The Catch-22 that ensures we don’t know why Elena Ferrante chooses to keep her “real” identity a secret because we can’t ask her, or even construct our own theories from extraneous biographical information, isn’t much offset by the explanations she gives in occasional written interviews. Her reasons – she’d prefer to let her work speak for itself, she doesn’t wish to court notoriety, hasn’t she done enough by writing the books in the first place? – are impeccable, impermeable and possibly even true. But, as her quartet of Neapolitan novels translated by Ann Goldstein, now concluded with The Story of the Lost Child, forcefully demonstrates, the truth is often only half the story.

More than two decades ago, Ferrante observed in a letter to her publisher that “I very much love those mysterious volumes, both ancient and modern, that have no definite author”. If that nudges us towards more productive territory, her novels lead us deep into it. The story of childhood friends Elena and Lila – for all that it chronicles a particular stratum of postwar southern Italian society and foregrounds the painful struggle to reconcile motherhood and self-determination within an inherited, rigidly gendered and seemingly invulnerable framework, for all its tentacular reach into radical politics, corruption, organised crime and intellectual elites – is essentially a story of the uncertainty of identity, of competing and contested and spawning narratives. Ferrante is not concerned with establishing a dominant version, or even a polymorphous stew of stories; rather, she brings the whole idea of authorship into question. Who, after all, is the real teller of Elena and Lila’s lives: the woman who writes the words, or the woman she writes them about, and for?

That ambiguity is stitched into the novels, emblematised by the pair’s variant names (Lenuccia, Lenù, Raffaella, Lina), by the tension between Italian and dialect, and by the terrifying, recurrent episodes of dissociation that Lila suffers, and calls “dissolving boundaries”. These are attacks in which her heartbeats become so powerful that they seem “capable of exploding the interlocking solidity of objects”, on one occasion causing her to fear for her baby son: “I have to get him away, she thought; the closer he is to me, the more likely he’ll break.” Lila, elsewhere charismatic, controlling and prideful, experiences her mental energy as potentially destructive, destabilising. But for Elena, it is also generative – and always has been.

In the prologue to My Brilliant Friend, the first book in the series – before we’re plunged into the pair’s childhood, with its vivid vignettes and its atmosphere of fairytale menace – present-day Elena tells us that Lila, now in her 60s, has disappeared without a trace. Elena is not alarmed, because she believes that Lila is simply making good on a long-held promise to absent herself (“The day will come,” she says in the third book, by which time she is expert in the nascent language of computing, “when I reduce myself to diagrams, I’ll become a perforated tape and you won’t find me any more”). But the news prompts Elena to write their story, from the moment in which they fling each other’s dolls, in a mixture of bravado and spite, into a darkened cellar, to their adult lives, when they repeatedly separate and reunite, absorb and repudiate one another.

On a material level, their paths diverge early on: Elena becomes an academic prodigy, and then a published author, physically and emotionally removing herself from Naples and her family. Even when she flouts convention, her story is something of a stereotype, particularly a fictional one; but Lila’s is more resistant to interpretation. Is she subservient to the violent husband she marries early on, or made wretched by the fickle lover she leaves him for, or impoverished by the brutal labour of factory work? Is she the victim of these situations or is she covertly orchestrating them? Is she, as the local gangsters are said to believe, “behind everything”, and “especially” behind Elena herself, manipulating the content of the celebrated portraits of Neapolitan life that she writes?

Elena’s books attempt to decode her friend, to plot the dividing line between them. But she is continually frustrated by the inherent expectations of unity and order that such a task involves: “I’m wrong, I said to myself in confusion, to write as I’ve done until now, recording everything I know. I should write the way she speaks, leave abysses, construct bridges and not finish them, force the reader to establish the flow.”

Lila, who can look at a beautiful night sky and instead smell rotten eggs and taste in her mouth “poisoned egg stars”, who emerges from an earthquake as if “directly from the churning guts of the earth”, is the ultimate disruptor, the insistent voice in Elena’s mind that pits authenticity against artifice and asks, “But if the coherence isn’t there, why pretend?” Or, as Lila has it: “The only problem has always been the disquiet of my mind. I can’t stop it. I always have to do, redo, uncover, reinforce and then suddenly undo, break.”

To escape one’s origins, or to fantasise that one has, is to construct a new version of the self; to remain escaped, that version cannot be assailed too frequently, it must attain some kind of consistency. Elena’s new version, a mixture of herself, her aspiration and what she feels is required of her, is fairly successful: “Every night I improvised successfully, starting from my own experience,” she writes of her public life as an author. But the former self and the past are not inalienable; they continue to exist, for Elena in the shape of Lila, who in this final instalment lures her back to the neighbourhood where they both grew up.

In earlier volumes, as this great psychic drama was being slowly assembled, there remained room for a more inclusive narrative; hence, for example, long and satisfying episodes such as the second book’s summer spent on Ischia, in which the young women compete for the love of romantic hero Nino Sarratore. Here, despite the continuation of various strands – including their serial obsession with Nino, the ascendancy of the local criminals, Elena’s agonised relationship with her own mother and her difficulties with her daughters, and new developments such as the horrifying “story” of the title – Ferrante has moved into more overtly psychological territory. A portrait of the dynamic of a friendship has mutated into a weightier, more uncanny exploration of the antipathy of love, of our compulsion to create one another, over and over again.

Many fans have described the Neapolitan novels as addictive, compulsive; whether that appeal lies more in the simpler pleasures that are an undoubted component of the narrative than in this more sinister aspect remains to be seen. They need time to settle in the imagination. But I am not sure I have read a more frightening account of a friendship, or a more unsentimental view of the uses that human beings have for one another, even in the presence of mutual attachment. “I loved Lila,” as Elena writes. “I wanted her to last. But I wanted it to be I who made her last.”

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