Paine addressed himself to an 18th-century world of unemancipated toilers ruled by monarchical despots and privileged oligarchies; a world in which children were hanged for stealing a few shillings, and women routinely died in childbirth (as Paine's first wife did); in which America was little more than a scattering of East Coast settlements, Britain had a population of barely ten million, and Islington - where Paine wrote his most famous work, The Rights of Man, in a pub close to what is now Angel tube - was an outlying village tucked away from the bustle and filth of central London.
Yet what is startling about Keane's account is not the difference, but the kinship. In politics, precious little has altered, and the debates that concerned Paine and his close contemporaries - about the definition of liberty and the institutions needed to protect it, about the desirable extent of equality, and about the legitimacy and use of state violence - are identical.
Much of the political landscape of the 18th century, too, is recognisable. "I am, Mr Dundas," wrote Paine to the Home Secretary of his day, after he had been accused of seditious libel because of The Rights of Man, "not your obedient humble servant, but the contrary." Paine would have laughed uproariously, with a strong sense of dj vu, at the Major government's frightened attacks on impertinent BBC interviewing.
The most striking aspect of Paine's world is the flow of ideas, as though the lack of satellite and Internet made no difference. In those incendiary times, a book or a pamphlet published in Pennsylvania could light fires across Europe almost before the ink was dry.
Of the trumpet blast there can be no doubt. Demagogues massage the vernacular, appealing to base instincts: Paine elevated it, reminding people of the best that was in them. He was a populariser, but never a vulgariser. Like many of the greatest thinkers and writers, he was not an innovator in the strictest sense. But there was a whirlwind freshness about him. He was a writer of eternal truths with a canny knack of surf-boarding the tide of fashionable longings. His continuing resonance comes both from what Keane calls the "diamond-hard" quality of his colloquial prose, and from a non-theorising, practical immediacy.
Indeed, if the extraordinary, passionate, limpid, still exciting Paine had one overriding capacity, it was an ability to be, physically and intellectually, at precisely the place where history was being made at any moment - whatever the barriers of distance and language. "My country is the world, and my religion is to do good," he wrote in The Rights of Man. Paine's country had no fixed point; his religion was addressed to what he saw as universal feelings. He was a philosopher above nations who, none-theless, became embroiled in the politics and ideas of three. Many people think of him as an American: in fact, he was one of Britain's most potent imperial exports. Keane makes much of Paine's birth in 1737 at Thetford, Norfolk, in the shadow of the local gibbet, symbol of oppressive state power, and of his youthful resentment of authority. Yet his early years seem unsettled rather than oppressed, and - as the persistent holder of minor office in the service of one government or another - he retained an ambivalent attitude to authority. The ambivalence may have had religious, as much as political, roots: he was the product of a split marriage, with a Quaker (dissenting, pacifist) father, and an Anglican mother.
According to Keane, Paine "reckoned life was a daring adventure or nothing" - and the streak of recklessness in him began early. Yet much of his young manhood remains a mystery. His grammar school education ended at 12, and he apprenticed in his father's trade as a corset-maker. At 20 he ran away to sea, but soon returned to an unremarkable existence in London and nearby counties, moving from job to job working in the female undergarments trade and as an Excise officer. The second occupation seems to have been most responsible for politicising him, as a trade union organiser on behalf of his fellow civil servants.
Personal unhappiness may also have sparked his public concern. In 1771, he married for a second time, and - amid rumours of non-consummation - rapidly separated from his wife, whom he never saw again. (Keane points out that the justice and liberty in Paine's idealism did not extend to the status of women, who featured little in his writing, or - for that matter - to the status of non-whites.) Perhaps if this marriage had succeeded, Paine would have stayed in England, eking out an existence as a minor pamphleteer: his life, before his emigration, contained little to set him apart from other coffee-house debaters and scribblers chafing at the restrictions of Hanoverian Whiggish rule. Indeed, Paine's early years on native soil are almost unfathomably dull, and there is still no easy explanation for his sudden transition, when already middle- aged, from English drifter into American polemicist of world renown.
Timing had something to do with it. Paine cannot have been completely unknown in London radical society: not every penniless immigrant arrived in America with letters of introduction from Benjamin Franklin. Nor did they arrive at such a key moment. For a political writer to turn up in Philadelphia at the end of 1774 - just as the colonists were getting worked up about taxation without representation - required a kind of opportunistic genius.
It also indicated an unquenchable curiosity. It is here that Keane's tale takes off, into an astonishing romance: as if the fantasy of every run-of-the-mill lingerie salesman and tax clerk had miraculously come true.
Breathing American air, Paine instantly began to identify in his own mind a nationhood that did not yet exist in the consciousness of those around him. To be a newcomer and a convert, as great events unfold before your eyes ... Paine's sense of his own destiny and the destiny of his adopted compatriots can be imagined. "As each day passed," says Keane, "the cause became his passion." Uncluttered by the reservations of established settlers, Paine was soon ahead of current thinking, and began to predict outright independence. His first clear voice can be heard in the pages of the Pennsylvania Magazine. As Keane shows, the directness of his writing for this journal led straight to the compelling invective of Common Sense, the work which made his name, and perhaps helped to cause the American Revolution.
The pen is mightier than the sword: within a month, Common Sense had set the country alight. To anticipate was the thing. Paine was saying what the colonists were beginning angrily to feel, but dared not say. "Independence a year ago could not have been publicly mentioned with impunity," wrote one Bostonian. "Nothing else is now talked of, and I know not what can be done by Great Britain to prevent it." If not a cause, then Paine's book was a catalyst: urgent, modernist, irreverent, outrageous, blaspheming against the verities of religion and Monarchy. Paine took George III's madness as the symbol of an insane political system, and dismissed him as a ruler whose tenuous legitimacy came from William the Conqueror, "a French bastard landing with an armed banditti and establishing himself King of England against the consent of the natives".
The adventure now began in earnest. Paine witnessed burnings and sackings, and was forced to flee from Philadelphia, under siege from the British. Briefly, he served as Secretary of the new American Committee of Foreign Affairs. His enthusiasm and insight, however, were not matched by the political judgement needed in such volatile times. He was forced to resign, and embarked on the next phase of his life, in Europe.
"I am a farmer of thoughts," wrote Paine, "and all the crops I raise I give away." Arriving at the Court of Louis XVI as a member of an American diplomatic mission (though he spoke no French), his fame had preceded him. For a time, he was part industrious clerk, drafting official letters, but also part celebrity - feted for his writings, which were directed against the French monarchy's enemy, monarchical Britain. There was a heavy irony here, not lost on contemporaries, for Paine seemed almost a caricature of the villainous agitator. His interpreter recalled his "brimstone odour", and that he was "coarse and uncouth in his manners, loathsome in his appearance, and a disgusting egoist; rejoicing most in talking of himself, and reading the effusions of his own mind".
Back in Philadelphia, Paine found himself frustratingly marginalised. In 1787, he returned to England to visit his parents in Thetford, and also to follow up a madcap scheme for a single-span wrought iron bridge over the Schuylkill River in Philadelphia. The plan failed: instead, Paine wrote The Rights of Man, the finest manifesto Islington has yet given birth to.
It was also the best-selling book in the history of publishing, ricocheting around the whole of the Eurocentric world, and speaking, not just to, but with and on behalf of, the common man. "As it is my design to make those that can scarcely read understand," wrote Paine, "I shall therefore avoid every literary ornament and put it in language as plain as the alphabet." He described despotic governments as a fungus growing out of a corrupt civil society, and vigorously demanded a welfare state (an aspect of Paine's writings Lady Thatcher neglects to mention when she quotes him in her speeches). Keane suggests that the fight for extending the rights of citizenship required a fresh political syntax: it is also true that Paine, unlike later writers chasing popular truths, showed that basic social and political realities are painfully simple, and can be simply expressed.
The best test of a book's power is the fury it arouses among those it attacks. In Britain, The Rights of Man caused uproar: the authorities had good reason to be worried about a man whose work had already helped to lose them half an empire. Paine was quickly forced into exile, once again getting the moment just right. Seldom has an Anglo-Saxon been so welcomed in France. Arriving in Calais in 1792, he was immediately offered French citizenship, and elected as the town's representative to the exploding National Convention.
"Vive la Nation! Vive Thomas Paine!", cried his constituents. It was to be a poisoned chalice. Forgetting England, almost forgetting America, Paine - the new citizen - was sucked into the vortex, involving himself utterly in the turmoil of a culture he barely understood. Yet, as Keane movingly reminds us, he retained his sense of proportion, and his humanity. Though he voted to condemn Louis XVI, he fought to save him from execution. ("Citizen President," he courageously declared, against the ominous tide. "My hatred and abhorrence of monarchy are sufficiently known ... but my compassion for the unfortunate, whether friend or enemy, is equally lively and sincere.") As the Terror mounted, Paine took to his desk, and savaged organised Christianity in another great and lasting work, The Age of Reason.
Once again, desperate events overtook him, this time almost fatally. Having escaped lynching at the hands of the mob, he was thrown into gaol by a paranoid government and, hourly fearing his own death, watched fellow prisoners dragged off to the guillotine. Despite this trauma, Paine stayed in his latest adopted country, his pen continuously active against enemies on both sides of the ocean. George Washington was the subject of special calumny, for allegedly conniving at his imprisonment, and for having "acted towards me the part of a cold-blooded traitor". In 1802 Paine returned to America to spend his last seven years in growing ill health and squalor, denouncing unreason to the last.
What did he leave? A vibrant energy pervades this book, and this record of the life of Tom Paine - smelly, cantankerous, privately selfish - characterises a particularly uplifting kind of public virtue. Yet Paine's deeds counted for little compared with his words. Today they are treated as a holy text. Paine's echoing prose has become as much a part of our modern idiom as some of the passages from Milton, on whom he draws. The best books, George Orwell noted in Nineteen Eighty-Four, are those that tell you what you knew already, and would have said, if you could get your thoughts together. A unique capacity to encapsulate the emerging beliefs of a fragmented, newly secular world was the secret of Paine's popularity and force.
There have been scores of studies of Tom Paine. Another might have been very dry. This one is the reverse. The author uses the skills of a political philosopher, as well as a biographer's empathy, to reveal the personality and ideas of a great crusader for humanity, and to show how much, in our equally chaotic and fast transforming age, we still have to learn from his great call.
! `Tom Paine: A Political Life' by John Keane is published tomorrow by Bloomsbury at £25