Objection 1. It would seem that sloth is not a special vice. For that which is common to all vices does not constitute a special kind of vice. But every vice makes a man sorrowful about the opposite spiritual good: for the lustful man is sorrowful about the good of continence, and the glutton about the good of abstinence. Since then sloth is sorrow for spiritual good, as stated above (Article 1), it seems that sloth is not a special sin.
Objection 2. Further, sloth, through being a kind of sorrow, is opposed to joy. Now joy is not accounted one special virtue. Therefore sloth should not be reckoned a special vice.
Objection 3. Further, since spiritual good is a general kind of object, which virtue seeks, and vice shuns, it does not constitute a special virtue or vice, unless it be determined by some addition. Now nothing, seemingly, except toil, can determine it to sloth, if this be a special vice; because the reason why a man shuns spiritual goods, is that they are toilsome, wherefore sloth is a kind of weariness: while dislike of toil, and love of bodily repose seem to be due to the same cause, viz. idleness. Hence sloth would be nothing but laziness, which seems untrue, for idleness is opposed to carefulness, whereas sloth is opposed to joy. Therefore sloth is not a special vice.
On the contrary, Gregory (Moral. xxxi, 45) distinguishes sloth from the other vices. Therefore it is a special vice.
I answer that, Since sloth is sorrow for spiritual good, if we take spiritual good in a general way, sloth will not be a special vice, because, as stated above (I-II, 71, 1), every vice shuns the spiritual good of its opposite virtue. Again it cannot be said that sloth is a special vice, in so far as it shuns spiritual good, as toilsome, or troublesome to the body, or as a hindrance to the body's pleasure, for this again would not sever sloth from carnal vices, whereby a man seeks bodily comfort and pleasure.
Wherefore we must say that a certain order exists among spiritual goods, since all the spiritual goods that are in the acts of each virtue are directed to one spiritual good, which is the Divine good, about which there is a special virtue, viz. charity. Hence it is proper to each virtue to rejoice in its own spiritual good, which consists in its own act, while it belongs specially to charity to have that spiritual joy whereby one rejoices in the Divine good. On like manner the sorrow whereby one is displeased at the spiritual good which is in each act of virtue, belongs, not to any special vice, but to every vice, but sorrow in the Divine good about which charity rejoices, belongs to a special vice, which is called sloth. This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)