IN 1749, while the philosopher Denis Diderot was imprisoned at Vincennes, his friend Jean-Jacques Rousseau regularly took the long walk from Paris to visit him. Reading on the road, Jean-Jacques one day saw the newspaper announcement of a prize offered for the best answer to the question: "Has the progress of the arts and sciences done more to corrupt or to purify morals?" He later described his reaction in a famous letter.

"I sank down under one of the trees in the avenue and passed the next half hour in such a state of agitation that when I got up I found that the front of my jacket was wet with tears, although I had no memory of shedding any. Ah . . . if ever I had been able to write down what I saw and felt as I sat under that tree, with what clarity would I have exposed the contradictions of our social system, with what force would I have demonstrated all the abuses of our institutions, with what simplicity would I have demonstrated that man is naturally good, and has only become bad because of those institutions. All that blazed in my mind for a quarter of an hour under that tree has been thinly scattered in my . . . principal works."

Thus erupted, from the bitter wellsprings of personal experience, the "great and melancholy system" that transformed Rousseau's life and revoluntionized modern consciousness. The resounding negative with which he answered the question posed by the Academy of Dijon (and won the prize) turned a literary exercise into a passionate denunciation of the moral and political corruption inherent in civilized society. Intellectually, the reactions to his paradoxical diagnosis drove him to probe more deeply into the origins and implications of the evils of social existence, and to conceive of ideal alternatives to them that have never ceased to haunt the modern imagination. He did so in a powerful succession of works that increasingly set him apart from his friends among the would-be reformers of the Enlightenment, even as he demolished the principles of the traditional social order. Existentially, he found himself condemned to live out his system through increasingly paranoid efforts to maintain the independence of a virtuous self within a corrupt society. Estranged both philosophically and personally, Rousseau therefore offered his life -- and his celebrated autobiography -- in evidence for his indictment of modern society.

And what an amazing life it is, as Maurice Cranston presents it in this admirable biography, which is as meticulous, calm, reasonable, and judicious as its subject is passionate and tumultuous. Drawing on the substantial advances in Rousseau scholarship in recent years, subjecting the author's own Confessions wherever possible to the test of independent evidence (which is offered in abundance), this first volume follows the "Citizen of Geneva" from his unhappy early years in that city to his triumphal (but temporary) return as a philosopher some 40 years later.

The road was a tortuous one, inscribed with themes of exile and loss, poverty, humiliation, and displacement. From Calvinist Geneva, where Rousseau lost his mother at birth and found himself abandoned by his father before he was 10, it led to neighboring Savoy. There, survival dictated entry into the bosom of a Catholic church always ready to reclaim converts from the reformed religion, while fortune conferred entry into the more enticing bosom of the famous Mmd. de Warens. Eventually displaced in her affections, Rousseau found himself again obliged to move on, this time to Lyon (as tutor in the family of the enlightened M. de Mably, brother of the philosophers Condillac and Mably), to Paris (long enough to secure a post as secretary to the French ambassador to the Venetian Republic), to Venice (a stay marked by a passionate affair with Venetian music and stormy relations with his employer), and back to Paris, to the afflictions of literary fame and the strange solace of the liaison with the illiterate servant girl, Therese Levasseur.

Following Rousseauu along this road in marvelously compelling detail, Cranston is an enormously sympathetic biographer. He has little inclination to blame a subject made so vulnerable by his own Confessions, and still less taste for "the kind of patronizing psychological explanation which takes the place of blame in contemporary deterministic culture." He avoids the temptation to cry hypocrite against a philosopher who justified abandoning five illegitimate children to the foundling hospital by citing the provisions of Plato's Republic regarding communal education, preferring to point instead to the prevalence of the practice of child abandonment among the poor of 18th-century Paris. And while he does not neglect the complex development of Rousseau's sexuality, he is perhaps more revealing on the importance of the philosopher's passion for music -- which emerges in this account as one of the fundamental themes of his life and thought.

Cranston is also very suggestive in his discussion of Rousseau's experiences as a domestic servant. To eat, the future philosopher served the rich and noble in a variety of positions -- as valet, as secretary, as tutor, as cashier, as "research assistant." Reading Cranston's description of these successive situations, one is struck by the porosity of the society they reveal, as by the extent to which Rousseau's mature philosophy must have been shaped by them. Certainly, the philosopher owed much of his education, as well as the contacts that brought him into enlightened circles, to the encouragement or needs of his employers. He seems also to have contracted a disdain for the wealthy parvenus he came to serve, which was given explosive force in the condemnation of the rich he offered in his later essay on the origins of inequality. But, above all, one wonders how much of his experience of domestic employment -- with its extraordinary combination of intimacy and exploitation, and its dialectic of dissimulation and dependence -- Rousseau distilled in his understanding of the essential nature of social relations, as he came to see them in the moment of revelation on the road to Vincennes.

Cranston leaves Rousseau in 1754, after he has returned to Geneva to recover the rights to citizenship lost upon his conversion to Catholicism. Restored (rather casually) to his earlier faith, feted by fellow citizens now eager (for the moment) to reclaim him, the philosopher vows to return for good once he has set his Parisian affairs in order.

But the fantasy of belonging is not to be realized: we know that Rousseau is obliged to remain in exile, where his greatest works will be written and his quarrels with society will become yet more bitter. We therefore await with anticipation the concluding volume of Cranston's masterly study.