of research by tracing those results back to the first indemonstrable principles. Hence thought pertains chiefly to judgment; and consequently the lack of right judgment belongs to the vice of thoughtlessness, in so far, to wit, as one fails to judge rightly through contempt or neglect of those things on which a right judgment depends. It is therefore evident that thoughtlessness is a sin.
Reply to Objection 1. Our Lord did not forbid us to take thought, when we have the opportunity, about what we ought to do or say, but, in the words quoted, He encourages His disciples, so that when they had no opportunity of taking thought, either through lack of knowledge or through a sudden call, they should trust in the guidance of God alone, because "as we know not what to do, we can only turn our eyes to God," according to 2 Chronicles 20:12: else if man, instead of doing what he can, were to be content with awaiting God's assistance, he would seem to tempt God.
Reply to Objection 2. All thought about those things of which counsel takes cognizance, is directed to the formation of a right judgment, wherefore this thought is perfected in judgment. Consequently thoughtlessness is above all opposed to the rectitude of judgment.
Reply to Objection 3. Thoughtlessness is to be taken here in relation to a determinate matter, namely, that of human action, wherein more things have to be thought about for the purpose of right judgment, than in speculative matters, because actions are about singulars.
Source: Summa Theologica (New Advent)