Objection 1. It would seem that charity increases by addition. For just as increase may be in respect of bodily quantity, so may it be according to virtual quantity. Now increase in bodily quantity results from addition; for the Philosopher says (De Gener. i, 5) that "increase is addition to pre-existing magnitude." Therefore the increase of charity which is according to virtual quantity is by addition.
Objection 2. Further, charity is a kind of spiritual light in the soul, according to 1 John 2:10: "He that loveth his brother abideth in the light." Now light increases in the air by addition; thus the light in a house increases when another candle is lit. Therefore charity also increases in the soul by addition.
Objection 3. Further, the increase of charity is God's work, even as the causing of it, according to 2 Corinthians 9:10: "He will increase the growth of the fruits of your justice." Now when God first infuses charity, He puts something in the soul that was not there before. Therefore also, when He increases charity, He puts something there which was not there before. Therefore charity increases by addition.
On the contrary, Charity is a simple form. Now nothing greater results from the addition of one simple thing to another, as proved in Phys. iii, text. 59, and Metaph. ii, 4. Therefore charity does not increase by addition.
I answer that, Every addition is of something to something else: so that in every addition we must at least presuppose that the things added together are distinct before the addition. Consequently if charity be added to charity, the added charity must be presupposed as distinct from charity to which it is added, not necessarily by a distinction of reality, but at least by a distinction of thought. For God is able to increase a bodily quantity by adding a magnitude which did not exist before, but was created at that very moment; which magnitude, though not pre-existent in reality, is nevertheless capable of being distinguished from the quantity to which it is added. Wherefore if charity be added to charity we must presuppose the distinction, at least logical, of the one charity from the other.
Now distinction among forms is twofold: specific and numeric. Specific distinction of habits follows diversity of objects, while numeric distinction follows distinction of subjects. Consequently a habit may receive increase through extending to objects to which it did not extend before: thus the science of geometry increases in one who acquires knowledge of geometrical matters which he ignored hitherto. But this cannot be said of charity, for even the slightest charity extends to all that we have to love by charity. Hence the addition which causes an increase of charity cannot be understood, as though the added charity were presupposed to be distinct specifically from that to which it is added.
It follows therefore that if charity be added to charity, we must presuppose a numerical distinction between them, which follows a distinction of subjects: thus whiteness receives an increase when one white thing is added to another, although such an increase does not make a thing whiter. This, however, does not apply to the case in point, since the subject of charity is none other than the rational mind, so that such like an increase of charity could only take place by one rational mind being added to another; which is impossible. Moreover, even if it were possible, the result would be a greater lover, but not a more loving one. It follows, therefore, that charity can by no means increase by addition of charity to charity, as some have held to be the case.
Accordingly charity increases only by its subject partaking of charity more and more subject thereto. For this is the proper mode of increase in a form that is intensified, since the being of such a form consists wholly in its adhering to its subject. Consequently, since the magnitude of a thing follows on its being, to say that a form is greater is the same as to say that it is more in its subject, and not that another form is added to it: for this would be the case if the form, of itself, had any quantity, and not in comparison with its subject. Therefore charity increases by being intensified in its subject, and this is for charity to increase in its essence; and not by charity being added to charity.
Reply to Objection 1. Bodily quantity has something as quantity, and something else, in so far as it is an accidental form. As quantity, it is distinguishable in respect of position or number, and in this way we have the increase of magnitude by addition, as may be seen in animals. But in so far as it is an accidental form, it is distinguishable only in respect of its subject, and in this way it has its proper increase, like other accidental forms, by way of intensity in its subject, for instance in things subject to rarefaction, as is proved in Phys. iv, 9. On like manner science, as a habit, has its quantity from its objects, and accordingly it increases by addition, when a man knows more things; and again, as an accidental form, it has a certain quantity through being in its subject, and in this way it increase in a man who knows the same scientific truths with greater certainty now than before. On the same way charity has a twofold quantity; but with regard to that which it has from its object, it does not increase, as stated above: hence it follows that it increases solely by being intensified.
Reply to Objection 2. The addition of light to light can be understood through the light being intensified in the air on account of there being several luminaries giving light: but this distinction does not apply to the case in point, since there is but one luminary shedding forth the light of charity.
Reply to Objection 3. The infusion of charity denotes a change to the state of "having" charity from the state of "not having it," so that something must needs come which was not there before. On the other hand, the increase of charity denotes a change to "more having" from "less having," so that there is need, not for anything to be there that was not there before, but for something to be more there that previously was less there. This is what God does when He increases charity, that is He makes it to have a greater hold on the soul, and the likeness of the Holy Ghost to be more perfectly participated by the soul.
Source: Summa Theologica (New Advent)