Alexis de Toqueville, by Hugh Brogan

Alexis de Tocqueville (1805-1859) was a pioneering sociologist, a political philosopher and a great historian in an age of great historians. Born into an aristocratic family, he studied law, was a magistrate at 22 and seemed destined to be a pillar of the restored Bourbon establishment. But he soon realised that France's ultra-right-wing monarchy was doomed. It had failed to anticipate the struggle between royal and popular power which produced a new Revolution in July 1830 and then replaced Charles X with Louis-Philippe, a "bourgeois monarch".

Though he swore allegiance to the new regime, his true loyalty was to the freedoms it failed to deliver. Alienated from the Bourbon legitimists who saw him as a traitor, but unable to work up much sympathy for the new regime, he did not become a republican (of sorts) until 1848. Though viscerally attached to aristocratic values, he became intellectually convinced that the future of France lay in the "democratic spirit" which he first discovered in America.

He travelled there with a friend in 1831 to study the prison system and their joint report suggested useful ways in which the American model could be adapted in France. But for Tocqueville the real lesson lay elsewhere, in the liberal society he found in the United States. He was impressed by the egalitarian spirit which allowed any citizen to write a letter to the President beginning "Dear Sir", and by the degree of local self-determination.

These and other insights were developed in De la Démocratie en Amérique (1835: part II appeared in 1840), a work of sociology with a sharp political edge. It was much admired at home and abroad, although what he meant by democracy is not always clear. Whatever it was, it was worth having because it meant liberalism and the opposite of tyranny. It was badly served by Louis-Philippe's regime and, after 1848, almost annihilated by the growing authoritarianism of Louis-Napoleon, who made himself Emperor by the coup d'état of 1851. The power of government now expanded at the expense of the freedoms and rights of citizens. The events which had excluded Tocqueville from public life now spurred him to take stock of the present by seeking answers in the past.

According to the accepted view, the French Revolution, which had swept away the ancien regime and invented government by the people, was an explosion, a sudden break with the past. In L'Ancien régime et la revolution (1856) Tocqueville argued that the Revolution had merely pushed at an open door. By 1789 the aristocracy and the parlements had been rendered impotent by the absolute monarchy, and justice and local affairs were controlled from the centre. The despotism of the crown was inherited by the Revolution which was therefore neither an end nor a beginning but a sudden acceleration.

While the historical process exerted a levelling, egalitarian effect on society, it also encouraged the centralisation of power. Now, if too much is given to the centre and too little to the individual or the locality, history could well return France to a form of absolutist tyranny. Indeed, since 1856, France has had its share of Caesarist and democratico-despotic moments. A mixture of eternal vigilance, events and elections has saved the day, and current decentralising policies are reassuringly Tocquevillean. But the danger always lurks.

Hugh Brogan has been fascinated by Tocqueville for nearly half a century. His industrious, rich biography sets his ideas in the context of his life and times. Tocqueville is not the most promising subject: he was a private man, diffident in public, but capable of deep friendships. Brogan's witty, affectionate but far from uncritical portrait humanises this intellectual biography, which even has what Brogan detects in L'ancien regime: "the charm of conversation".

David Coward is emeritus professor of French at Leeds University; his translation of Hedi Kaddour's 'Waltenberg' will be published this year