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 (Q. 66, A. 1) how it is with the virtues.
Reply Obj. 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 Obj. 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 Obj. 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). ________________________
SECOND ARTICLE [I-II, Q. 52, Art. 2]
Whether Habit Increases by Addition?
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.