Objection 1. It would seem that the increase of habits is by way of addition. For the word "increase," as we have said, is transferred to forms, from corporeal quantities. But in corporeal quantities there is no increase without addition: wherefore (De Gener. i, text. 31) it is said that "increase is an addition to a magnitude already existing." Therefore in habits also there is no increase without addition.
Objection 2. Further, habit is not increased except by means of some agent. But every agent does something in the passive subject: for instance, that which heats, causes heat in that which is heated. Therefore there is no increase without addition.
Objection 3. Further, as that which is not white, is in potentiality to be white: so that which is less white, is in potentiality to be more white. But that which is not white, is not made white except by the addition of whiteness. Therefore that which is less white, is not made more white, except by an added whiteness.
On the contrary, The Philosopher says (Phys. iv, text. 84): "That which is hot is made hotter, without making, in the matter, something hot, that was not hot, when the thing was less hot." Therefore, in like manner, neither is any addition made in other forms when they increase.
I answer that, The solution of this question depends on what we have said above (Article 1). For we said that increase and decrease in forms which are capable of intensity and remissness, happen in one way not on the part of the very form considered in itself, through the diverse participation thereof by the subject. Wherefore such increase of habits and other forms, is not caused by an addition of form to form; but by the subject participating more or less perfectly, one and the same form. And just as, by an agent which is in act, something is made actually hot, beginning, as it were, to participate a form, not as though the form itself were made, as is proved in Metaph. vii, text. 32, so, by an intense action of the agent, something is made more hot, as it were participating the form more perfectly, not as though something were added to the form.
For if this increase in forms were understood to be by way of addition, this could only be either in the form itself or in the subject. If it be understood of the form itself, it has already been stated (1) that such an addition or subtraction would change the species; even as the species of color is changed when a thing from being pale becomes white. If, on the other hand, this addition be understood as applying to the subject, this could only be either because one part of the subject receives a form which it had not previously (thus we may say cold increases in a man who, after being cold in one part of his body, is cold in several parts), or because some other subject is added sharing in the same form (as when a hot thing is added to another, or one white thing to another). But in either of these two ways we have not a more white or a more hot thing, but a greater white or hot thing.
Since, however, as stated above (Article 1), certain accidents are of themselves susceptible of more or less, in some of these we may find increase by addition. For movement increases by an addition either to the time it lasts, or to the course it follows: and yet the species remains the same on account of the oneness of the term. Yet movement increases the intensity as to participation in its subject: i.e. in so far as the same movement can be executed more or less speedily or readily. In like manner, science can increase in itself by addition; thus when anyone learns several conclusions of geometry, the same specific habit of science increases in that man. Yet a man's science increases, as to the subject's participation thereof, in intensity, in so far as one man is quicker and readier than another in considering the same conclusions.
As to bodily habits, it does not seem very probable that they receive increase by way of addition. For an animal is not said to be simply healthy or beautiful, unless it be such in all its parts. And if it be brought to a more perfect measure, this is the result of a change in the simple qualities, which are not susceptible of increase save in intensity on the part of the subject partaking of them. How this question affects virtues we shall state further on (66, 1).
Reply to Objection 1. Even in bodily bulk increase is twofold. First, by addition of one subject to another; such is the increase of living things. Secondly, by mere intensity, without any addition at all; such is the case with things subject to rarefaction, as is stated in Phys. iv, text. 63.
Reply to Objection 2. The cause that increases a habit, always effects something in the subject, but not a new form. But it causes the subject to partake more perfectly of a pre-existing form, or it makes the form to extend further.
Reply to Objection 3. What is not already white, is potentially white, as not yet possessing the form of whiteness: hence the agent causes a new form in the subject. But that which is less hot or white, is not in potentiality to those forms, since it has them already actually: but it is in potentiality to a perfect mode of participation; and this it receives through the agent's action.
Source: Summa Theologica (New Advent)