The author of about 500 books, most of them written in less than a fortnight, including nearly 80 Inspector Maigret volumes and over 100 romans durs or “hard novels”, Georges Simenon began keeping notebooks in 1960, when he was nearing 60 and beginning to feel old. The three volumes that are published here run from June of that year up to February 1963. By December 1969, when he wrote the preface to the book, he was able to declare: “I have not felt old for a long time. I no longer feel the need to write in notebooks, and those that I did not use I’ve given to my children.”

Why Simenon wrote the notebooks when he did is not entirely clear. At first, they may have been intended principally as family reading. He writes that he wanted to show his children their father as he really was – an ordinary human being with normal human foibles. He also mentions that he was finding it increasingly difficult to achieve the concentration required to produce his novels: whereas he used to write them (the popular novels, at least) in three or four days, then 12 a year (at the time of the Maigrets), then six a year, “Now it is down to four.” Incredibly, Simenon may have been suffering from a form of writer’s block. Producing the notebooks may have served to distract him from this condition.

But the thoughts recorded here serve another purpose. These notebooks contain his most explicit account of his goals and methods of writing and of the view of human beings that his work expressed. As he puts it:

Like the great naturalists, I would like to focus on certain human mechanisms. Not on grand passions. Not on questions of ethics or morality. Only to study the minor machinery which may appear secondary. That is what I try to do in my books. For this reason I choose characters who are ordinary rather than exceptional . . . the naked man in contrast to the clothed man.

In Simenon’s stories, the appearances of everyday life are costumes that are quickly discarded. The catalyst may be an unexpected event, or an impulse that seems to come from nowhere. Either way, what emerges is the bare human animal.

In The Man Who Watched the Trains Go By (1938), a quiet clerk for a respectable Dutch shipping company discovers that his boss has looted the firm in order to fund an affair. Having lost his life savings, the clerk boards a train to Paris, contacts his boss’s mistress, goes on a wild spree and (almost by accident) commits a murder. In Monsieur Monde Vanishes (1945), the prosperous protagonist leaves his business, wife and family without warning, exchanges his expensive suit in a second-hand clothes shop for a shabby anonymous outfit, and disappears into the demi-monde. M Monde has no clear idea why he leaves his life behind:

He had not thought about it beforehand . . . He was following a preordained plan, for which he was not responsible. Nor had he taken any decision the day before. It all came from much further back, from the beginning of things.

Some of the best examples of what is commonly described as crime fiction – the novels of Patricia Highsmith, for instance – are studies in character which show why the protagonists act as they do by probing their states of mind. In Simenon, human beings are the sum of their impulses and behaviours; there is no enduring self behind the façade of habit. No one authors their own life; the belief that they are responsible for their actions is an illusion.

“My very first Maigrets,” Simenon writes, “were imbued with the sense, which has always been with me, of man’s irresponsibility. This is never stated openly in my writings. But Maigret’s attitude to the criminal makes it quite clear.” Simenon would have dismissed any suggestion that his romans durs were novels of ideas. He believed that ideas count for very little in human life. But the idea – or fact, as he would have called it – of human irresponsibility is at the centre of nearly everything he wrote.

That is one reason why Simenon’s work does not belong in the genre of crime fiction. In the romans durs, criminal acts are important only in signifying a final break with society. Even in the Maigrets, the question is not why a crime was committed, but how the person who committed the crime departed from a settled routine of living, and the detective resolves the conundrum by imaginatively entering into the life of the suspect. Identifying the criminal is rarely the principal focus of the story, though this fact has been obscured for English readers by the uneven quality of the versions of those Maigrets that have been available to date, in some of which the endings were altered in an effort to make the novels more closely resemble crime fiction. The new and freshly translated versions of Simenon’s novels that Penguin is publishing give us, for the first time, the opportunity to read them as he wanted them to be read.

Simenon’s work has some affinities with a French tradition of realism, exemplified in the novels of Balzac, which aim to depict human life as it is lived. But this tradition has generally produced commonplace tales of ambition and divorce – bourgeois novels that treat human lives as more or less successful careers. In contrast, Simenon’s focus was on individual fate. However firmly established it might appear, society, for him, is a makeshift that easily breaks down. When it does, the solitary human being confronts the fundamental difficulty of being human.

Simenon formed this view of things from experience. Born in Liège, Belgium, in 1903, into a French-speaking middle-class family, he witnessed the effect on the city’s inhabitants of the German occupation during the First World War. In the daily struggle for existence, morality was discarded and forgotten. Everyone chiselled and cheated, lied and betrayed. Simenon formed a coolly realistic attitude to how the world works, giving him what he describes in the notebooks as “a somewhat caustic serenity”. By January 1919, after a number of casual jobs, the teenage Simenon was working for the local newspaper as a junior reporter – a role he enjoyed because it enabled him to explore the city’s nightlife. By then he considered himself a writer. He quickly emerged as a successful author who insisted on controlling every aspect of his work, from the book jackets and publicity to the contractual arrangements with publishers. By the late 1920s he was married and a high-earning author of pulp fiction.

It was around this time that he became involved with the black singer and cabaret dancer Josephine Baker, then 19 years old and starring at the Théâtre des Champs-Élysées in Paris. Simenon found Baker irresistible, and as well as being one of her lovers became for a while her part-time secretary. As a result, the pace of his writing slackened. During 1927, when the affair was at its height, he produced only a collection of short stories and a mere 11 popular novels. Later he wrote that he would have married her if he hadn’t been afraid of being known as Monsieur Baker. His hard-headed lucidity prevailed, the affair ended and in 1928 he returned to his usual level of productivity, publishing 44 novels.

Simenon liked to think of himself as one of the “little people” whose lives he chronicled, but there was nothing little about him. By normal standards he was a prodigy. Among the hundreds of novels he produced, at least a dozen are of the first rank. André Gide was not exaggerating when he described Simenon as one of the greatest 20th-century writers of fiction. (Other admirers include T S Eliot, Henry Miller, William Faulkner and John Banville.) By the time he died in 1989, he was one of the most widely published writers in history, with world sales of over 500 million books and many film adaptations to his credit. In the intervals between bouts of intensive writing he travelled widely, visiting Africa, the South Pacific, eastern Europe and the Soviet Union, Cuba and the United States. He married twice, conducted lengthy affairs with household maids and had sex, by his own account, with thousands of women, often in exchange for money. He never allowed these diversions to interfere with his work – indeed, he tells us that he used sex to relieve the strain of writing – and the notebooks show him to be devoted to his family. But perhaps he, too, was moved by the impulse to escape that impels so many of his central characters.

Simenon may have come from the small bourgeois world in which many of his stories begin; it was not one whose pretensions he accepted, however. For his protagonists, bourgeois life is a form of confinement from which they struggle to break free. But they usually fail, and Simenon himself did not try – as soon as he could afford it, he simply lived as he pleased. The freedom that M Monde looked for on the fringes of society was as illusory as the seeming stability of bourgeois life. A series of imaginative illustrations of this truth, the romans durs are a genre in their own right. The Maigrets also fall into a category of their own.

Contentedly uxorious, devoted to the pleasures of the table and displaying little interest in politics or social change, Jules Maigret is archetypally bourgeois in his manner of living. Nonetheless, he stands at some distance from the morality that underpins the bourgeois way of life. The patiently plodding detective regards criminals and their victims with the same unwavering, dispassionate sympathy. It’s an attitude that refuses to condemn, but at the same time isn’t at all sentimental. If Maigret does not see criminals as evil, neither does he think they have any moral superiority as casualties of social oppression. Like the rest of us, those who commit crimes are caught up in knots of circumstance that cannot be untied.

One after another, actors have tried, with varying success, to re-create the character in film and television. In France he was played first by Pierre Renoir in the 1930s, and later by Jean Gabin, among others. In Britain, Rupert Davies presented a small-screen version (much admired by Simenon) in the 1960s, Michael Gambon gave a forceful rendition in the 1990s and a new adaptation has been announced with Rowan Atkinson playing the detective. All who attempt the role have to resolve a conundrum in capturing Maigret’s complex motives.

In Maigret’s First Case (1948), set in 1913, when the detective is working in a junior position at a small police station in Paris, Simenon tells us that Maigret only joined the police force when the studies he was pursuing in order to become a doctor were disrupted by the death of his father. In fact, Maigret seems to have wanted to become something between a doctor and a priest. That was why he joined the police. Simenon shows Maigret asking, “Are not policemen actually repairers of destinies sometimes?”

His question is intriguing. In general, religion is absent from Simenon’s work. It’s not that he rejects religious belief: he doesn’t even consider it. Like Maigret, Simenon was brought up in a conservative Catholic family. Again like his character, Simenon left Christianity behind in his teens and never looked back. A part of the interest of his work is that it illustrates a view of the human world that is authentically post-theist. A doctor of souls who does not believe in the soul, Maigret understands human beings in thoroughly naturalistic terms. The autonomous individual – so revered by liberal humanists haunted by religious notions of free will – is nowhere to be seen. Rather than shaping their lives according to their personal choices, human beings simply act, and then deal with the consequences. Without knowing why, Maigret wants to help those he encounters in his work come to terms with their fate.

Simenon is sometimes read as a writer who offers no hope. It is true that he holds out no prospect of redemption, whether for his characters or for humankind. Yet there is nothing in Simenon’s work of the horror at the human condition that is expressed in some of the stories of Guy de Maupassant – in other respects a comparable writer. Those of Simenon’s protagonists who are not destroyed in the course of attempting to escape their lives return rid of their illusions and readier to enjoy what the world has to offer.

The final effect of Simenon’s work is not depressing, but liberating. Before he left his life behind, M Monde “was a man who, for a long time, had endured man’s estate without being conscious of it, as others endure an illness of which they are unaware. He had always been a man living among other men and like them he had struggled, jostling among the crowd, now feebly and now resolutely, without knowing whither he was going.” When he finds his way back to his family and business, he is more relaxed. “He was part of life, as flexible and fluid as life itself.” M Monde may be the man Simenon wanted to be, and in some degree did become: “a man who had laid all ghosts, lost all shadows, and who stared you in the eyes with cold serenity”.

When I Was Old by Georges Simenon, translated by Helen Eustis, is published by Penguin Classics (464pp, £8.99)

John Gray will be discussing Simenon at the LSE literature festival on Saturday 27 Febuary