He was a brilliant lawyer and philosopher who first brought Albert Einstein to Britain, inspired the creation of Imperial College and crusaded for social welfare. He transformed the British Army, became a confidant of King Edward VII and the archbishops of York and Canterbury, and befriended Oscar Wilde in prison.

He wryly claimed to have been the only English member of the German cabinet after meeting Kaiser Wilhelm II and his ministers at Windsor in 1907 — and subsequently entertained the kaiser and his generals at his London home. Raymond Asquith described him as having the body of Nero and brain of Socrates. More than a century ago he campaigned for hereditary membership of the House of Lords to be replaced by “an elite of talent”.

Richard Haldane is among the most remarkable and admirable figures to have graced Britain’s body politic. John Campbell — a 72-year-old City financier, not to be confused with the more well-known biographer — is wrong to describe him as forgotten, because every historian recognises his achievement as the secretary of state for war from 1905 to 1912 in the Liberal government. He is right, however, that few people today appreciate the magnificence of Haldane’s contribution to British life.

Born in 1856, he came from a line of prosperous and earnest Scots, was educated at Edinburgh Academy, and afterwards the city’s university. A term spent at Göttingen proved life-changing, for he became a passionate Germanophile. He never married, following the crushing disappointment of being jilted in 1890 by Val Munro Ferguson, the love of his life. That year he became a precociously young QC, earning a huge income, having already entered the House of Commons.

Haldane’s bachelor status enabled him to sustain a volume of work, social life and sybaritic dining that would have sufficed for 10 lesser and thinner men: he made John Buchan seem lazy and unambitious. As secretary of state for war he recast the British Army to participate in a modern conflict, and founded the Territorial Army.

Beatrice Webb described how this lonely, driven man confided to her at the outset, after groping through a fog so dense that he was obliged to quit his carriage and walk to the War Office, clutching his seals: “I shall spend three years observing and thinking. I shall succeed: I have always succeeded in everything I have undertaken.”

An admiring general wrote: “When I … think of the jungle filled with hissing adders which Haldane broke up into a symmetrical and delectable garden, I do really feel uplifted to think that I was privileged to watch his address, his artistry, his perseverance.”

The editor and historian AG Gardiner wrote of Haldane’s army reform blitz: “You cannot resist a man who bursts with such enjoyment into the mess, smokes bigger and stronger cigars than anyone else … knows as much about explosives as he does about the Westminster Confession, and with all these accomplishments does you the delicate honour of discussing his scheme with you as if your approval were the one thing in the world necessary to his complete happiness.”

Even while he was conducting his military revolution, Haldane laboured at promoting philanthropic projects, aided by close friendship with the Rothschilds and others. He argued that education was the best investment for any country’s money; that all decisions should be taken as close as possible to the people they affect; that realism must be balanced with idealism. He strove to gain cross-party support for important policies.

He helped found the London School of Economics, was a keen supporter of Toynbee Hall, and latterly chancellor of Bristol and St Andrews universities. A guest at Cloan, his Scottish baronial family home in Perthshire, marvelled at the lofty conversation: “New and unfamiliar words like ‘quantum’ and ‘atom-splitting’ found their way into my vocabulary.” At a time when bridge was a country-house obsession, Haldane denounced the game as a menace to the nation’s intellectual life.

Yet his hubris, founded upon certainty that he was doing what was best for everybody, made him vulnerable. Beatrice Webb again: “His ideal has no connection with the ugly rough and tumble workaday world of the average sensual man, who is compelled to earn his livelihood by routine work and bring up a family of children on narrow means.”

In 1912 he left the War Office to become lord chancellor. That year he led an unsuccessful peace mission to Germany, which foundered on the kaiser’s insistence that, in return for his halting the naval arms race, Britain must promise to remain neutral in a European war.

With the coming of Armageddon two years later, Lord Northcliffe and Lord Beaverbrook launched one of their most deplorable press campaigns, vilifying Haldane as a German stooge — a travesty, given his resolute patriotism. In 1915 he was dismissed from government.

This humiliation reduced a man who was addicted to the wielding of authority to “a wreck”, in Beatrice Webb’s words. While Churchill and other colleagues offered private sympathy, they did not dare face down the mob publicly. Haldane went off to translate Schopenhauer, pen philosophical works of which the best known is The Reign of Relativity, and urge compromise in the bitter Anglo-Irish struggle. In 1924 he again served briefly as lord chancellor, having joined the Labour Party. Viscount Haldane, OM, as he had become, lived on until 1928, but the high gloss was gone from his career.

Campbell’s book is not a biography, but rather an act of homage to a man for whom he asserts lifelong admiration: he has been visiting Cloan since boyhood. He approaches his idol’s life thematically rather than chronologically, which must confuse those unfamiliar with the story.

Yet this intelligent book fulfils a purpose that the author defines in his introduction: “It aims to offer an example of statesmanship that we so vitally need today.” The supreme seriousness of Haldane’s career emphasises the triviality of his modern successors. Though an insensitive politician — rash enough to name his favourite dog Kaiser when anti-German sentiment was rising — his was a life of public service. Violet Bonham Carter wrote: “I have never known anyone who gave one such sense of greatness of mind and heart.”

While keenly ambitious, he was happy to fulfil himself through making his country a better place. In the second half of the 20th century more than a few people of his kind were still found in parliament. In the 21st century, however, how many members of British governments have shown themselves capable of thinking further than Monday, or of advancing the most meagre objectives beyond self-interest?