Objection 1. It would seem that the fruits of the Holy Ghost are not contrary to the works of the flesh, which the Apostle enumerates (Galatians 5:19, seqq.). Because contraries are in the same genus. But the works of the flesh are not called fruits. Therefore the fruits of the Spirit are not contrary to them.
Objection 2. Further, one thing has a contrary. Now the Apostle mentions more works of the flesh than fruits of the Spirit. Therefore the fruits of the Spirit and the works of the flesh are not contrary to one another.
Objection 3. Further, among the fruits of the Spirit, the first place is given to charity, joy, and peace: to which, fornication, uncleanness, and immodesty, which are the first of the works of the flesh are not opposed. Therefore the fruits of the Spirit are not contrary to the works of the flesh.
On the contrary, The Apostle says (Galatians 5:17) that "the flesh lusteth against the spirit, and the spirit against the flesh."
I answer that, The works of the flesh and the fruits of the Spirit may be taken in two ways. First, in general: and in this way the fruits of the Holy Ghost considered in general are contrary to the works of the flesh. Because the Holy Ghost moves the human mind to that which is in accord with reason, or rather to that which surpasses reason: whereas the fleshly, viz. the sensitive, appetite draws man to sensible goods which are beneath him. Wherefore, since upward and downward are contrary movements in the physical order, so in human actions the works of the flesh are contrary to the fruits of the Spirit.
Secondly, both fruits and fleshly works as enumerated may be considered singly, each according to its specific nature. And in this they are not of necessity contrary each to each: because, as stated above (3, ad 4), the Apostle did not intend to enumerate all the works, whether spiritual or carnal. However, by a kind of adaptation, Augustine, commenting on Galatians 5:22-23, contrasts the fruits with the carnal works, each to each. Thus "to fornication, which is the love of satisfying lust outside lawful wedlock, we may contrast charity, whereby the soul is wedded to God: wherein also is true chastity. By uncleanness we must understand whatever disturbances arise from fornication: and to these the joy of tranquillity is opposed. Idolatry, by reason of which war was waged against the Gospel of God, is opposed to peace. Against witchcrafts, enmities, contentions, emulations, wraths and quarrels, there is longsuffering, which helps us to bear the evils inflicted on us by those among whom we dwell; while kindness helps us to cure those evils; and goodness, to forgive them. In contrast to heresy there is faith; to envy, mildness; to drunkenness and revellings, contingency."
Reply to Objection 1. That which proceeds from a tree against the tree's nature, is not called its fruit, but rather its corruption. And since works of virtue are connatural to reason, while works of vice are contrary to nature, therefore it is that works of virtue are called fruits, but not so works of vice.
Reply to Objection 2. "Good happens in one way, evil in all manner of ways," as Dionysius says (Div. Nom. iv): so that to one virtue many vices are contrary. Consequently we must not be surprised if the works of the flesh are more numerous than the fruits of the spirit.
The Reply to the Third Objection is clear from what has been said.
Source: Summa Theologica (New Advent)