From Ron Belgau, at First Things:
In his treatise On Spiritual Friendship, Aelred of Rievaulx, a 12th-century Cistercian abbot, insists that we need to test our beliefs about friendship with Scripture. The treatise is a series of dialogues in which three monks join Aelred to examine their ideas about friendship in light of their Christian faith.
One of the most important passages in the treatise is the discussion of the three kinds of friendship—carnal, worldly, and spiritual—found in Book I, paragraphs 33-49. (This division of different kinds of friendship is not original to Aelred: Aristotle drew similar distinctions in the Nicomachean Ethics, Book 8, chapters 2-6.)
We might think that Aelred is talking about kinds of friendship in the same way that we think of planes, trains, and automobiles as three different kinds of transport vehicles. Although a car is very different from a plane, and both are very different from a train, each is a legitimate kind of vehicle.
This is not Aelred’s idea, however. He thinks that only spiritual friendship represents a true form of friendship. Carnal and worldly friendship are not real friendship, although many think they are. In speaking of different kinds of friendship, then, Aelred means to distinguish between true friendship and two different kinds of false friendship….To make love of God and neighbor our highest good, then, does not involve denying that there is any good in pleasure, any more than loving God above all precludes loving our neighbor. Instead, it involves recognizing each good in its proper place, and never choosing lesser goods in preference to greater goods. In The Abolition of Man, C. S. Lewis writes:
St. Augustine defines virtue as ordo amoris, the ordinate condition of the affections in which every object is accorded that kind or degree of love which is appropriate to it. (cf. De Civ. Dei, xv. 22; ix. 5; xi. 28)
Spiritual friendship involves properly ordered affection. God must be loved above all, and all other loves subordinated to Him. Friends should encourage each other to love God and neighbor rightly, and cherish each others’ love in the degree proper to their situation. Friendship should not be an excuse to avoid duties of charity; Aelred is very clear that we must love everyone, including our enemies (paragraph 32; cf. Matthew 5:44, Luke 6:27-35).