1265–1273 · Thomas Aquinas, Summa Theologica II-II, q. 35, a. 2, co.
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 (FS, Q[71], A[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. In 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.