Objection 1. It would seem that imprudence is not a special sin. For whoever sins, acts against right reason, i.e. against prudence. But imprudence consists in acting against prudence, as stated above (Article 1). Therefore imprudence is not a special sin.
Objection 2. Further, prudence is more akin to moral action than knowledge is.
But ignorance which is opposed to knowledge, is reckoned one of the general causes of sin. Much more therefore should imprudence be reckoned among those causes.
Objection 3. Further, sin consists in the corruption of the circumstances of virtue, wherefore Dionysius says (Div. Nom. iv) that "evil results from each single defect." Now many things are requisite for prudence; for instance, reason, intelligence docility, and so on, as stated above (Q48;49). Therefore there are many species of imprudence, so that it is not a special sin.
On the contrary, Imprudence is opposed to prudence, as stated above (Article 1). Now prudence is a special virtue. Therefore imprudence too is one special vice.
I answer that, A vice or sin may be styled general in two ways; first, absolutely, because, to wit, it is general in respect of all sins; secondly, because it is general in respect of certain vices, which are its species. On the first way, a vice may be said to be general on two counts: first, essentially, because it is predicated of all sins: and in this way imprudence is not a general sin, as neither is prudence a general virtue: since it is concerned with special acts, namely the very acts of reason: secondly, by participation; and in this way imprudence is a general sin: for, just as all the virtues have a share of prudence, in so far as it directs them, so have all vices and sins a share of imprudence, because no sin can occur, without some defect in an act of the directing reason, which defect belongs to imprudence.
If, on the other hand, a sin be called general, not simply but in some particular genus, that is, as containing several species of sin, then imprudence is a general sin. For it contains various species in three ways. First, by opposition to the various subjective parts of prudence, for just as we distinguish the prudence that guides the individual, from other kinds that govern communities, as stated above (48; 50, 07), so also we distinguish various kinds of imprudence. Secondly, in respect of the quasi-potential parts of prudence, which are virtues connected with it, and correspond to the several acts of reason. Thus, by defect of "counsel" to which euboulia (deliberating well) corresponds, "precipitation" or "temerity" is a species of imprudence; by defect of "judgment," to which synesis (judging well according to common law) and gnome (judging well according to general law) refer, there is "thoughtlessness"; while "inconstancy" and "negligence" correspond to the "command" which is the proper act of prudence. Thirdly, this may be taken by opposition to those things which are requisite for prudence, which are the quasi-integral parts of prudence. Since however all these things are intended for the direction of the aforesaid three acts of reason, it follows that all the opposite defects are reducible to the four parts mentioned above. Thus incautiousness and incircumspection are included in "thoughtlessness"; lack of docility, memory, or reason is referable to "precipitation"; improvidence, lack of intelligence and of shrewdness, belong to "negligence" and "inconstancy."
Reply to Objection 1. This argument considers generality by participation.
Reply to Objection 2. Since knowledge is further removed from morality than prudence is, according to their respective proper natures, it follows that ignorance has the nature of mortal sin, not of itself, but on account either of a preceding negligence, or of the consequent result, and for this reason it is reckoned one of the general causes of sin. On the other hand imprudence, by its very nature, denotes a moral vice; and for this reason it can be called a special sin.
Reply to Objection 3. 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. Hence the lack of those things which are requisite for prudence, does not constitute a diversity of species, except in so far as they are directed to different acts of reason, as stated above.
Source: Summa Theologica (New Advent)