Objection 1. It would seem that we ought not to reckon seven capital vices, viz. vainglory, envy, anger, sloth, covetousness, gluttony, lust. For sins are opposed to virtues. But there are four principal virtues, as stated above (Question 61, Article 2). Therefore there are only four principal or capital vices.
Objection 2. Further, the passions of the soul are causes of sin, as stated above (Question 77). But there are four principal passions of the soul; two of which, viz. hope and fear, are not mentioned among the above sins, whereas certain vices are mentioned to which pleasure and sadness belong, since pleasure belongs to gluttony and lust, and sadness to sloth and envy. Therefore the principal sins are unfittingly enumerated.
Objection 3. Further, anger is not a principal passion. Therefore it should not be placed among the principal vices.
Objection 4. Further, just as covetousness or avarice is the root of sin, so is pride the beginning of sin, as stated above (Question 84, Article 2). But avarice is reckoned to be one of the capital vices. Therefore pride also should be placed among the capital vices.
Objection 5. Further, some sins are committed which cannot be caused through any of these: as, for instance, when one sins through ignorance, or when one commits a sin with a good intention, e.g. steals in order to give an alms. Therefore the capital vices are insufficiently enumerated.
On the contrary, stands the authority of Gregory who enumerates them in this way (Moral. xxxi, 17).
I answer that, As stated above (Question 84, Article 3), the capital vices are those which give rise to others, especially by way of final cause. Now this kind of origin may take place in two ways. First, on account of the condition of the sinner, who is disposed so as to have a strong inclination for one particular end, the result being that he frequently goes forward to other sins. But this kind of origin does not come under the consideration of art, because man's particular dispositions are infinite in number. Secondly, on account of a natural relationship of the ends to one another: and it is in this way that most frequently one vice arises from another, so that this kind of origin can come under the consideration of art.
Accordingly therefore, those vices are called capital, whose ends have certain fundamental reasons for moving the appetite; and it is in respect of these fundamental reasons that the capital vices are differentiated. Now a thing moves the appetite in two ways. First, directly and of its very nature: thus good moves the appetite to seek it, while evil, for the same reason, moves the appetite to avoid it. Secondly, indirectly and on account of something else, as it were: thus one seeks an evil on account of some attendant good, or avoids a good on account of some attendant evil.
Again, man's good is threefold. For, in the first place, there is a certain good of the soul, which derives its aspect of appetibility, merely through being apprehended, viz. the excellence of honor and praise, and this good is sought inordinately by "vainglory." Secondly, there is the good of the body, and this regards either the preservation of the individual, e.g. meat and drink, which good is pursued inordinately by "gluttony," or the preservation of the species, e.g. sexual intercourse, which good is sought inordinately by "lust." Thirdly, there is external good, viz. riches, to which "covetousness" is referred. These same four vices avoid inordinately the contrary evils.
Or again, good moves the appetite chiefly through possessing some property of happiness, which all men seek naturally. Now in the first place happiness implies perfection, since happiness is a perfect good, to which belongs excellence or renown, which is desired by "pride" or "vainglory." Secondly, it implies satiety, which "covetousness" seeks in riches that give promise thereof. Thirdly, it implies pleasure, without which happiness is impossible, as stated in Ethic. i, 7; x, 6,7, and this "gluttony" and "lust" pursue.
On the other hand, avoidance of good on account of an attendant evil occurs in two ways. For this happens either in respect of one's own good, and thus we have "sloth," which is sadness about one's spiritual good, on account of the attendant bodily labor: or else it happens in respect of another's good, and this, if it be without recrimination, belongs to "envy," which is sadness about another's good as being a hindrance to one's own excellence, while if it be with recrimination with a view to vengeance, it is "anger." Again, these same vices seek the contrary evils.
Reply to Objection 1. Virtue and vice do not originate in the same way: since virtue is caused by the subordination of the appetite to reason, or to the immutable good, which is God, whereas vice arises from the appetite for mutable good. Wherefore there is no need for the principal vices to be contrary to the principal virtues.
Reply to Objection 2. Fear and hope are irascible passions. Now all the passions of the irascible part arise from passions of the concupiscible part; and these are all, in a way, directed to pleasure or sorrow. Hence pleasure and sorrow have a prominent place among the capital sins, as being the most important of the passions, as stated above (Question 25, Article 4).
Reply to Objection 3. Although anger is not a principal passion, yet it has a distinct place among the capital vices, because it implies a special kind of movement in the appetite, in so far as recrimination against another's good has the aspect of a virtuous good, i.e. of the right to vengeance.
Reply to Objection 4. Pride is said to be the beginning of every sin, in the order of the end, as stated above (Question 84, Article 2): and it is in the same order that we are to consider the capital sin as being principal. Wherefore pride, like a universal vice, is not counted along with the others, but is reckoned as the "queen of them all," as Gregory states (Moral. xxxi, 27). But covetousness is said to be the root from another point of view, as stated above (Question 84, Article 1, Question 84, Article 2).
Reply to Objection 5. These vices are called capital because others, most frequently, arise from them: so that nothing prevents some sins from arising out of other causes. Nevertheless we might say that all the sins which are due to ignorance, can be reduced to sloth, to which pertains the negligence of a man who declines to acquire spiritual goods on account of the attendant labor; for the ignorance that can cause sin, is due to negligence, as stated above (Question 76, Article 2). That a man commit a sin with a good intention, seems to point to ignorance, in so far as he knows not that evil should not be done that good may come of it.
Source: Summa Theologica (New Advent)