To the fourth one proceeds thus. It seems that it is not necessary that there be habitudes. For habitude is that by which something is disposed well or badly to something, as was said. But something is disposed well or badly by its form, for something is good according to its form, as also being. Therefore it is not a necessity that there is habitude.
Further, habitude involves order to act. But power sufficiently involves principle of act, for even natural powers without habitudes are principles for act. Therefore it is not necessary that there be habitude.
Further, just as power has itself toward good and bad, so also habitude, and just as power does not always act, so neither does habitude. Powers existing, therefore, it is superfluous that there be habitude.
But contrariwise is that habitudes are sorts of completions, as is said in Phys. VII, but completion is maximally necessary for a thing, because it has the notion of an end. Therefore it is necessary that there be habitude.
I reply that it must be said that, just as it was said above, that habitude involves a sort of disposition in order to the nature of a thing, and to its operation or end, according to which something is disposed well or badly to it. But to this, that something needs to be disposed to another, three things are required.
(1) First, that what is disposed be other than that to which it is disposed, and thus that it have itself to it as potential to actual. Thus if there is something whose nature is not composed of potential and actual, and whose substance is its working, so that it is for itself, then habitude or disposition has no place, as is clear with God.
(2) Second, it is required that what is potential to another is determined in many ways and to diverse things. Thus if something is potential to another, but in such a way that it is not potential except to the same thing, then disposition and habitude have no place, because such subject from its nature has due having [habitudinem] to such act. Thus if heavenly body is composed from matter and form, then because that matter is not potential to another form, as was said in the first place, then disposition or habitude to form, or even to working, has no place there, because the nature of heavenly body is not potential except to one determinate change.
(3) Third, it is required that several things, which are able to be commensurated in diverse ways, concur to disposing the subject to one of the things to which it is disposed, so that it is disposed well or badly to form or to working. Thus the simple qualities of the elements, which concur [conveniunt] in one determinate way to the natures of the elements, we do not call dispositions or habitudes, but simple qualities; but we call dispositions or habitudes health, or beauty, or suchlike, which involve a sort of commensuration of several things that can be commensurated in diverse ways. Because of this, the Philosopher says in Metaphys. V, that habitude is disposition, and disposition is order of what has parts either according to place or according to power or according to species; as was said above.
Therefore because there are many beings to whose natures and workings it is necessary for several things to concur that can be commensurated in diverse ways, it is therefore necessary that there be habitudes.
Therefore to the first it must be said that the nature of a thing is completed by form, but it is necessary that in order to the form the subject be disposed by some disposition. But the form itself is further ordered to working, which is either an end or a way to an end. And if a form has only one determinate such working, no other disposition is required for the working, beyond the form itself. But if it is a form of such kind that it can work in diverse ways, as is a soul, it must be disposed to its workings by some habitudes.
To the second it must be said that power sometimes has itself toward many things, and then it must be determined by something other; but if there is some power that does not have itself toward many things, it does not need a determining habitude, as was said. And because of this natural forces do not enact their workings by way of some habitudes, because according to themselves they are determinate to one.
To the third it must be said that it is not the same habitude that has itself toward good and bad, as will be clear below, but the same power has itself toward good and bad. And therefore habitudes are necessary so that the powers may be determined to good.
[Thomas Aquinas, ST 2-1.49.4, my translation. The Latin is here, the Dominican Fathers translation is here.]
Thus the definition of habitude that we have at this point is something like, Habitude is an acquired quality ordering the nature of some subject (either directly or by way of some power) either well or badly to some specific act (either form or operation) by mutually disposing several things to one out of several possibilities. In this definition, accidental quality involving order to act, seems to be the primarily formal cause; the act seems the final cause; the quasi-material cause seems to be the several things disposed; and the quasi-efficient cause the subject (as the principle of the accidental quality). But there are several ambiguities and thus parts that are still unclear. Note for instance, that this argument tells us that the following things don't need habitudes:
(a) simple things, like God
(b) things determinate to one action, like celestial bodies
(c) things where action does not arise from several mutually adjustable things, like natural forces or simple elemental qualities
But it's not clear that this is exhaustive, and there are several other questions Aquinas will have to answer to clarify the matter. Angels, for instance, are simple, but are they simple enough? (Aquinas will argue that they are not; angels also require habitudes, although in a different way than we do.) Beasts and plants can be classed as 'determinate to one' or not 'determinate to one', depending on how strictly we take that. Do they have habitudes? (This is a more complicated question; very briefly, Aquinas will say that plants don't, and beasts only incompletely sometimes, if we are talking about habitude in strict sense.) And what about cases, like the intellect, the will, or, for that matter, angels again, in which we don't have parts in the ordinary sense? (Aquinas will argue that integral parts are not necessary; potential parts are sufficient.)
