After actions and passions, the principles of human acts must be considered, and first, intrinsic principles, second, extrinsic principles. Now, intrinsic principles are powers and habitudes, but because powers were discussed in the first part, now it remains to consider habitudes. And first, of course, in general, and second, virtues and vices and other habitudes of this sort, which are principles of human acts. Concerning the habitudes in general, four things are to be considered: first, the substance of the habitudes; second, their subject; third, the cause of their generation, growth, and corruption; fourth, their distinction. Regarding the first, there are four things to investigate: first, whether habitude is quality; second, whether it is a determinate species of quality; third, whether habitude involves order to act; fourth, the necessity of habitude.
The first is approached in this way. It seems that habitude is not quality, for Augustine says (LXXXIII Quaest. 73) that this name, habitude, is derived from the verb habere (to have). But to have does not pertain only to quality, but also to other genera, for we are said to have quantity, and money, and other things of this sort. Therefore habitude is not quality.
Further, habitude is placed in one category, as is clear from the book of Categories. But one category is not contained under another. Therefore habitude is not quality.
Further, every habitude is a disposition, as is said in the Categories. But disposition is an order of what has parts, as is said in Metaph. V. But this pertains to the category of posture [situs]. Therefore habitude is not quality.
But contrariwise is what the Philosopher says in the Categories, that habitude is a quality of the difficult to change.
I respond that it must be said that this name, 'habitude' is taken from having, from which the name of habitude derives in two ways: [A] in one way, according as a man or some other thing is having something; [B] in another way, according as some thing is having itself in some way to itself or to something else.
[A] But about the first, it must be considered that having, according as it is said to with respect to whatever is had, is common to different genera; thus the Philosopher puts it in the postpredicaments, which are, to wit, those following on different genera of things, just as are opposites, and prior and posterior, and such like things. But between things that are had, there seems to be this distinction, that (A1) there are some in which there is nothing mediating between haver and what is had, just as there is nothing mediating between subject and quantity or quality. (A2) Then there are those in which something mediates between them, but only a relativity, just as someone is said to have an associate or friend. (A3) And further there are some in which something mediates, not quite as action or passion, but something by way of action and passion, as, for instance, in one adorning and covering and another adorned and covered, and therefore these constitute a special genus of things, namely, the category of habit; thus the Philosopher says in Metaph. V that between having clothing and the clothing that is had is the mediation of habit.
[B] But if we take having in the way something is said to have itself in some way to itself or to something else, because this way of having itself is according to some quality, in this way habitude is a certain quality, of which the Philosopher says in Metaph. V that habitude is said to be a disposition according to which what is disposed is disposed well or badly, and either according to itself or to another, as health is a certain habitude. And thus is the habitude of which we speak now. Wherefore it is to be said that habitude is quality.
To the first therefore it must be said that that objection proceeds from having taken in general, so therefore it is common to many genera, as was said.
To the second it must be said that that reason proceeds from habit insofar as it is understood to be something mediating between having and what is had, so it is therefore a kind of category, as was said.
To the third it must be said that disposition always involves order of something having parts, but this happens three ways, as the Philosopher immediately adds, to wit, either according to place, or according to power, or according to species. In which, as Simplicius says in his Commentary on the Categories, he includes all dispositions, such as corporeal ones, in what he says according to place, and to this pertains the category of posture [situs], which is order of parts in place; but what he says according to power includes those dispositions that are preparatory and not perfectly suitable, such as inchoate knowledge and virtue; but what he says according to species includes perfect dispositions, which are called habitudes, such as complete knowledge and virtue.
[Thomas Aquinas, ST 2-1.49.1, my translation. The Latin is here; the Dominican Fathers translation is here.]