Objection 1. It would seem that sloth ought not to be accounted a capital vice. For a capital vice is one that moves a man to sinful acts, as stated above (Question 34, Article 5). Now sloth does not move one to action, but on the contrary withdraws one from it. Therefore it should not be accounted a capital sin.
Objection 2. Further, a capital sin is one to which daughters are assigned. Now Gregory (Moral. xxxi, 45) assigns six daughters to sloth, viz. "malice, spite, faint-heartedness, despair, sluggishness in regard to the commandments, wandering of the mind after unlawful things." Now these do not seem in reality to arise from sloth. For "spite" is, seemingly the same as hatred, which arises from envy, as stated above (Question 34, Article 6); "malice" is a genus which contains all vices, and, in like manner, a "wandering" of the mind after unlawful things is to be found in every vice; "sluggishness" about the commandments seems to be the same as sloth, while "faint-heartedness" and "despair" may arise from any sin. Therefore sloth is not rightly accounted a capital sin.
Objection 3. Further, Isidore distinguishes the vice of sloth from the vice of sorrow, saying (De Summo Bono ii, 37) that in so far as a man shirks his duty because it is distasteful and burdensome, it is sorrow, and in so far as he is inclined to undue repose, it is sloth: and of sorrow he says that it gives rise to "spite, faint-heartedness, bitterness, despair," whereas he states that from sloth seven things arise, viz. "idleness, drowsiness, uneasiness of the mind, restlessness of the body, instability, loquacity, curiosity." Therefore it seems that either Gregory or Isidore has wrongly assigned sloth as a capital sin together with its daughters.
On the contrary, The same Gregory (Moral. xxxi, 45) states that sloth is a capital sin, and has the daughters aforesaid.
I answer that, As stated above (I-II, 84, 3,4), a capital vice is one which easily gives rise to others as being their final cause. Now just as we do many things on account of pleasure, both in order to obtain it, and through being moved to do something under the impulse of pleasure, so again we do many things on account of sorrow, either that we may avoid it, or through being exasperated into doing something under pressure thereof. Wherefore, since sloth is a kind of sorrow, as stated above (2; I-II, 85, 8), it is fittingly reckoned a capital sin.
Reply to Objection 1. Sloth by weighing on the mind, hinders us from doing things that cause sorrow: nevertheless it induces the mind to do certain things, either because they are in harmony with sorrow, such as weeping, or because they are a means of avoiding sorrow.
Reply to Objection 2. Gregory fittingly assigns the daughters of sloth. For since, according to the Philosopher (Ethic. viii, 5,6) "no man can be a long time in company with what is painful and unpleasant," it follows that something arises from sorrow in two ways: first, that man shuns whatever causes sorrow; secondly, that he passes to other things that give him pleasure: thus those who find no joy in spiritual pleasures, have recourse to pleasures of the body, according to the Philosopher (Ethic. x, 6). Now in the avoidance of sorrow the order observed is that man at first flies from unpleasant objects, and secondly he even struggles against such things as cause sorrow. Now spiritual goods which are the object of the sorrow of sloth, are both end and means. Avoidance of the end is the result of "despair," while avoidance of those goods which are the means to the end, in matters of difficulty which come under the counsels, is the effect of "faint-heartedness," and in matters of common righteousness, is the effect of "sluggishness about the commandments." The struggle against spiritual goods that cause sorrow is sometimes with men who lead others to spiritual goods, and this is called "spite"; and sometimes it extends to the spiritual goods themselves, when a man goes so far as to detest them, and this is properly called "malice." On so far as a man has recourse to eternal objects of pleasure, the daughter of sloth is called "wandering after unlawful things." From this it is clear how to reply to the objections against each of the daughters: for "malice" does not denote here that which is generic to all vices, but must be understood as explained. Nor is "spite" taken as synonymous with hatred, but for a kind of indignation, as stated above: and the same applies to the others.
Reply to Objection 3. This distinction between sorrow and sloth is also given by Cassian (De Instit. Caenob. x, 1). But Gregory more fittingly (Moral. xxxi, 45) calls sloth a kind of sorrow, because, as stated above (Article 2), sorrow is not a distinct vice, in so far as a man shirks a distasteful and burdensome work, or sorrows on account of any other cause whatever, but only in so far as he is sorry on account of the Divine good, which sorrow belongs essentially to sloth; since sloth seeks undue rest in so far as it spurns the Divine good. Moreover the things which Isidore reckons to arise from sloth and sorrow, are reduced to those mentioned by Gregory: for "bitterness" which Isidore states to be the result of sorrow, is an effect of "spite." "Idleness" and "drowsiness" are reduced to "sluggishness about the precepts": for some are idle and omit them altogether, while others are drowsy and fulfil them with negligence. All the other five which he reckons as effects of sloth, belong to the "wandering of the mind after unlawful things." This tendency to wander, if it reside in the mind itself that is desirous of rushing after various things without rhyme or reason, is called "uneasiness of the mind," but if it pertains to the imaginative power, it is called "curiosity"; if it affect the speech it is called "loquacity"; and in so far as it affects a body that changes place, it is called "restlessness of the body," when, to wit, a man shows the unsteadiness of his mind, by the inordinate movements of members of his body; while if it causes the body to move from one place to another, it is called "instability"; or "instability" may denote changeableness of purpose.
Source: Summa Theologica (New Advent)