Today we discuss with Thomas the sin of sloth. What is it? When does it become a mortal sin?

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Sloth, according to Damascene (De Fide Orth. ii, 14) is an oppressive sorrow, which, to wit, so weighs upon man's mind, that he wants to do nothing; thus acid things are also cold. Hence sloth implies a certain weariness of work, as appears from a gloss on Psalm 106:18, "Their soul abhorred all manner of meat," and from the definition of some who say that sloth is a "sluggishness of the mind which neglects to begin good."

Now this sorrow is always evil, sometimes in itself, sometimes in its effect. For sorrow is evil in itself when it is about that which is apparently evil but good in reality, even as, on the other hand, pleasure is evil if it is about that which seems to be good but is, in truth, evil. Since, then, spiritual good is a good in very truth, sorrow about spiritual good is evil in itself. And yet that sorrow also which is about a real evil, is evil in its effect, if it so oppresses man as to draw him away entirely from good deeds. Hence the Apostle (2 Corinthians 2:7) did not wish those who repented to be "swallowed up with overmuch sorrow."

Accordingly, since sloth, as we understand it here, denotes sorrow for spiritual good, it is evil on two counts, both in itself and in point of its effect. Consequently it is a sin, for by sin we mean an evil movement of the appetite, as appears from what has been said above (II-II:10:2; I-II:74:4).

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mortal sin is so called because it destroys the spiritual life which is the effect of charity, whereby God dwells in us. Wherefore any sin which by its very nature is contrary to charity is a mortal sin by reason of its genus. And such is sloth, because the proper effect of charity is joy in God, as stated above (II-II:28:1), while sloth is sorrow about spiritual good in as much as it is a Divine good. Therefore sloth is a mortal sin in respect of its genus. But it must be observed with regard to all sins that are mortal in respect of their genus, that they are not mortal, save when they attain to their perfection. Because the consummation of sin is in the consent of reason: for we are speaking now of human sins consisting in human acts, the principle of which is the reason.

Wherefore if the sin be a mere beginning of sin in the sensuality alone, without attaining to the consent of reason, it is a venial sin on account of the imperfection of the act. Thus in the genus of adultery, the concupiscence that goes no further than the sensuality is a venial sin, whereas if it reach to the consent of reason, it is a mortal sin. So too, the movement of sloth is sometimes in the sensuality alone, by reason of the opposition of the flesh to the spirit, and then it is a venial sin; whereas sometimes it reaches to the reason, which consents in the dislike, horror and detestation of the Divine good, on account of the flesh utterly prevailing over the spirit. On this case it is evident that sloth is a mortal sin.

ST II-II, Q. 35, A. 1;3.

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