After he discusses the causes of habitudes, St. Thomas goes into the important question of the intension and remission of habitude. The importance of this question cannot be overestimated. In working out intension and remission of habitudes, scholastics tried to clarify the matter by expanding their view to the intension and remission of dispositions generally, and it is out of this that the late medieval scholastics discovered the Mean Speed Theorem, the Calculatores first began applying crude geometrical tools to explore how acceleration works, and the first tentative shifts away from Aristotelian physics began to pick up steam so that the early modern experimental revolution should begin to happen. The topic that particularly precipitated all of this, however, was originally not a physical one but an extremely important one for spiritual life: the increase and decrease of the virtue of charity. Thus it is quite important. It also leads to some relatively technical discussions. So I've decided to do here what I did with the topic of the subject of habitudes: I'll mix commentary and selective translation.
Increase and Decrease of Habitude
The first question that has to be asked, of course, is whether habitude admits of intension and remission. Intension is a kind of increase, and remission a kind of decrease, and we mostly associate increase and decrease with quantity. However, says Aquinas, we transfer the idea to capture that is true about quality: that just as there is a kind of distinguishable completeness that is tracked by quantitative increase, so there is a kind of distinguishable completeness for quality. We can recognize greatness not just of (say) size, but also goodness. The quantitative analogue is exactly that, only an analogue, but the analogy can sometimes be quite tight. This is a point that distinguishes both quality and quantity from substance, for instance. Not all qualities have this feature, however; it is a feature of quality that arises from how the quality relates to other things. As St. Thomas says:
If any form, or anything whatsoever, gets the notion of the species from itself or from something of itself, it has a determinate notion, which is able neither to exceed by more nor to fail by less, and such are hotness and whiteness, and other suchlike qualities, which are not said by ordering to another, and even more so substance, which is being per se. But those which receive their species from something to which they are ordered can be diversified in themselves by more or by less, and nonetheless be the same species, according to the unity of that to which they are ordered, from which they receive their species. [ST 2-1.52.1]
Health, for instance, varies according to more and less, because it is a disposition that concerns something other than itself, and can be related to it in various ways (generally by various excesses and deficiencies) while still being health. If we only called health what was completely healthy, then there would be no increase or decrease of health, by definition; but 'health' would then be the maximum, or the most perfect balance, of something that did admit of more and less.
This, however, is only one of the ways a disposition can increase or decrease, namely, by the very nature of its form as related to other things. Dispositions can also vary by how their subjects participate that form. If a form consitutes the very species of a thing, then that thing does not have a participation that admits of more and less; this is the case with substantial forms. This is also with quantitative forms or qualitative forms like shapes that are derive very closely from substances and quantities, because they are not just divisible in a way that admits of more and less. Actions that are more associated with actions and passions, however, are 'farther away' from substance and quantity; their subjects may participate them to a greater or lesser degree.
Thus habitudes may increase or decrease (1) in themselves or (2) according to participation by subject.
The Manner of the Increase and Decrease
This increase and decrease, however, that we find in intension and remission of qualities, cannot be by addition (which would effectively make it reducible to quantity. If something is more intensely Q, this is not the same as having more of Q. As Aquinas likes to put it, more and less white is not the same as larger and smaller white. If we consider intension and remission of the quality in itself, any addition or subtraction would actually change the kind of thing we are talking about; we would have a new thing that was not the previous quality. This doesn't rule out there being a kind of addition or subtraction for quality. You can for instance, know more or fewer things just as you can know them more or less well. But this is not guaranteed either; Aquinas points out that bodily habitudes like health, while admitting greater and lesser degree, do not themselves admit of larger and smaller amounts, at least if we're not just using a metaphor.
We've seen, however, that habitudes can be caused by multiplication of acts, and so we can ask if they are increased in some kind of one-to-one way with those acts. Aquinas's answer is interesting:
Because the use of habitudes consists in human willing, as is obvious from what was said above, then as one who has the habitude might not use it, or even act contrarily to it, so also can it happen that the habitude is used according to an act not proportionally corresponding to the intensity of the habitude. Thus if the intension of the act is proportionally equated to the intension of the habitude, or even exceeds it, then each act either increases the habitude or disposes to its increase, so that we may speak of the increase of the habitudes on a similarity to animal increase. For not all food taken in actually increases the animal, as not every drop hollows out a stone, but food being multiplied eventually makes an increase. So also, with multiplication of acts, the habitude grows. But if the intension of the act proportionally falls short of the intension of the habitude, such an act does not dispose to the increase of the habitude, but rather to its decrease. [ST 2-1.52.3]
'Intension' could also be translated as 'intensity'. Thus, for instance, if we have a virtue, let's say generosity, that is of such-and-such intensity, acts of generosity that are less intense than that will eventually reduce the intensity of the generosity. To increase in virtue, or knowledge, or such, the intensity of the acts matters. And much the same is true of remission or decrease, mutatis mutandis.
Corruption of Habitude
Forms perish, or are corrupted, either by their contraries or the corruption of their subjects. Your health can break down either by a sickness being introduced or you dying. In an incorruptible subject, of course, the latter sort of loss of form cannot occur. What this means is that whether or not a habitude can be lost simply depends, in the case of corruption by subject, on the corruptibility of their subject; habitudes depend for their existence on the existence of their subjects.
Corruption by contrary is a somewhat more complicated matter. It of course depends first and foremost on whether the habitude has a contrary. Intelligible species in the agent or potential intellect do not have a contrary, so any intelligible species caused in the latter by the former is incorruptible. Examples of this are first principles, both of the theoretical and of the practical intellect, "which by no oblivion or deception are able to be corrupted" (ST 2-1.53.1). However, habitudes concerned with conclusions do admit of contraries, either because they depend on assumptions that are not necessarily known, or because false reasoning can lead to a different conclusion. So habitudes like knowledge or opinion are corruptible and can be lost. Moral virtues likewise can be lost, as we know all too well, because they presuppose the movement of reason, and so they can be erased, whether through ignorance or the influence of the passions or deliberate choices; fortunately, moral vices can also be lost, for essentially the same reason.
In some cases habitudes can be lost not merely by acts themselves that are contrary, but simply through the cessation of some sustaining act. This occurs when the action is removing some impediment to the habitude, and therefore removing the action results in an external contrary being imposed. This is especially true in the case of both moral virtues and intellectual virtues, which begin to erode if you stop using them. You eventually stop knowing things that you don't actively know; you eventually lose opinions just from not doing anything with them; you eventually stop being honest by no longer doing honest things. How quickly this happens, of course, depends on the opposing forces and the extent of one's exposure to them.
