When I began my career as a historian in the 1960s, biography fell into the category of "unserious" stuff written by amateurs. Not any more. Big biographies of Adolf Hitler, Benito Mussolini, Francisco Franco, Winston Churchill, Lyndon B. Johnson and many others pour from the pens of the most distinguished academic historians. What has changed? Why has biography become respectable as a form of history?

In the 1960s, the discipline's prevailing paradigms came from the social sciences. History had to build sociological models. It had to measure, count and verify. It had to study structures and functions of the social order, drawn from Marxist analysis or Weberian sociology. Anything else seemed dangerously uncertain, ill-defined and, worse, "subjective".

The collapse of the Soviet Union brought down the whole edifice of social science. Nobody in the spectrum of social studies had a clue that the USSR and its vast empire could vaporise in two years as if it had been a mirage; anything with "social" in its terminology lost purchase along with socialism.

The gap left in the set of tools available to historians has not yet been filled. Even I, educated in Parsonian structural-functional analysis and a dedicated social scientific historian, had noticed an absurd contrast between my models and a 20th-century reality dominated by hugely charismatic individuals: David Lloyd George, Georges Clemenceau, Woodrow Wilson, V.I. Lenin, Joseph Stalin, Churchill, Franklin D. Roosevelt, Mussolini, Franco, Hitler, Mao Zedong, Fidel Castro, Margaret Thatcher, LBJ and Ronald Reagan.

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Biography established itself, I think, because the social science models ignored the power of human personality. Serious historians of National Socialism had realised for a long time that they had to solve the "Hitler problem". The great Hitler biographer Ian Kershaw begins his massive two-volume biography with a section titled "Reflecting on Hitler" with these words: "The legacy of Hitler belongs to all of us. Part of that legacy is the continuing duty to seek understanding of how Hitler was possible...the character of his power - the power of the F¨¹hrer...a social construct, a creation of social expectations and motivations vested in Hitler by his followers."

Kershaw makes a fundamental and helpful distinction between the life of the man called Hitler and the interaction of that life with the category of rule associated with the term "Fuhrer" (leader) - a political, objective reality, which we can study as we can the growth of modern industry or changes in population.

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My biography of Otto von Bismarck works on the same principle. For the past four decades since I first lectured on the Iron Chancellor as a (very) junior research fellow at the University of Cambridge, his achievement had puzzled me. How had he done it? The answer I offer rests on an assessment of Bismarck's personality. He achieved his feats because his powerful personality disarmed and dominated supporters and opponents alike for nearly 40 years. Contemporaries testified to this power: those who knew him said he "bewitched", "enchanted", "charmed", "delighted" and "fascinated" them. Others called him "diabolical" or "demonic". None disputed the magnetism of his presence.

Yet personality alone cannot be enough. Every individual, no matter how great, works within real parameters. Changes in the international balance of power over which Bismarck had no control made his success possible. The semi-absolute structure of the Kingdom of Prussia after the Revolution of 1848 left the monarch with considerable power. The reform of the Prussian Army and its use of the railroads, over which Bismarck as a civilian could by definition have no say, made his victories possible.

As a result of his civilian status, Bismarck needed a general as an ally and a go-between to the Kaiser on military matters. Albrecht von Roon, a lieutenant general, became his military partner and supporter. Finally he had to manipulate the old king, Kaiser Wilhelm I, who had to live a long time, which he did. The relationship between Bismarck and Wilhelm needs a biographer, not a social scientist, to explain. I see it as a conflict between Bismarck as the adopted son of the king and the monarch's wife, Kaiserin Augusta, who hated Bismarck as he hated her. In that quasi-family triangle, the politician unfolded his genius.

Biography can, therefore, be proper history if it asks the kinds of questions that an academic historian can define and offers evidence to support the answers. Now clearly, the argument that Bismarck's personality explains his success cannot be "falsified" in the terms that philosopher Karl Popper defined scientific proof, but it can rest on a different order of epistemology: a type of knowledge that I call "human knowledge". That knowledge allows us to understand each other by a tone of voice, a gesture, a smile, a raised eyebrow. You sit at a meeting: a speaker says something absurd. You catch the eye of a colleague and both know that you agree. You get a phone call from your partner; you know by the sound of her voice that she is annoyed with you. This is a form of human knowledge that we cannot verify but cannot live without.

Why should such human knowledge not apply to the figures we study in the past? I offer an example. Roon made Bismarck's career possible, and he knew it. In a letter written in 1864 to his best friend, Clemens Theodor Perthes, he put it this way: "Bismarck is an extraordinary man, whom I can certainly help, whom I can support and here and there correct, but never replace. Yes, he would not be in the place he now has without me, that is an historical fact, but even with all that he is himself."

Without Roon, Bismarck would have had no career in the Prussian monarchy. The latter tried to get out of compulsory military service (German historian Ernst Engelberg published the documents on this unheroic episode in January 1838, which the editors had omitted from the official publication of the Bismarck papers in 1933) and had no military credentials beyond a year as a reserve officer in a modest regiment. In Prussia, the landed gentry and aristocracy all "served" - and "served" meant service in the army. They went first to the Kadettenanstalt, the military school, and then to their regiment. Bismarck went to a bourgeois gymnasium and then to university to become a lawyer. He was, as Baron von Osten sneers in Theodor Fontane's novel Irrungen, Wirrungen (On Tangled Paths) (1888), "nothing but a pen pusher".

Roon was exactly the opposite: an upright soldier of modest origins and no private means; he and his wife Anna "lived on his salary". In 1848, he had turned down a personal invitation by the Kaiserin (at that time the Crown Princess) to become military tutor to her son, the future Emperor Fried-rich III. Roon, a junior officer of no fortune, refused the opportunity of a lifetime, service with the House of Hohenzollern, which the agile, socially polished (and equally penniless) Helmuth von Moltke the Elder used as his route to the top. Roon declined because he considered his views too reactionary for a young prince; he also disapproved of how the Crown Prince and Crown Princess were raising Friedrich and had the nerve to say so. This integrity impressed the royals.

In 1857, Kaiser Friedrich Wilhelm IV had a stroke and surrendered the crown to his brother, Crown Prince Wilhelm. The Regent asked Roon on 25 June 1858 to draw up plans for the reform of the army and in 1859 appointed him minister of war to carry them out. Roon, who had first met Bismarck as a teenager in the summer of 1832, immediately pressed Wilhelm (who became Kaiser in January 1861) to appoint him and continued to do so until the antagonism between Crown and Parliament over the cost of Roon's reforms threatened revolution. In September 1862, Roon arranged for Bismarck to be in Berlin so that Wilhelm could summon him to be Minister-President.

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Roon was a reactionary, a Prussian general and an anti-Semite, but in spite of myself I admire him. When he died in 1879, his son Waldemar edited and published his papers, which appeared in three volumes in 1892. Aristocrat and diarist Hildegard Freifrau Hugo von Spitzemberg recorded on 7 August that year that she had been reading Roon's Denkwurdigkeiten (memoirs), just published: "What a pious, decent, competent man, how loyal and yet how frank. One reads how much annoyance he had to swallow from high and highest persons. And how charming his travel descriptions, how touching his relationship to his wife, and his friends Perthes and Blanckenburg."

That two people from different worlds and times - an obscure academic in the 21st century and a grand society lady of the 19th century who actually knew Roon - saw the same character traits encouraged in me the hope that my "feel" for Bismarck's personality and that of his contemporaries was accurate.

Take another case. Richard Bosworth's brilliant Mussolini (2002) reveals the witty and cynical aspects of the dictator's personality, a side of the Duce that his pompous outer appearance would never suggest. Mussolini once said that "the extent of credulity which can be found in any man of whatever class or intelligence is extraordinary...lies always win against the truth". Luigi Federzoni, the Nationalist leader, Mussolini wrote, was "the sort of old man who put on a dark suit before going out to buy a roll of toilet paper". On his relationship with King Victor Emanuel III, Mussolini said: "the king and I share a bedroom without a double bed in it".

Later in the book, the Australian scholar emphasises an emptiness at the heart of Mussolini's triumphs. After the March on Rome and the seizure of power, he writes of "a rancour (and the nervous fear) at the heart of fascism" and speculates about Mussolini's "deepening sense of the hollowness of life". He adds that "happiness and satisfaction continued to elude Benito Mussolini"; "both private and public life still stubbornly refused to bring contentment". The Duce's "arrogance...only partly cloaked his own sense of inadequacy".

How does Bosworth know what went on in Mussolini's mind? How can the reader who disagrees say that his assessment is "wrong"? Any judgement that any of us make at any time of another person can be called "wrong", and we cannot prove or deny it. That "emptiness" operates in Bosworth's Mussolini as a central explanatory device. The good biographer makes his or her case and the readers form their own opinions. It works that way in life, and biography attempts to portray "a life".

Paul Preston's splendid Franco: A Biography (1993) shows us a political general rather than a charismatic leader, a cunning, closed and competent man, hard to assess and interpret. He ascribes some of Franco's attitudes to the culture of his native Galicia and those of us who have never been there must take that view on trust.

US journalist Robert Caro, author of the multi-volume biography of LBJ, took the issue of trustworthiness to an extreme rarely seen in biography. In order to understand and describe Johnson's early years, he and his wife went to the Texas "hill country" and lived there for quite a long time to evoke that world as part of the former president's nature.

Human knowledge forms only a part of any biography and, in a wider sense, of history more generally. Historians work within rules and procedures that govern the subjectivity of such judgements. As University of Chicago professor Dipesh Chakrabarty writes, history is "a disciplined and institutionally regulated form of collective memory". Each of us in the profession observes those regulated forms: assertions must be supported by evidence; we try to give precise dates and places for the events we describe.

The exchange of ideas among practitioners produces a consensus on "what actually happened" about an aspect of the past for a particular society at a particular time in its past. This changes when new people ask new questions in a new age. There is, therefore, no "truth of the matter" about historical narrative or analysis, although certain facts and dates can be considered true. It is not random but, as Chakrabarty says, "disciplined and institutional".

Biography fits into this modest account of what history is and can do. The discipline is not the past but a systematic, "institutionally regulated form" of thinking about that past. Human knowledge of past actors can be wrong, as can knowledge of present actors whom we know personally; it can be systemically wrong because we no longer understand the meaning of terms important to past actors such as "honour"; perhaps human nature itself has changed over time. Biography can be "wrong" in all sorts of ways about the people described, their world, their values and their unspoken assumptions, but if it is wrong, it fails in the way that all history does. However, if it succeeds, it does so in a way that only biography can, showing us what extraordinary human beings have done and what they were like. The biographer hopes that he or she has "got it right", but can never be sure: that is what it means to be human and know each other in the way we do.

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