Objection 1. It would seem that habit is not a distinct species of quality. Because, as we have said (1), habit, in so far as it is a quality, is "a disposition whereby that which is disposed is disposed well or ill." But this happens in regard to any quality: for a thing happens to be well or ill disposed in regard also to shape, and in like manner, in regard to heat and cold, and in regard to all such things. Therefore habit is not a distinct species of quality.
Objection 2. Further, the Philosopher says in the Book of the Predicaments (Categor. vi), that heat and cold are dispositions or habits, just as sickness and health. Therefore habit or disposition is not distinct from the other species of quality.
Objection 3. Further, "difficult to change" is not a difference belonging to the predicament of quality, but rather to movement or passion. Now, no genus should be contracted to a species by a difference of another genus; but "differences should be proper to a genus," as the Philosopher says in Metaph. vii, text. 42. Therefore, since habit is "a quality difficult to change," it seems not to be a distinct species of quality.
On the contrary, The Philosopher says in the Book of the Predicaments (Categor. vi) that "one species of quality is habit and disposition."
I answer that, The Philosopher in the Book of Predicaments (Categor. vi) reckons disposition and habit as the first species of quality. Now Simplicius, in his Commentary on the Predicaments, explains the difference of these species as follows. He says "that some qualities are natural, and are in their subject in virtue of its nature, and are always there: but some are adventitious, being caused from without, and these can be lost. Now the latter," i.e. those which are adventitious, "are habits and dispositions, differing in the point of being easily or difficultly lost. As to natural qualities, some regard a thing in the point of its being in a state of potentiality; and thus we have the second species of quality: while others regard a thing which is in act; and this either deeply rooted therein or only on its surface. If deeply rooted, we have the third species of quality: if on the surface, we have the fourth species of quality, as shape, and form which is the shape of an animated being." But this distinction of the species of quality seems unsuitable. For there are many shapes, and passion-like qualities, which are not natural but adventitious: and there are also many dispositions which are not adventitious but natural, as health, beauty, and the like. Moreover, it does not suit the order of the species, since that which is the more natural is always first.
Therefore we must explain otherwise the distinction of dispositions and habits from other qualities. For quality, properly speaking, implies a certain mode of substance. Now mode, as Augustine says (Gen. ad lit. iv, 3), "is that which a measure determines": wherefore it implies a certain determination according to a certain measure. Therefore, just as that in accordance with which the material potentiality [potentia materiae] is determined to its substantial being, is called quality, which is a difference affecting the substance, so that, in accordance with the potentiality of the subject is determined to its accidental being, is called an accidental quality, which is also a kind of difference, as is clear from the Philosopher (Metaph. v, text. 19).
Now the mode of determination of the subject to accidental being may be taken in regard to the very nature of the subject, or in regard to action, and passion resulting from its natural principles, which are matter and form; or again in regard to quantity. If we take the mode or determination of the subject in regard to quantity, we shall then have the fourth species of quality. And because quantity, considered in itself, is devoid of movement, and does not imply the notion of good or evil, so it does not concern the fourth species of quality whether a thing be well or ill disposed, nor quickly or slowly transitory.
But the mode of determination of the subject, in regard to action or passion, is considered in the second and third species of quality. And therefore in both, we take into account whether a thing be done with ease or difficulty; whether it be transitory or lasting. But in them, we do not consider anything pertaining to the notion of good or evil: because movements and passions have not the aspect of an end, whereas good and evil are said in respect of an end.
On the other hand, the mode or determination of the subject, in regard to the nature of the thing, belongs to the first species of quality, which is habit and disposition: for the Philosopher says (Phys. vii, text. 17), when speaking of habits of the soul and of the body, that they are "dispositions of the perfect to the best; and by perfect I mean that which is disposed in accordance with its nature." And since the form itself and the nature of a thing is the end and the cause why a thing is made (Phys. ii, text. 25), therefore in the first species we consider both evil and good, and also changeableness, whether easy or difficult; inasmuch as a certain nature is the end of generation and movement. And so the Philosopher (Metaph. v, text. 25) defines habit, a "disposition whereby someone is disposed, well or ill"; and in Ethic. ii, 4, he says that by "habits we are directed well or ill in reference to the passions." For when the mode is suitable to the thing's nature, it has the aspect of good: and when it is unsuitable, it has the aspect of evil. And since nature is the first object of consideration in anything, for this reason habit is reckoned as the first species of quality.
Reply to Objection 1. Disposition implies a certain order, as stated above (1, ad 3). Wherefore a man is not said to be disposed by some quality except in relation to something else. And if we add "well or ill," which belongs to the essential notion of habit, we must consider the quality's relation to the nature, which is the end. So in regard to shape, or heat, or cold, a man is not said to be well or ill disposed, except by reason of a relation to the nature of a thing, with regard to its suitability or unsuitability. Consequently even shapes and passion-like qualities, in so far as they are considered to be suitable or unsuitable to the nature of a thing, belong to habits or dispositions: for shape and color, according to their suitability to the nature of thing, concern beauty; while heat and cold, according to their suitability to the nature of a thing, concern health. And in this way heat and cold are put, by the Philosopher, in the first species of quality.
Wherefore it is clear how to answer the second objection: though some give another solution, as Simplicius says in his Commentary on the Predicaments.
Reply to Objection 3. This difference, "difficult to change," does not distinguish habit from the other species of quality, but from disposition. Now disposition may be taken in two ways; in one way, as the genus of habit, for disposition is included in the definition of habit (Metaph. v, text. 25): in another way, according as it is divided against habit. Again, disposition, properly so called, can be divided against habit in two ways: first, as perfect and imperfect within the same species; and thus we call it a disposition, retaining the name of the genus, when it is had imperfectly, so as to be easily lost: whereas we call it a habit, when it is had perfectly, so as not to be lost easily. And thus a disposition becomes a habit, just as a boy becomes a man. Secondly, they may be distinguished as diverse species of the one subaltern genus: so that we call dispositions, those qualities of the first species, which by reason of their very nature are easily lost, because they have changeable causes; e.g. sickness and health: whereas we call habits those qualities which, by reason of their very nature, are not easily changed, in that they have unchangeable causes, e.g. sciences and virtues. And in this sense, disposition does not become habit. The latter explanation seems more in keeping with the intention of Aristotle: for in order to confirm this distinction he adduces the common mode of speaking, according to which, when a quality is, by reason of its nature, easily changeable, and, through some accident, becomes difficultly changeable, then it is called a habit: while the contrary happens in regard to qualities, by reason of their nature, difficultly changeable: for supposing a man to have a science imperfectly, so as to be liable to lose it easily, we say that he is disposed to that science, rather than that he has the science. From this it is clear that the word "habit" implies a certain lastingness: while the word "disposition" does not.
Nor does it matter that thus to be easy and difficult to change are specific differences (of a quality), although they belong to passion and movement, and not the genus of quality. For these differences, though apparently accidental to quality, nevertheless designate differences which are proper and essential to quality. In the same way, in the genus of substance we often take accidental instead of substantial differences, in so far as by the former, essential principles are designated.
Source: Summa Theologica (New Advent)