The Canon: Agape and Eros by Anders Nygren

Agape and Eros was written in the 1930s, just as philosophers were beginning to realise that language doesn't fit transparently over reality, but alters the way reality is conceived. Swedish theologian Anders Nygren commented that numerous books were written on the doctrinal details of Christianity, but few tackled the idea at its heart - love.

Far from being "self-evident and unambiguous", said Nygren, love is a concept that has changed incessantly throughout Christianity's history and is not as such contained in the original Greek text of the New Testament. "Agape", the word most often used there, refers to a particular sort of "unmotivated" love that does not depend on the value of the love object, but creates value in it. St Paul, Christianity's second founder, revolutionised ethics by focusing on agape as the epitome of Christian love. God's selfless, unmotivated agape for humans was manifest in the Crucifixion - no god of Greece or the mystery religions would have descended into flesh or suffering - and is the model for how we should love one another.

But the ethics of the ancient world had been dominated by "eros" (egocentric love awakened by beauty, and seeking to capture its object, sometimes sexually), and mystically inclined Christian thinkers have tended to incorporate eros into the meaning of love. Eros, however, as in Plato's Symposium, rests on an idea of human nature as "at once akin to the Divine and at enmity with it", containing sparks of the divine and striving to purify itself from matter and rise upward. For Nygren, its self-seeking aspirational neediness is at odds with agape's selfless, downward outpouring. What has bedevilled Christianity, he argues, is that thanks to the word "love", these two incommensurable forms of feeling have been confused.

Nygren traces the variety of confusions from St Paul to Martin Luther. He disparages St Augustine's attempt to combine eros and agape in the notion of "caritas" - a synthesis favoured by the Catholic Church and reiterated in an encyclical of 2006 - and applauds Luther for disentangling agape from eros and reinstating its unique centrality.

His complex idea of the body hatred in eros is fascinating and makes sense of the mysterious early Christian doctrine of the resurrection of the body at the Last Judgement. As my enemy's enemy, matter and the body become good; so that even the misogynistic, sex-hating Tertullian held a totus-homo (whole-man) view and a "materialistic doctrine of the soul". Nygren denudes eros of its tantalising glory and makes agape desirable - except that he gives no clue as to how we can actually desire it, how this disinterested selflessness can ever get off the ground, or how self-love (which St Augustine at least caters for) can be abrogated.

Anyone writing on love, religious or secular, since the publication of Agape and Eros has had to address this scholarly analysis. Yet seminal and much-referenced as it has been, it is currently out of print. Perhaps it is Nygren's neglect of the psychology and inspiration of agape that has led to the regrettable neglect of this book.