Intension and remission of habitude plays an important role in the theory of virtue, so it's worthwhile to jump over to some of St. Thomas's discussions of intension and remission of virtue.
Intension of Virtues Generally
In considering whether virtues are all equal, he notes that virtues can be greater than other kinds of virtue by being more closely tied to reason, so that prudence, for instance, is the highest moral virtue. However, he goes on to say, we can also find virtues being greater than other virtues when dealing with the same species of virtue:
And so, according to what was said above, when considering the intensions of habitudes, virtue can be greater and lesser in two ways: in one way, according to itself, in another way, on the part of the participating subject. Therefore, if it is considered according to itself, its greatness and smallness is directed according to that to which it extends. But whoever has some virtue, such as temperance, has it inasmuch as it extends to all that to which temperance extends. This does not happen with knowledge or productive skill, for not everyone who is grammatical knows all that pertains to the grammatical. And according to this, the Stoics said well, as Simplicius says in the commentary on the categories, that virtue does not admit of more and less, just like knowledge and productive skill, because the nature of virtue consists in a maximum.
But if virtue is considered on the part of the participating subject, virtue can happen to be greater or lesser, either according to diverse times in the same person, or in diverse human beings. Because in reaching the mean of virtue, which is according to right reason, one is better disposed than another, either because of accustomedness, or because of better disposition of nature, or because of more perspicacious judgment of reason, or even because of greater gift of grace.... [ST 2-1.66.1]
Intension of the Virtue of Charity
Union with God is by virtue of the infused virtue of charity, so as one 'draws nigh' to God, one's charity must increase in intensity. This cannot be by addition, because then it would be a matter of adding new charities on top of distinct old charities; rather, it increases in essence (i.e., being): the person with the virtue of charity participates charity more and more fully.
The spiritual growth of charity is in a way similar to the growth of the body. But bodily growth in animals and plants is not continuous change, that is, such that if something grows so much in so much time, it is necessary that it change proportionally in each temporal part, as happens in place-change, but through some time nature works by disposing to growth and not actually growing anything, and afterwards produces in effect that to which it had been disposed, actually growing the animal or the part. So also not every act of charity actually grows charity, but every act of charity disposes to the growth of charity, inasmuch as from one act of charity a human being is rendered more prompt to act again according to charity, and, ability increasing, a human being breaks out into more fervent act of love, by which he endeavors to advance in charity, and then charity actually grows. [ST 2-2.24.6]
Charity, however, since it is directed to God has no limit; that is, in itself it can grow indefinitely without ever reaching a maximum, and as it grows it gives us the ability to endure even greater charity.
