To many, however, he is known not for his philosophical work but for his political campaigning. During his extraordinarily long and active life (he died in 1970 at the age of 97), he led and inspired several waves of popular protest, ranging from the movement against conscription in the First World War to the campaigns against the nuclear deterrent in the Fifties and against the Vietnam War in the Sixties. Others know him from his appearances on the radio and the television, where, as a panellist on The Brains Trust, or as an interviewee on John Freeman's classic series Face To Face, his popular image was fixed as "the great philosopher on the telly", the urbane, witty and brilliant octogenarian with silvery white hair, an impossibly old-fashioned aristocratic voice and an impish and knowing twinkle in his eye that suggested that he would, given half the chance, "basilisk your girlfriend".
Popular images are notoriously difficult to shift, and Russell's has clung to his reputation ever since his death: the sceptical satyr, the embodiment of a kind of confident rationality that seems to belong more to the 18th century than to our own, more pessimistic age. He is, it seems, destined forever to be seen as a latter-day Voltaire. Indeed, I once interviewed an elderly woman who had known him well in the Forties, who ended almost every story she told about him with the exclamation: "He was Voltaire - voila!"
Russell himself encouraged and played up to this popular image. Wherever he lived, a bust of Voltaire stood on his mantlepiece, and in 1958 he published (in French) an article called Voltaire's Influence On Me that emphasised the connections.
And yet, over the past six years, most of which I have spent researching and writing my biography of Russell, it has become increasingly clear to me that being Voltaire was, for Russell, an ideal rather than a reality. He wanted to be Voltaire, but actually felt himself to be more like a character from a Dostoevsky novel. He wanted to be ruled by cold reason precisely because he felt himself to be driven by deep, irrational fears and impulses, often so powerful and disturbing that they persuaded him he was going insane.
Russell had an extraordinary and disturbing ability to hide even his strongest feelings from those around him. When he was irritated, he could appear charming. When he was passionately aroused, he could appear coldly indifferent; when he was burning with hatred, he could appear jolly and even loving. It was a trait that had been built up from long years of isolation as a child, when, as he said later, "the most important hours of my day were those that I spent alone in the garden, and the most vivid part of my existence was solitary".
He was brought up by his grandmother, Countess Russell, the widow of the great Victorian Prime Minister, Lord John Russell, his mother having died when he was two, his father when he was three and his grandfather when he was six. Lady Russell was pious and sentimental, and Russell quickly learnt to conceal from her those thoughts and feelings of which she might disapprove, which included almost all the thoughts and feelings most dear to him.
Thus, he wrote: "I acquired the habit of deceit, in which I persisted up to the age of 21. It became second-nature to me to think that whatever I was doing had better be kept to myself, and I have never quite overcome the impulse to concealment which was thus generated. I still have an impulse to hide what I am reading when anybody comes into the room, and to hold my tongue generally as to where I have been, and what I have done."
Ironically, the result of this concealment is that Russell left a written record of his inner life that is matched perhaps only by the diaries of Virginia Woolf in its detailed self-absorption and its determination to document everything. For, denied the opportunity to express himself orally, Russell took to writing everything down. Of the vast body of writing he produced in his lifetime (he published 70 books, 2,000 articles, and wrote well over 4,000 letters), an enormous amount is concerned with himself, revealing the feelings he kept hidden from those around him and trying to make sense of the conflicts that characterised his intellectual and emotional life. In his youth, for example, he kept a diary in which he set down his earliest philosophical thinking and the religious doubts that he hid from his grandmother (who, all this time, was persuaded that he was as pious as she was). And, later on, when his first marriage to Alys Pearsall Smith had deteriorated to a hollow shell, he kept another diary in which he expressed all the anger, irritation and even hatred towards his wife that he successfully concealed from her by an outwardly cold demeanour.
To read these latter diaries is often a distressing and shocking experience. One cannot help but be amazed at Russell's impassiveness in the face of his wife's suffering. For example, in 1902, when he told Alys he no longer loved her, he described how she lay in her bedroom at night while he was in the adjoining study, and "her loud, heart-rending sobs, while I worked at my desk next door". For nine years after that, Russell and Alys lived together, with Russell growing increasingly resentful of her, while doing his best to be outwardly kind and sympathetic (it is no coincidence that his greatest philosophical work was written during these nine years).
Again, his diary allowed him to release the feelings that were thus imprisoned, and, again and again, one reads of occasions such as a dinner they both attended, during which, Russell writes, "Most of the people jarred me, misanthropy and misogyny settled on me like a cloud. At night, I didn't kiss Alys often enough, and she began to cry when I put out the light, but I did nothing to comfort her."
The dam burst in 1911, when Russell fell in love with Ottoline Morrell and finally left Alys, releasing in the process a torrent of pent-up passion of alarming intensity. Again, everything was committed to writing, since Ottoline would not leave her husband to live with Russell and therefore their affair had to be conducted principally through correspondence. At the height of his passion - that is, for about two years - Russell wrote to Ottoline every single day, often two or three times a day, long letters, full of expressions of love, of self-analysis as well as detailed accounts of his day-to-day life. When things with Ottoline started to go wrong, however, Russell would write to her about how he did not want to feel anything, he wanted only to think. At such times, he would return to abstruse and technical issues in philosophy and try to put his passionate love for Ottoline out of his mind.
Linked to this pendulum-swing between abstract thought and exuberant passion is the tension that existed between Russell's two greatest fears: the fear of loneliness and the fear of madness. For much of his life Russell felt, as he often put it, like a ghost, a quasi-substantial being, unable to make real contact with the flesh-and-blood creatures around him. His often desperate searches for love were a series of attempts to escape this spectral existence.
But, just as there are advantages of keeping one's deepest thoughts and feelings hidden, so there are advantages in being untouchable, and counterbalancing Russell's dread of remaining in ghostly isolation was his fear of the forces within him that would be unleashed if he let himself be touched, if he made real contact with another. "It doesn't do for me to relax too much," he once wrote to Ottoline, "the forces inside are too wild - some of them must be kept chained up ... I had thought possibly now I might let all the dogs have an outing, but some of them are mad dogs and are not safe to leave at large."
The intense passion that Russell kept locked up was, he often thought, akin to madness and frequently, when emotionally aroused, he thought himself on the brink of insanity. His Uncle Willy had gone insane and, after murdering a complete stranger, had spent the rest of his life hidden away in an asylum. Russell did not know this until he was 21, and ever afterwards his very deepest fear was of reliving his uncle's fate. This fear, he wrote later, "caused me, for many years, to avoid all deep emotion and live, as nearly as I could, a life of intellect tempered by flippancy".
Among the "mad dogs" that Russell considered unsafe to let loose were his often extraordinarily intense hatreds. As he himself once put it: "There is a well of fierce hate in me." One of the few people to see this was DH Lawrence, who once wrote to Russell with devastating insight and frankness. "You are too full of devilish repressions to be anything but lustful and cruel .... It is not the hatred of falsity which inspires you. It is the hatred of people, of flesh and blood. It is a perverted mental blood-lust."
How near to the truth Lawrence came can be seen in a short story Russell wrote late in life called Satan in the Suburbs, the writing of which he said, was "a great release of my hitherto unexpressed feelings". The central character in the story, Dr Mallako, might be seen as a literal personification of Lawrence's phrase "devilish repression". "There is no human being, no, not one, whom I do not hate," Mallako declares. "There is no being, no, not one, whom I do not wish to see suffering the extremity of torment." When he explains why he feels this "well of hatred", Mallako describes a childhood that is, in essence, Russell's.
At the age of six, he says, he lost both his parents and was put into the care of a pious old lady, who "was persuaded that I was a good little boy. She adopted me, and educated me. For the sake of these benefits, I put up with the almost intolerable boredom that she inflicted upon me in the shape of prayers and church-goings and moral sentiments, and twittering sentimental softness to which I often longed to retort with something biting and bitter, with which to wither her foolish optimism ... never for one moment have I been able to forget those early years ... the friendlessness, the dark despair, the complete absence of hope."
The picture of Russell's inner life one gets from this story is disturbing. And yet, as Russell realised, his well of hatred was, as he put it to Ottoline, "also a well of life and energy - it would not really be good if I ceased to hate". We are often told that repression is a bad thing, but it is massively to Russell's credit that he did not follow Lawrence's advice and release his "mad dogs", for, by keeping them enchained, he was able to redirect their frightening and fierce energy so that it was channelled into producing his unrivalled corpus of work, not only in philosophy, but also in politics, journalism and education.
Being Voltaire was an ideal, a mask, but when one gets a glimpse of what lay behind it, one can only be thankful that the mask was so rarely taken off and grateful for the work that was produced under its cover.
'Bertrand Russell: The Spirit of Solitude', by Ray Monk, is published by Jonathan Cape on 18 April, pounds 25.Reuse content