Objection 1. It would seem that the fruits of the Holy Ghost, enumerated by the Apostle (Galatians 5:22-23), are not acts. For that which bears fruit, should not itself be called a fruit, else we should go on indefinitely. But our actions bear fruit: for it is written (Wisdom 3:15): "The fruit of good labor is glorious," and (John 4:36): "He that reapeth receiveth wages, and gathereth fruit unto life everlasting." Therefore our actions are not to be called fruits.
Objection 2. Further, as Augustine says (De Trin. x, 10), "we enjoy ['Fruimur', from which verb we have the Latin 'fructus' and the English 'fruit'] the things we know, when the will rests by rejoicing in them." But our will should not rest in our actions for their own sake. Therefore our actions should not be called fruits.
Objection 3. Further, among the fruits of the Holy Ghost, the Apostle numbers certain virtues, viz. charity, meekness, faith, and chastity. Now virtues are not actions but habits, as stated above (Question 55, Article 1). Therefore the fruits are not actions.
On the contrary, It is written (Matthew 12:33): "By the fruit the tree is known"; that is to say, man is known by his works, as holy men explain the passage. Therefore human actions are called fruits.
I answer that, The word "fruit" has been transferred from the material to the spiritual world. Now fruit, among material things, is the product of a plant when it comes to perfection, and has a certain sweetness. This fruit has a twofold relation: to the tree that produces it, and to the man who gathers the fruit from the tree. Accordingly, in spiritual matters, we may take the word "fruit" in two ways: first, so that the fruit of man, who is likened to the tree, is that which he produces; secondly, so that man's fruit is what he gathers. Yet not all that man gathers is fruit, but only that which is last and gives pleasure. For a man has both a field and a tree, and yet these are not called fruits; but that only which is last, to wit, that which man intends to derive from the field and from the tree. In this sense man's fruit is his last end which is intended for his enjoyment.
If, however, by man's fruit we understand a product of man, then human actions are called fruits: because operation is the second act of the operator, and gives pleasure if it is suitable to him. If then man's operation proceeds from man in virtue of his reason, it is said to be the fruit of his reason: but if it proceeds from him in respect of a higher power, which is the power of the Holy Ghost, then man's operation is said to be the fruit of the Holy Ghost, as of a Divine seed, for it is written (1 John 3:9): "Whosoever is born of God, committeth no sin, for His seed abideth in him."
Reply to Objection 1. Since fruit is something last and final, nothing hinders one fruit bearing another fruit, even as one end is subordinate to another. And so our works, in so far as they are produced by the Holy Ghost working in us, are fruits: but, in so far as they are referred to the end which is eternal life, they should rather be called flowers: hence it is written (Sirach 24:23): "My flowers are the fruits of honor and riches."
Reply to Objection 2. When the will is said to delight in a thing for its own sake, this may be understood in two ways. First, so that the expression "for the sake of" be taken to designate the final cause; and in this way, man delights in nothing for its own sake, except the last end. Secondly, so that it expresses the formal cause; and in this way, a man may delight in anything that is delightful by reason of its form. Thus it is clear that a sick man delights in health, for its own sake, as in an end; in a nice medicine, not as in an end, but as in something tasty; and in a nasty medicine, nowise for its own sake, but only for the sake of something else. Accordingly we must say that man must delight in God for His own sake, as being his last end, and in virtuous deeds, not as being his end, but for the sake of their inherent goodness which is delightful to the virtuous. Hence Ambrose says (De Parad. xiii) that virtuous deeds are called fruits because "they refresh those that have them, with a holy and genuine delight."
Reply to Objection 3. Sometimes the names of the virtues are applied to their actions: thus Augustine writes (Tract. xl in Joan.): "Faith is to believe what thou seest not"; and (De Doctr. Christ. iii, 10): "Charity is the movement of the soul in loving God and our neighbor." It is thus that the names of the virtues are used in reckoning the fruits.
Source: Summa Theologica (New Advent)