What is the essential division of love? St Thomas presents it as the division between ‘love of friendship’ (amor amicitiae) and ‘love of desire’ (amor concupiscentiae).

As the Philosopher says (Rhet. ii, 4), ‘to love is to wish good to someone.’ Hence the movement of love 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. Accordingly, man has love of concupiscence towards the good that he wishes to another, and love of friendship towards him to whom he wishes good (1a 2ae 26, 4).

This allows St Thomas to solve the problem raised in Plato’s Lysis and only partially answered in books 8 and 9 of the Nicomachean Ethics, of whether likeness or unlikeness is the cause of love. He answers that the cause of love of friendship is an actual likeness between the friends:

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.

By contrast, the love of desire is caused by a potential likeness. It derives from the ‘likeness’ that exists between potentiality and its act; akin to the similarity that exists between a hole in a jig-saw puzzle and the piece that fits into it. Thus, thirst is like the quenching of thirst rather than like, say, the satisfaction of curiosity. And this is only ‘likeness’ in an analogous sense, and indeed includes a contrariety within itself, and so seems to be an explanation of whatever truth there is in the proverb, already found in antiquity, that opposites attract. Thus, a silent person and a loquacious one might be mutually attracted insofar as each best realised the other’s potentiality for conversation.

So, we have two basic forms of love, corresponding to the relations of act-act and potency-act. But this raises the question: is there another basic form, corresponding to the relation of act-potency? That is, if someone has a perfection that he can communicate to another, does that give rise to a third kind of love?

We might think for example of the love of a parent for a little child. The parent may have some ‘love of desire’ for the child, for example, as he thinks of how the child may grow up to shed lustre on the family. And the parent will have the ‘love of friendship’ for the child: because of the similarity between them – the child being, as Aristotle says, ‘a sort of other himself’ – the parent spontaneously wills good for the child for the child’s own sake.

But there seems to be another kind of love, not obviously reducible to the first two, by which the very helplessness of the child endears him to the parent. We might even call this ‘love of endearment’. Older writers might have called it ‘love of condescension’, though the term would be hopeless nowadays. At any rate, it seems to correspond to the relation of act-potency.

And with what kind of love does God love rational creatures when He offers them His friendship? Can it already be with the love of friendship? Yet surely they do not have a similarity to Him sufficient to call forth a love of friendship.

But thou wast cast out upon the face of the earth in the abjection of thy soul, in the day that thou wast born. And passing by thee, I saw that thou wast trodden under foot in thy own blood. and I said to thee when thou wast in thy blood: Live.

So, does this mean that our basic division of love should be into three, and not two?