Call of Cthulhu, a tabletop role-playing game based on the work of HP Lovecraft, includes in its rules a system called “sanity points”. In most other role-playing games, such as Dungeons and Dragons, you become more resilient with each adventure. But in Call of Cthulhu, you can be driven out of your wits by your experiences. According to the original rulebook, published in 1981, sanity points, once lost, can never be regained. Later editions permit you to try psychoanalysis.

The sanity points system is an adaptation of one of the quintessential premises of Lovecraft’s fiction: that sanity is not a fixed personal attribute but rather a scarce resource that must be carefully guarded. “Life is a hideous thing,” he writes in one story, “and from the background behind what we know of it peer daemonical hints of truth which make it sometimes a thousandfold more hideous … Its reserve of unguessed horrors could never be borne by mortal brains.” Many of his protagonists end up in asylums. Danforth, one of the Antarctic explorers in At the Mountains of Madness, glimpses an ancient monstrosity out of an aeroplane window and suffers a breakdown in which “his shrieks were confined to the repetition of a single, mad word … ‘Tekeli-li! Tekeli-li!’”

That, of course, is how mad people in books tend to behave. In Ford Madox Ford’s The Good Soldier – my favourite novel, celebrating its centenary this year – a character called Nancy Rufford meets a similar fate after she learns of a loved one’s suicide. “In a darkened room, my poor girl, sitting motionless, with her wonderful hair about her, looking at me with eyes that did not see me, and saying distinctly: ‘Credo in unum Deum omnipotentem.... Credo in unum Deum omnipotentem’ ... those are the only words, it appears, that she ever will utter.” Like Danforth, she has used up all her sanity points.

In fact, all the characters in The Good Soldier seem to be playing by Call of Cthulhu’s system. “Unguessed horrors” are an ever-present threat. If they don’t drive you insane, they’ll give you a heart attack. The American narrator, John Dowell, is told by doctors that if his wife, Florence, “became excited over anything or if her emotions were really stirred her little heart might cease to beat. For 12 years I had to watch every word that any person uttered in any conversation and I had to head it off what the English call ‘things’ – off love, poverty, crime, religion and the rest of it.” Mere attention, mere sensation, is enough to end you. No violence is necessary. It is as if the world is full of basilisks and Medusas.

Much like many of Lovecraft’s stories, The Good Soldier is narrated by a survivor – broken, faithless, sitting alone in a dark house, unable to comprehend what he has seen. “I know nothing – nothing in the world – of the hearts of men,” Dowell says. “I only know that I am alone – horribly alone. No hearthstone will ever again witness, for me, friendly intercourse. No smoking room will ever be other than peopled with incalculable simulacra amidst smoke wreaths.” Here, for comparison, is the narrator of Lovecraft’s original “The Call of Cthulhu”, published 11 years later: “I have looked upon all that the universe has to hold of horror, and even the skies of spring and the flowers of summer must ever afterward be poison to me.”

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Ford and Lovecraft are not often discussed in the same breath. But in fact they are very similar. The difference is that Lovecraft appears to be writing about cosmic horror but is really writing about sex, whereas Ford appears to be writing about sex but is really writing about cosmic horror. Another way of putting it is that they are writing about exactly the same thing: the feeling that if you peel back the skin of everyday reality, what you will see underneath is something so alien that it will burn away all your sanity points in an instant.

What has driven Nancy Spufford to madness – and what has also driven Dowell, if not to madness, at least to distraction – is finding out what life is really like at the extremes. Nancy’s beloved guardian, Edward Ashburnham, has cut his throat out of despair. Dowell’s wife Florence has turned out to have been sleeping with Ashburnham, Dowell’s best and only friend, for nine years. Neither of our two innocents ever suspected that any such dreadfulness was possible. “If everything is so nebulous about a matter so elementary as the morals of sex,” says Dowell, baffled and despairing, “what is there to guide us in the more subtle morality of all other personal contacts, associations and activities? Or are we meant to act on impulse alone? It is all a darkness.”

A darkness that a hundred years of sexual liberation has scarcely illuminated. One of the reasons why The Good Soldier is such a timeless book is that nearly all of us have Dowell’s experience at least once: when we first discover, probably by accident and in the most bathetic circumstances, that while we have been playing the game of sex, as it were, according to the original rulebook, some of those close to us seem to have been using a newer edition in which very different tactics are permitted – that we are all of a sudden in a world of realpolitik, krav maga, free-market economics, and our old assumptions are now very much out of date. We are righteously indignant and painfully envious at the same time. And we hurry to catch up. Because we still have time. Unlike the 46-year-old Dowell. For him, it’s too late ever to come to terms with sex.

You expect a boring novel about Edwardian marri­ages. But The Good Soldier is a gothic melodrama with a high body count

Actually Ford seems to have been pretty relaxed about what he once wryly called “the one unmentionable thing”. In terms of personality type, the better comparison is between Lovecraft, the real person, and Dowell, the fictional character. Stephen King has argued that Lovecraft’s stories “are about sex and little else, and when Cthulhu makes one of its appearances... we are witnessing a gigantic, tentacle-equipped, killer vagina from beyond space and time.”

Alan Moore observed in an interview that “Lovecraft was sexually squeamish and would only talk of ‘certain nameless rituals’. Or he’d use some euphemism: ‘blasphemous rites’. It was pretty obvious, given that a lot of his stories detailed the inhuman offspring of these ‘blasphemous rituals’, that sex was probably involved somewhere along the line.” In 2011, Moore published a Lovecraftian comic called Neomonicon, in which an FBI agent investigating a serial killer is captured by a Dagonite cult. Trapped in the sewers of Salem, Massachusetts, she is forced to take part in a horrific sexual Walpurgis. This is precisely John Dowell’s nightmare: that beneath the genteel surface of the Protestant north-east, his friends and neighbours are having orgies day and night.

And yet fear of sex, much as we may mock it, is in some respects an existentialist fear. Sex is a reminder that we are nothing but bodies, nothing but livestock, nothing but atoms in space. In The Good Soldier, one of the prodromes of Nancy’s madness is a revelation she has in front of a fireplace in Ashburnham’s estate that “the burning logs were just logs that were burning and not the comfortable symbols of an indestructible mode of life”. When Dowell is obliged for the first time in his life to imagine his intimates undressed and rutting, he is having a revelation of just the same kind. To quote Lovecraft’s poem “Nemesis”, he has “seen the dark universe yawning / Where the black planets roll without aim / Where they roll in their horror unheeded / Without knowledge or lustre or name”. Sex is a tiding of something much greater. Anarchy in sex, anarchy in the universe.

Facebook Twitter Pinterest ‘Are we meant to act on impulse alone? It is all a darkness’ … Ford Madox Ford. Photograph: Hulton Archive/Getty Images

I probably never would have read The Good Soldier if I hadn’t been given it as a birthday present. Glance at the Penguin Classics version and you expect a boring novel about Edwardian marriages. This could not be more misleading. It’s a gothic melodrama with a high body count. It’s a construction of weird geometry and deep shadow, like the extraterrestrial city of At the Mountains of Madness. Above all, it’s a work of shattering modernist nihilism that could fit on the shelf between Beckett and Céline, and should therefore be repackaged in an edition that intense young men will want to be seen reading in cafes. “Are all men’s lives like the lives of us good people,” Dowell asks, “like the lives of the Ashburnhams, of the Dowells, of the Ruffords – broken, tumultuous, agonied, and unromantic, lives, periods punctuated by screams, by imbecilities, by deaths, by agonies? Who the devil knows?”

Halfway through the novel, we find Nancy’s monstrous father in conversation with “an Italian baron who had had much to do with the Belgian Congo. They must have been talking about the proper treatment of natives, for I heard him say: ‘Oh, hang humanity!’” Naturally, our minds go straight to Heart of Darkness, the 1899 novel by Ford’s great friend and collaborator Joseph Conrad. Marlow, the narrator of that book, admits to his relief when he is so preoccupied by his maritime duties that “the reality – the reality, I tell you – fades. The inner truth is hidden – luckily, luckily. But I felt it all the same; I felt often its mysterious stillness watching me.” Marlow may draw back his “hesitating foot” from this inner truth. But not the ivory trader Kurtz, who lies on his deathbed with a “stare, that could not see the flame of the candle, but was wide enough to embrace the whole universe, piercing enough to penetrate all the hearts that beat in the darkness. He had summed up – he had judged. ‘The horror!’”

Facebook Twitter Pinterest Sexually squeamish … HP Lovecraft. Photograph: Pictorial Press Ltd/Alamy

What Ford means by his Italian baron is that, with all due to respect to Conrad, you don’t need to go all the way to the jungle to find the heart of darkness. You only need to go as far as a spa in Hesse, or a country house in Hampshire – or indeed anywhere there are human beings. In fact, Conrad comes close to acknowledging this with Marlow’s famous observation that London, too, “has been one of the dark places of the earth”, because of course the reader is not so naive as to think that London has stopped being a dark place just because it’s on all the maps now. And then Ford quotes that line right back at Conrad in The Good Soldier, when Dowell suggests that his wife’s attempts to educate Ashburnham in high culture are “Florence clearing up one of the dark places of the earth”. The inner truth, the unguessed horrors, are inside every one of us. Dowell is Kurtz. By an agony of stammering and circumlocution The Good Soldier drags out to 200 pages that final croak: “The horror! The horror!” “Tekeli-li! Tekeli-li!” “Credo in unum Deum omnipotentem … Credo in unum Deum omnipotentem.”