Practical men, who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist. Madmen in authority, who hear voices in the air, are distilling their frenzy from some academic scribbler of a few years back.

John Maynard Keynes, The General Theory of Employment, Interest and Money (1936)

Most political observers can trot out Keynes’s remark but it does not follow that political leaders are mere ventriloquists. Even the most unintellectual and anti-intellectual have to decide to which academic scribbler or scribblers they are going to enslave themselves. And how politicians put different ideas together makes a critical difference to the politics they pursue. Just compare varieties of liberalism and conservatism – and communism and fascism – under different leaders.

Moreover, many of the most successful political leaders are not “exempt from any intellectual influence”; on the contrary, for some, intellectualism is a central part of their political personality and appeal. As the Guardian columnist Jonathan Freedland once wrote, “People don’t believe in ideas: they believe in people who believe in ideas.”

All of this is prompted by grappling with the career of François Mitterrand, courtesy of Mitterrand: a Study in Ambiguity (Bodley Head, £30), an excellent new book by the former BBC Paris correspondent Philip Short. His previous subjects as a biographer were Pol Pot and Mao Zedong. An odd trio but, as with the other two, Short brings to the former French president great insight without undue sympathy, qualities admired by Mitterrand, who said the most essential attribute in politics is “indifference”.

The reason for grappling with Mitterrand is simple enough: he is the most successful left-wing leader of any of the three leading western European countries (France, Britain and Germany), measured by longevity in power and arguably also by electoral dominance. A front-rank politician by the age of 30 in 1946 and a senior minister in successive governments of the Fourth Republic while in his thirties, he went on to lead today’s Socialist Party in 1971, then to win two presidential elections (in 1981 and 1988) and two parliamentary elections. Having condemned Charles de Gaulle’s strong Fifth Republic presidency as “a permanent coup d’état” when the general assumed power in the late 1950s, he occupied the post in full plenitude for 14 years (1981-95), ruling for longer than de Gaulle – longer indeed than any leader of France since Napoleon III – in a political career spanning half a century.

However, the explanation for Mitterrand’s success is anything but simple; also complex are the lessons for today’s left as it struggles to win and hold power across Europe, not least in France, where François Hollande evinces little of the mastery of Tonton (“Uncle”).

Short’s biography is subtitled A Study in Ambiguity but it could equally be described as “a study in deception”, because there was nothing ambiguous about the massive falsehoods and carefully constructed but entirely bogus images that litter every part of Mitterrand’s career. Short begins the biography with an electric account of the “observatory affair” of 1959, when Mitterrand faked an assassination attempt on himself as a ploy to regain the political initiative the year after de Gaulle buried the Fourth Republic and most of its political inmates. The fake was exposed and it is extraordinary that he ever recovered.

Yet the greatest deceptions were still to come. Throughout his presidency, he lied (and ordered his doctors to lie) about his health. Diagnosed with advanced prostate cancer within months of taking power in 1981 and expected to live for only three more years, he told his urologist: “It’s a state secret; you are bound by this secret.” When the cancer went into remission, he not only stood for re-election while maintaining the secret but struggled on for the full seven years although the cancer returned and, by the end, became undeniable.

His personal life was similarly full of falsehood. While his wife, Danielle, and their two sons were his public family, they coexisted with a secret second family of his mistress (who was 27 years younger than him) and their daughter, with Mitterrand shuttling between the two in Paris and the country, again unknown to the public until the end of his presidency. His daughter, Mazarine, was named after Cardinal Mazarin, Louis XIV’s wily and secretive first minister, whose precepts for the politician were taken deeply to heart by her father: “Be sparing with your gestures, walk with measured steps . . . Simulate, dissimulate, trust nobody.”

It is no surprise that it is hard to pin down what Mitterrand believed. Partly this is because his ideas changed so much and so often. Starting out as an official of the Vichy regime and an admirer of Philippe Pétain, he was elected in the Fourth Republic for a shifting array of parties of the centre right. As a minister in the mid-1950s, he was a voice not only of conservatism but of outright reaction and repression in respect of Algeria and the French colonies.

Throughout the 1960s, his big idea was anti-Gaullism. He championed liberalism in the face of overweening personal and presidential power. Socialism entered his vocabulary only as he sought a viable anti- and post- Gaullist political grouping, which, as a result of his artful machinations, came together in the Socialist Party in 1971.

Mitterrand then rose to power on the back of an alliance with the still-strong Communists. He fashioned this as the tribune of a leftism that included wholesale nationalisation, a war on the rich and a huge expansion of welfare spending without any regard for conventional economics, which he professed to despise.

This led to the “common programme”, which was put into action in 1981. Elected on the rhetoric of a “complete rupture” with capitalism and the slogan “Change life”, Mitterrand appointed Communist ministers to a pan-left coalition that embarked on the most radical and frenetic programme of nationalisation, state spending and cultural reform attempted by any western European government since the early postwar years.

Barely a year later, Mitterrand put most of this into rapid reverse. With the franc collapsing and the financial markets in revolt, economic orthodoxy returned, state spending was slashed and the Communists were ejected. Nationalisation was rolled back after the right won the parliamentary elections of 1986. Scotching the notion that he should resign in the face of this debacle, Mitterrand instead fashioned a new concept of “cohabitation” between a president of the left and a government of the right. He proceeded to outwit the then prime minister, Jacques Chirac, fighting him on the slogan of “Opening to the centre” in the 1988 presidential and parliamentary elections, while Chirac scrapped with Le Pen and the National Front – whose potency was largely a creation of Mitterrand’s manoeuvre to change the electoral system to proportional representation before the 1986 elections, specifically to strengthen the far right in relation to the centre right.

Re-elected as a centrist, Mitterrand appointed a government under the centrist social democrat Michel Rocard, including a large number of non-aligned ministers and even a handful of centre-right former ministers, before succumbing to another “cohabitation”, this time under Edouard Balladur (who had been the finance minister in the first “cohabitation”), which saw out his final two years of office.

“It was not in my interests to oppose the trend of public opinion,” said Mitterrand, abdicating any role in leading opinion as he drifted, with increasing physical and political infirmity and growing controversy – not least about his Vichy past, coming fully into the open for the first time – to the end of his second term.

Shortly before his replacement as prime minister in 1991, Rocard described his rival and nemesis as “cynicism in its purest sense”. During his 14 years in the Élysée, Mitterrand got through seven prime ministers, each the product of labyrinthine political calculations, the subtlety of which was often lost on the participants.

In all these manoeuvres, over five decades, ideology and political language were as often as not the servants of short-term political advantage. Simulate, dissimulate. Once he became a “socialist” after 1970, for instance, varieties of leftism were deployed to outwit party rivals on all sides and to make possible (and later to destroy) the alliance with the Communists.

The ceaseless shifting of Mitterrand’s ideas is a dominant theme of Short’s biography. Having sought out the essence of Mitterrand’s credo as a lesson for left-wing rejuvenation, I am instead bewildered by the endlessly turning kaleidoscope. I cannot think of a modern democratic leader who has made so successful a career trading rival ideas and policies to suit immediate political convenience. In the British context, over the course of his career, he was Tony Benn, Clement Attlee, Harold Macmillan, Harold Wilson, Tony Blair and Enoch Powell, all rolled into one.

All of this ideological somersaulting was donewith immense intellectual engagement and fine calibration. In Mitterrand, the academic scribbler was more the servant of the practical politician than vice versa. A big part of “brand Mitterrand” was an ostentatious intellectualism giving apparent depth and sincerity to whichever creed he was peddling at any given time, however great the difference with the last one.

Mitterrand said he needed to read for two hours or more a day “to oxygenate the brain”. Many of his major shifts in ideas were accompanied by a book or pamphlet by the maestro – including his remarkable 47-page “Letter to all the French”, written as a manifesto for his “opening to the centre” for his 1988 re-election, advertised with little modesty as covering “all the big subjects which are worth discussing and mulling over between French men and women”. (“The night before it was to be published [he] stayed up till 3am at the printing press correcting the proofs, like a neophyte brooding over a first novel” – a brilliant detail, as are Short’s revelations that during tedious cabinet presentations, Mitterrand annotated antiquarian book catalogues and on presidential flights would sometimes ask the pilot to circle before landing so he could finish a chapter.)

Is Mitterrand’s legacy an object lesson in intellectual manoeuvring, with no inner core, as the method of a politician supreme? It is more than that in four respects. First, however labyrinthine his methods, there is a substantial progressive legacy from which the French left takes inspiration, including the abolition of the death penalty, significantly raising the minimum wage, equal rights for women and minorities, decentralisation and numerous beneficial grands projets.

Second, the 1982-83 reversal had the effect of demonstrating to the European left that “socialism in one country” didn’t work; pragmatic social democracy is the successful face of “Mitterrandism”.

There wasn’t the clear break with the doctrinaire past of the German SPD in the late 1950s and the British Labour Party in the mid-1990s, which is part of François Hollande’s problem as he tries to play the centre and an unreconstructed left together. Yet the post-Mitterrand French Socialist Party is as broad a church as its British and German counterparts and knows how to govern from the centre.

Third, there was a Mitterrand core: peace with Germany and projects to entrench European peace and security, from the European Communities in the 1950s to the single currency in the 1990s. A survivor of European war and its horrors – including time as a prisoner of war – Mitterrand never allowed the central pillars of a pro-German and pro-US foreign policy to become part of the game of “simulate, dissimulate”. Ironically, it was de Gaulle who played dangerously in this arena.

With Communists in his government and the left triumphant, Mitterrand’s first move in 1981 was to assure Ronald Reagan in unequivocal words and actions that France was a reliable ally. He did the same with Margaret Thatcher during the Falklands war a year later and also with Helmut Kohl, after an initial wobble, on German reunification. It was ambiguity at home but clarity abroad – and clarity in the cause of European peace and stability. Hence the most enduring image of Mitterrand: hand in hand with Helmut Kohl before two huge wreaths at Verdun in 1984 at a ceremony to seal Franco-German reconciliation.

Fourth, throughout his life and career, Mitterrand had a patrician sympathy with the underdog. Although he was the son of a stationmaster who inherited a family vinegar business, he served in the ranks in the war, having failed the competition for a commission, and developed a contempt for hierarchy and authority (besides his own) and a social sympathy for the less fortunate that was genuine and lasting. His political initiation – and his early political power base – was in organisations for returning prisoners of war. This need not have led him to the socialist left but it helped him accomplish the transition with an authenticity born to some degree from experience.

No feats of intellectual and political gymnastics can substitute or detract from personal experience. In Mitterrand’s case, it was his intimate experience of a France prostrate, impoverished and divided, that dominated his twenties and shaped him fundamentally.

Philip Short suggests another attribute of Mitterrand the leader: natural authority rooted in an “inner solitude” – “a part of [his] being that was locked, inaccessible to others, which is one of the characteristics of uncommon leaders everywhere” – and which came in part from a long period in the political wilderness (the 23 years from 1958 to 1981). He draws the parallel with de Gaulle in the wilderness in the 1950s; Churchill in the 1930s also comes to mind.

Perhaps. Yet François Mitterrand showed himself to be a notable leader as a prisoner of war and an organiser of fellow returnees long before his wilderness years. Maybe it owed more to Cardinal Mazarin, whose further advice for politicians was to “maintain a posture at all times which is full of dignity . . . Each day spend a moment studying how you should respond to the events which might befall you.”

In the last months of his life, his doctor told him he was a mixture of Machiavelli, Don Corleone, Casanova and the Little Prince. When Mitterrand enquired, “In what proportions?” the physician replied, “That depends on which day.”