Objection 1. It would seem that negligence is not opposed to prudence. For negligence seems to be the same as idleness or laziness, which belongs to sloth, according to Gregory (Moral. xxxi, 45). Now sloth is not opposed to prudence, but to charity, as stated above (Question 35, Article 3). Therefore negligence is not opposed to prudence.
Objection 2. Further, every sin of omission seems to be due to negligence. But sins of omission are not opposed to prudence, but to the executive moral virtues. Therefore negligence is not opposed to prudence.
Objection 3. Further, imprudence relates to some act of reason. But negligence does not imply a defect of counsel, for that is "precipitation," nor a defect of judgment, since that is "thoughtlessness," nor a defect of command, because that is "inconstancy." Therefore negligence does not pertain to imprudence.
Objection 4. Further, it is written (Ecclesiastes 7:19): "He that feareth God, neglecteth nothing." But every sin is excluded by the opposite virtue. Therefore negligence is opposed to fear rather than to prudence.
On the contrary, It is written (Sirach 20:7): "A babbler and a fool [imprudens] will regard no time." Now this is due to negligence. Therefore negligence is opposed to prudence.
I answer that, Negligence is directly opposed to solicitude. Now solicitude pertains to the reason, and rectitude of solicitude to prudence. Hence, on the other hand, negligence pertains to imprudence. This appears from its very name, because, as Isidore observes (Etym. x) "a negligent man is one who fails to choose [nec eligens]": and the right choice of the means belongs to prudence. Therefore negligence pertains to imprudence.
Reply to Objection 1. Negligence is a defect in the internal act, to which choice also belongs: whereas idleness and laziness denote slowness of execution, yet so that idleness denotes slowness in setting about the execution, while laziness denotes remissness in the execution itself. Hence it is becoming that laziness should arise from sloth, which is "an oppressive sorrow," i.e. hindering, the mind from action [Cf. 35, 1; I-II, 35, 8].
Reply to Objection 2. Omission regards the external act, for it consists in failing to perform an act which is due. Hence it is opposed to justice, and is an effect of negligence, even as the execution of a just deed is the effect of right reason.
Reply to Objection 3. Negligence regards the act of command, which solicitude also regards. Yet the negligent man fails in regard to this act otherwise than the inconstant man: for the inconstant man fails in commanding, being hindered as it were, by something, whereas the negligent man fails through lack of a prompt will.
Reply to Objection 4. The fear of God helps us to avoid all sins, because according to Proverbs 15:27, "by the fear of the Lord everyone declineth from evil." Hence fear makes us avoid negligence, yet not as though negligence were directly opposed to fear, but because fear incites man to acts of reason. Wherefore also it has been stated above (I-II, 44, 2) when we were treating of the passions, that "fear makes us take counsel."
Source: Summa Theologica (New Advent)