Objection 1. It would seem that spiritual sins are unfittingly distinguished from carnal sins. For the Apostle says (Galatians 5:19): "The works of the flesh are manifest, which are fornication, uncleanness, immodesty, luxury, idolatry, witchcrafts," etc. from which it seems that all kinds of sins are works of the flesh. Now carnal sins are called works of the flesh. Therefore carnal sins should not be distinguished from spiritual sins.
Objection 2. Further, whosoever sins, walks according to the flesh, as stated in Romans 8:13: "If you live according to the flesh, you shall die. But if by the spirit you mortify the deeds of the flesh, you shall live." Now to live or walk according to the flesh seems to pertain to the nature of carnal sin. Therefore carnal sins should not be distinguished from spiritual sins.
Objection 3. Further, the higher part of the soul, which is the mind or reason, is called the spirit, according to Ephesians 4:23: "Be renewed in the spirit of your mind," where spirit stands for reason, according to a gloss. Now every sin, which is committed in accordance with the flesh, flows from the reason by its consent; since consent in a sinful act belongs to the higher reason, as we shall state further on (74, 7). Therefore the same sins are both carnal and spiritual, and consequently they should not be distinguished from one another.
Objection 4. Further, if some sins are carnal specifically, this, seemingly, should apply chiefly to those sins whereby man sins against his own body. But, according to the Apostle (1 Corinthians 6:18), "every sin that a man doth, is without the body: but he that committeth fornication, sinneth against his own body." Therefore fornication would be the only carnal sin, whereas the Apostle (Ephesians 5:3) reckons covetousness with the carnal sins.
On the contrary, Gregory (Moral. xxxi, 17) says that "of the seven capital sins five are spiritual, and two carnal."
I answer that, As stated above (Article 1), sins take their species from their objects. Now every sin consists in the desire for some mutable good, for which man has an inordinate desire, and the possession of which gives him inordinate pleasure. Now, as explained above (Question 31, Article 3), pleasure is twofold. One belongs to the soul, and is consummated in the mere apprehension of a thing possessed in accordance with desire; this can also be called spiritual pleasure, e.g. when one takes pleasure in human praise or the like. The other pleasure is bodily or natural, and is realized in bodily touch, and this can also be called carnal pleasure.
Accordingly, those sins which consist in spiritual pleasure, are called spiritual sins; while those which consist in carnal pleasure, are called carnal sins, e.g. gluttony, which consists in the pleasures of the table; and lust, which consists in sexual pleasures. Hence the Apostle says (2 Corinthians 7:1): "Let us cleanse ourselves from all defilement of the flesh and of the spirit."
Reply to Objection 1. As a gloss says on the same passage, these vices are called works of the flesh, not as though they consisted in carnal pleasure; but flesh here denotes man, who is said to live according to the flesh, when he lives according to himself, as Augustine says (De Civ. Dei xiv, 2,3). The reason of this is because every failing in the human reason is due in some way to the carnal sense.
This suffices for the Reply to the Second Objection.
Reply to Objection 3. Even in the carnal sins there is a spiritual act, viz. the act of reason: but the end of these sins, from which they are named, is carnal pleasure.
Reply to Objection 4. As the gloss says, "in the sin of fornication the soul is the body's slave in a special sense, because at the moment of sinning it can think of nothing else": whereas the pleasure of gluttony, although carnal, does not so utterly absorb the reason. It may also be said that in this sin, an injury is done to the body also, for it is defiled inordinately: wherefore by this sin alone is man said specifically to sin against his body. While covetousness, which is reckoned among the carnal sins, stands here for adultery, which is the unjust appropriation of another's wife. Again, it may be said that the thing in which the covetous man takes pleasure is something bodily, and in this respect covetousness is numbered with the carnal sins: but the pleasure itself does not belong to the body, but to the spirit, wherefore Gregory says (Moral. xxxi, 17) that it is a spiritual sin.
Source: Summa Theologica (New Advent)