Part 1 is here, and part 2 is here.
In this part of my discussion on the relation of love and unity, I focus on St. Thomas Aquinas's distinction between two different kinds of love, and the types of unity to which they are ordered.
Two Types of Love
In his answer to the question "whether in God there is love", Aquinas writes the following:
An act of love always tends towards two things; to the good that one wills, and to the person for whom one wills it: since to love a person is to wish that person good. Hence, inasmuch as we love ourselves, we wish ourselves good; and, so far as possible, union with that good. So love is called the unitive force, even in God, yet without implying composition; for the good that He wills for Himself, is no other than Himself, Who is good by His essence, as above shown (6, 1, 3). And by the fact that anyone loves another, he wills good to that other. Thus he puts the other, as it were, in the place of himself; and regards the good done to him as done to himself. So far love is a binding force [vis concretiva], since it aggregates another to ourselves [quia alium aggregat sibi], and refers his good to our own. And then again the divine love is a binding force, inasmuch as God wills good to others; yet it implies no composition in God. (ST I Q.20 a.1 ad.3)
This consideration of the nature of love reveals the basis for Aquinas's distinction between love of friendship and love of concupiscence (amorem amicitiae et amorem concupiscentiae). (ST I-II Q.26 a.4) First he refers to Aristotle's statement that "to love is to wish good to someone." From this Aquinas draws the conclusion that love by its very nature has a twofold tendency: "towards the good which a man wishes to someone (to himself or to another) and towards that to which he wishes some good." This twofold tendency intrinsic to love makes possible the difference between love of friendship and love of concupiscence. He explains the difference between these two kinds of love when he says that "man has love of concupiscence towards the good that he wishes to another, and love of friendship towards him to whom he wishes good." These two kinds of love are related as primary and secondary, in this way: "that which is loved with the love of friendship is loved simply and for itself (simpliciter et per se amatur); whereas that which is loved with the love of concupiscence, is loved, not simply and for itself, but for something else."
If we love something in the sense of wishing good to it for its own sake, that is the type of love Aquinas calls 'love of friendship'. But if we love something in the sense of wishing it to be the good of something or someone else, that is the type of love he calls 'love of concupiscence'. For example, if we love an apple, we love it in the sense of wishing it to be a good for someone (either our self or someone else) when it is eaten. The type of love we have for the apple, in that case, is 'love of concupiscence'. But if we love a friend with the 'love of friendship', we love our friend not as a means to a further good to ourselves, but simpliciter et per se, that is, for his own sake, and that is why we wish good to come to him.
Aquinas then adds: "the love with which a thing is loved, that it may have some good, is love simply; while the love, with which a thing is loved, that it may be another's good, is relative love." So love of concupiscence is relative love; it is directed toward a good with the intention that it may be another's good. Love of friendship, by contrast, is love simply; it is directed toward something with the intention that it have some good.
If we are loving something as a good for ourselves, it is plain that such love is unitive in this respect, that we seek to be united to that good such that it becomes our good. In the example of loving the apple, we desire the apple to be united to us through eating the apple, so that it provides the goods of nourishing our body, satisfying our hunger and pleasing our palate. So the type of unity aimed at by the 'love of concupiscence' is the having of the beloved good by the lover.
In what way then, does the love of friendship aim at unity? Given what I have said about the manner in which love of concupiscence is unitive, it may seem initially that, as directed toward other persons, love of friendship is unitive only in the sense that the lover wishes that what is good for his friend be united to his friend. What I wish to show here and in part 4 is how the the love of friendship also seeks union of the lover and the friend.
Unity as a Cause of Love
In order develop this argument, we need to consider the ways in which union is both a cause and an effect of love. First, let us consider briefly how, according to Aquinas, union is a cause of love. According to Aquinas, union is a cause of love in two ways. The type of union that causes the love with which one loves oneself is substantial union. (ST I-II Q.28 a.1 ad.2) This claim has its basis in what Aquinas said earlier, namely, that "everything has this aptitude towards its natural form, that when it has it not, it tends towards it; and when it has it, it is at rest therein. It is the same with every natural perfection, which is a natural good." (ST I Q.19 a.1 co.) And elsewhere he writes, "But it is common to every nature to have some inclination; and this is its natural appetite or love." (ST I Q.60 a.1 co.) The transcendental relation of being, unity and goodness shows that insofar as everything is naturally inclined to its perfection, so it is naturally inclined to its being and unity. Hence Aquinas says elsewhere, "[H]ence it is that everything guards its unity as it guards its being." (Et inde est quod unumquodque, sicut custodit suum esse, ita custodit suam unitatem.)(ST I Q.11 a.1 co.)
The second way in which unity causes love is by unity of likeness. We have seen this already in our discussion of connaturality in Part 2, where we saw that the principle of movement in the appetite is the appetitive subject's connaturality with the thing to which it tends. Connaturality is a kind of likeness of natures. According to Aquinas, there are two types of likeness between things, and each type of likeness causes a different type of love. One type of likeness between two things arises when each thing has the same quality actuality. The other type of likeness between two things arises when one thing has "potentially and by way of inclination, a quality which the other has actually". (ST I-II Q.27 a.3 co.) The first type of likeness, that is, the type of likeness that arises when each thing has the same quality actuality, is love of friendship. Aquinas writes, "For the very fact that two men are alike, having, as it were, one form, makes them to be, in a manner, one in that form: thus two men are one thing in the species of humanity, and two white men are one thing in whiteness. Hence the affections of one tend to the other, as being one with him; and he wishes good to him as to himself. But the second kind of likeness causes love of concupiscence, or friendship founded on usefulness or pleasure: because whatever is in potentiality, as such, has the desire for its act; and it takes pleasure in its realization, if it be a sentient and cognitive being." (ST I-II Q.27 a.3 co.) Thus we can be caused to love another person by likeness in two general ways. Either the other person has actually some good quality that we have potentiality, or the other person has actually some quality that we have actually. In the former case, either we love the other as a means to the actualization of our potentiality, or we love the other as a pleasing subjunctive depiction of what we would be were our potentiality actualized. But in the case where both the lover and the beloved share the likeness of actuality to actuality, that which is like need not be only a quality or set of qualities; it may also be a nature, for example, human nature. Love caused directly by apprehended likeness of this sort is love of friendship.
Two types of Unity
Aquinas teaches that the union of lover and beloved is fundamentally of two sorts. "The first is real union; for instance, when the beloved is present with the lover. The second is union of affection: and this union must be considered in relation to the preceding apprehension; since movement of the appetite follows apprehension." Aquinas explains this when he says, "The first of these unions [i.e. real union] is caused effectively by love; because love moves man to desire and seek the presence of the beloved, as of something suitable and belonging to him (quasi sibi convenientis et ad se pertinentis). The second union is caused formally by love; because love itself is this union or bond. In this sense Augustine says (De Trin. viii, 10) that "love is a vital principle uniting, or seeking to unite two together, the lover, to wit, and the beloved." For in describing it as "uniting" he refers to the union of affection, without which there is no love: and in saying that "it seeks to unite," he refers to real union." (ST I-II Q.28 a.1 co.)
Again, in the reply to the second objection he writes, "There is also a union which is essentially love itself. This union is according to a bond of affection, and is likened to substantial union, inasmuch as the lover stands to the object of his love, as to himself, if it be love of friendship; as to something belonging to himself, if it be love of concupiscence. Again there is a union, which is the effect of love. This is real union, which the lover seeks with the object of his love." (ST I-II Q.28 a.1 ad.2)
In this first article of question 28, Aquinas is making two points. First he is showing that the union of affection is essentially love itself, and this union is likened [assimilatur] to substantial union inasmuch as the lover stands to the beloved as to himself (i.e. in love of friendship), and to a lesser degree if the lover stands to the beloved as one stands toward something belonging to oneself (i.e. in love of concupiscence).
Aquinas's second point is that real union is the effect or goal of love; real union is what the union of affection seeks to bring about. Regarding real union as an effect of love he says, "Moreover this union [real union] is in keeping with the demands of love (convenientiam amoris: for as the Philosopher relates (Polit. ii, 1), "Aristophanes stated that lovers would wish to be united both into one," but since "this would result in either one or both being destroyed," they seek a suitable and becoming union--to live together, speak together, and be united together in other like things." (ST I-II Q.28 a.1 ad.2) In seeking real union love does not seek to destroy its own possibility, but seeks rather that which preserves its own actuality. Both types of love seek real union of the lover with the beloved.
In the middle of this article, Aquinas distinguishes between the two types of unity that are apprehended by the lover, and which give rise to the love of concupiscence and the love of friendship, respectively. He writes, "Now love being twofold, viz. love of concupiscence and love of friendship; each of these arises from a kind of apprehension of the oneness of the thing loved with the lover. For when we love a thing, by desiring it, we apprehend it as belonging to our well-being (quasi sibi convenientis et ad se pertinentis). In like manner when a man loves another with the love of friendship, he wills good to him, just as he wills good to himself: wherefore he apprehends him as his other self (vult ei bonum sicut et sibi vult bonum, unde apprehendit eum ut alterum se), in so far, to wit, as he wills good to him as to himself. Hence a friend is called a man's "other self" (Ethic. ix, 4), and Augustine says (Confess. iv, 6), "Well did one say to his friend: Thou half of my soul." (ST I-II Q.28 a.1 co.)
The kind of unity that when apprehended gives rise to love of concupiscence is one in which the lover stands to the beloved as to something belonging to himself (ut ad aliquid sui). This unity seems to be apprehended as a potential unity. The kind of unity that when apprehended gives rise to love of friendship is one in which the lover stands to the beloved as to himself (ut ad seipsum) [ST I-II Q.28 a.1 ad.2]. This unity seems to be apprehended as an actual unity, though this leaves open the discovery and dynamic creation of further actual unity, as well as the apprehension of potential unity.
So to summarize, first, substantial unity gives rise to self love. Second, there are two types of apprehended unity that give rise to the two types of love. These two types of apprehended unity are an accidental unity (i.e. of having or possessing a good for oneself) and a formal subjective unity of being another self. They give rise to love of concupiscence and love of friendship, respectively. In the former, the union aimed at by the appetite is the union of possessing the good loved, such that this good actualizes the lover's potential and thus perfects the lover. In the love of friendship, by contrast, the union aimed at by the appetite is a union of subject with subject, and this grounds the love of concupiscence towards the good that the lover wishes to his friend. The object of the love of friendship is not loved as a means to one's own perfection, but as another self. This is primarily a spiritual union of a self with another self, an outward extension of the love that one has for oneself to that which is another self. But the beloved as an object of the 'love of friendship' does not cease to be apprehended as a good any more than in self-love the self ceases to be apprehended as a good. On the contrary, just as the self is apprehended as a qualitatively greater good than all those goods perfecting of the self, so likewise the beloved as an object of the 'love of friendship' is apprehended as a qualitatively greater good than all those goods perfecting either oneself or the beloved (God excepted).
In the next part in this series I will examine the three ways in which love effects mutual indwelling, focusing most especially on the third way. I will be focusing particularly on the way in which love of friendship seeks not only that what is good for one's friend be united to one's friend, but also seeks the union of the lover with the friend. For this I will draw on Aquinas's commentary on chapter four of book nine of Aristotle's Nicomachean Ethics.