Thomas Aquinas

“ 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. ”

“ Properly speaking it is not taken as a negation, so as merely to signify the absence of prudence, for this can be without any sin. Taken as a privation, imprudence denotes lack of that prudence which a man can and ought to have, and in this sense imprudence is a sin by reason of a man's negligence in striving to have prudence. ”

“ There are vices opposed to every virtue, not only vices that are in manifest opposition to virtue, as temerity is opposed to prudence, but also vices which have a kind of kinship and not a true but a spurious likeness to virtue; thus in opposition to prudence we have craftiness. ”

“ When various circumstances are corrupted for the same motive, the species of sin is not multiplied: thus it is the same species of sin to take what is not one's own, where one ought not, and when one ought not. If, however, there be various motives, there are various species: for instance, if one man were to take another's property from where he ought not, so as to wrong a sacred place, this would constitute the species called sacrilege, while if another were to take another's property when he ought not, merely through the lust of possession, this would be a case of simple avarice. ”

“ Therefore inconstancy pertains not to imprudence but to envy.
Objection 3: Further, a man would seem to be inconstant who fails to persevere in what he has proposed to do. Now this is a mark of "incontinency" in pleasurable matters, and of "effeminacy" or "squeamishness" in unpleasant matters, according to Ethic. vii, 1. Therefore inconstancy does not pertain to imprudence.
On the contrary, It belongs to prudence to prefer the greater good to the lesser. Therefore to forsake the greater good belongs to imprudence. Now this is inconstancy. Therefore inconstancy belongs to imprudence. ”

“ It would seem that the aforesaid vices do not arise from lust. For inconstancy arises from envy, as stated above (A [5] , ad 2) . But envy is a distinct vice from lust.
Objection 2: Further, it is written (James 1:8) : "A double-minded man is inconstant in all his ways." Now duplicity does not seem to pertain to lust, but rather to deceitfulness, which is a daughter of covetousness, according to Gregory (Moral. xxxi, 45) . Therefore the aforesaid vices do not arise from lust.
Objection 3: Further, the aforesaid vices are connected with some defect of reason. ”

“ Accordingly we must consider first of all those vices which are in evident opposition to prudence, those namely which are due to a defect either of prudence or of those things which are requisite for prudence, and secondly those vices which have a false resemblance to prudence, those namely which are due to abuse of the things required for prudence. ”

“ Wherefore, if this should happen through aversion from the Divine Law, it will be a mortal sin, as when a man acts precipitately through contempt and rejection of the Divine teaching: whereas if he act beside the Law and without contempt, and without detriment to things necessary for salvation, it will be a venial sin. ”

“ 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 Paral 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. ”

“ Many things have to be considered in the research of reason; hence the Philosopher declares (Ethic. vi, 9) that "one should be slow in taking counsel." Hence precipitation is more directly opposed to rectitude of counsel than over slowness is, for the latter bears a certain likeness to right counsel.
Art. 4 - Whether thoughtlessness is a special sin included in prudence? 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 Ps. ”