Objection 1. It would seem that habits cannot increase. For increase concerns quantity (Phys. v, text. 18). But habits are not in the genus quantity, but in that of quality. Therefore there can be no increase of habits.
Objection 2. Further, habit is a perfection (Phys. vii, text. 17,18). But since perfection conveys a notion of end and term, it seems that it cannot be more or less. Therefore a habit cannot increase.
Objection 3. Further, those things which can be more or less are subject to alteration: for that which from being less hot becomes more hot, is said to be altered. But in habits there is no alteration, as is proved in Phys. vii, text. 15,17. Therefore habits cannot increase.
On the contrary, Faith is a habit, and yet it increases: wherefore the disciples said to our Lord (Luke 17:5): "Lord, increase our faith." Therefore habits increase.
I answer that, Increase, like other things pertaining to quantity, is transferred from bodily quantities to intelligible spiritual things, on account of the natural connection of the intellect with corporeal things, which come under the imagination. Now in corporeal quantities, a thing is said to be great, according as it reaches the perfection of quantity due to it; wherefore a certain quantity is reputed great in man, which is not reputed great in an elephant. And so also in forms, we say a thing is great because it is perfect. And since good has the nature of perfection, therefore "in things which are great, but not in quantity, to be greater is the same as to be better," as Augustine says (De Trin. vi, 8).
Now the perfection of a form may be considered in two ways: first, in respect of the form itself: secondly, in respect of the participation of the form by its subject. In so far as we consider the perfections of a form in respect of the form itself, thus the form is said to be "little" or "great": for instance great or little health or science. But in so far as we consider the perfection of a form in respect of the participation thereof by the subject, it is said to be "more" or "less": for instance more or less white or healthy. Now this distinction is not to be understood as implying that the form has a being outside its matter or subject, but that it is one thing to consider the form according to its specific nature, and another to consider it in respect of its participation by a subject.
In this way, then, there were four opinions among philosophers concerning intensity and remission of habits and forms, as Simplicius relates in his Commentary on the Predicaments. For Plotinus and the other Platonists held that qualities and habits themselves were susceptible of more or less, for the reason that they were material and so had a certain want of definiteness, on account of the infinity of matter. Others, on the contrary, held that qualities and habits of themselves were not susceptible of more or less; but that the things affected by them [qualia] are said to be more or less, in respect of the participation of the subject: that, for instance, justice is not more or less, but the just thing. Aristotle alludes to this opinion in the Predicaments (Categor. vi). The third opinion was that of the Stoics, and lies between the two preceding opinions. For they held that some habits are of themselves susceptible of more and less, for instance, the arts; and that some are not, as the virtues. The fourth opinion was held by some who said that qualities and immaterial forms are not susceptible of more or less, but that material forms are.
In order that the truth in this matter be made clear, we must observe that, in respect of which a thing receives its species, must be something fixed and stationary, and as it were indivisible: for whatever attains to that thing, is contained under the species, and whatever recedes from it more or less, belongs to another species, more or less perfect. Wherefore, the Philosopher says (Metaph. viii, text. 10) that species of things are like numbers, in which addition or subtraction changes the species. If, therefore, a form, or anything at all, receives its specific nature in respect of itself, or in respect of something belonging to it, it is necessary that, considered in itself, it be something of a definite nature, which can be neither more nor less. Such are heat, whiteness or other like qualities which are not denominated from a relation to something else: and much more so, substance, which is "per se" being. But those things which receive their species from something to which they are related, can be diversified, in respect of themselves, according to more or less: and nonetheless they remain in the same species, on account of the oneness of that to which they are related, and from which they receive their species. For example, movement is in itself more intense or more remiss: and yet it remains in the same species, on account of the oneness of the term by which it is specified. We may observe the same thing in health; for a body attains to the nature of health, according as it has a disposition suitable to an animal's nature, to which various dispositions may be suitable; which disposition is therefore variable as regards more or less, and withal the nature of health remains. Whence the Philosopher says (Ethic. x, 2,3): "Health itself may be more or less: for the measure is not the same in all, nor is it always the same in one individual; but down to a certain point it may decrease and still remain health."
Now these various dispositions and measures of health are by way of excess and defect: wherefore if the name of health were given to the most perfect measure, then we should not speak of health as greater or less. Thus therefore it is clear how a quality or form may increase or decrease of itself, and how it cannot.
But if we consider a quality or form in respect of its participation by the subject, thus again we find that some qualities and forms are susceptible of more or less, and some not. Now Simplicius assigns the cause of this diversity to the fact that substance in itself cannot be susceptible of more or less, because it is "per se" being. And therefore every form which is participated substantially by its subject, cannot vary in intensity and remission: wherefore in the genus of substance nothing is said to be more or less. And because quantity is nigh to substance, and because shape follows on quantity, therefore is it that neither in these can there be such a thing as more or less. Whence the Philosopher says (Phys. vii, text. 15) that when a thing receives form and shape, it is not said to be altered, but to be made. But other qualities which are further removed from quantity, and are connected with passions and actions, are susceptible of more or less, in respect of their participation by the subject.
Now it is possible to explain yet further the reason of this diversity. For, as we have said, that from which a thing receives its species must remain indivisibly fixed and constant in something indivisible. Wherefore in two ways it may happen that a form cannot be participated more or less. First because the participator has its species in respect of that form. And for this reason no substantial form is participated more or less. Wherefore the Philosopher says (Metaph. viii, text. 10) that, "as a number cannot be more or less, so neither can that which is in the species of substance," that is, in respect of its participation of the specific form: "but in so far as substance may be with matter," i.e. in respect of material dispositions, "more or less are found in substance."
Secondly this may happen from the fact that the form is essentially indivisible: wherefore if anything participate that form, it must needs participate it in respect of its indivisibility. For this reason we do not speak of the species of number as varying in respect of more or less; because each species thereof is constituted by an indivisible unity. The same is to be said of the species of continuous quantity, which are denominated from numbers, as two-cubits-long, three-cubits-long, and of relations of quantity, as double and treble, and of figures of quantity, as triangle and tetragon.
This same explanation is given by Aristotle in the Predicaments (Categor. vi), where in explaining why figures are not susceptible of more or less, he says: "Things which are given the nature of a triangle or a circle, are accordingly triangles and circles": to wit, because indivisibility is essential to the motion of such, wherefore whatever participates their nature must participate it in its indivisibility.
It is clear, therefore, since we speak of habits and dispositions in respect of a relation to something (Phys. vii, text. 17), that in two ways intensity and remission may be observed in habits and dispositions. First, in respect of the habit itself: thus, for instance, we speak of greater or less health; greater or less science, which extends to more or fewer things. Secondly, in respect of participation by the subject: in so far as equal science or health is participated more in one than in another, according to a diverse aptitude arising either from nature, or from custom. For habit and disposition do not give species to the subject: nor again do they essentially imply indivisibility.
We shall say further on (66, 1) how it is with the virtues.
Reply to Objection 1. As the word "great" is taken from corporeal quantities and applied to the intelligible perfections of forms; so also is the word "growth," the term of which is something great.
Reply to Objection 2. Habit is indeed a perfection, but not a perfection which is the term of its subject; for instance, a term giving the subject its specific being. Nor again does the nature of a habit include the notion of term, as do the species of numbers. Wherefore there is nothing to hinder it from being susceptible of more or less.
Reply to Objection 3. Alteration is primarily indeed in the qualities of the third species; but secondarily it may be in the qualities of the first species: for, supposing an alteration as to hot and cold, there follows in an animal an alteration as to health and sickness. In like manner, if an alteration take place in the passions of the sensitive appetite, or the sensitive powers of apprehension, an alteration follows as to science and virtue (Phys. viii, text. 20).
Source: Summa Theologica (New Advent)