The original, French edition of this book appeared in 2004 with Gallimard under the title Le Système du monde. Pierre Simon Laplace – un itinéraire dans la science (Paris, 2004). The ‘itinerary’ in question was the lengthy and tortuous one that took Laplace from Beaumont-en-Auge in Normandy, where he was born into comfortable (though far from wealthy) rural obscurity in 1749, to national and international eminence as one of the most powerful scientists of his generation. Through the vicissitudes of a long life, as Roger Hahn argues, Laplace was profoundly marked by his experiences and beliefs as a young man. Even his scientific programme was formulated early. It was a programme, Newtonian in its thrust, that led many to regard him as the Newton of France; even the timing of his death in 1827, just a century after Newton's, lent force to an accolade whose true foundation was rather his host of achievements in celestial mechanics, terrestrial physics and probability theory.

Hahn's own pursuit of Laplace has been long and tortuous too. But it has yielded hard-won documentary evidence that earlier biographers had assumed to be unavailable, especially about Laplace's early life, family and religious opinions. The result is an intricately woven study that not only illuminates our perception of Laplace himself but also gives insights into the many worlds – social, political, scientific – that he frequented. Hahn finds in Normandy some of the richest clues to the man that Laplace was to become. The fourth of five children, Laplace lost his mother early, and his education owed more to his time at the college of Beaumont and the University of Caen than it did to his family. At the university his encounter with a modern-minded teacher of mathematics, Christophe Gadbled, seems to have been decisive in turning his aspirations from the priesthood to science. Leaving family and ecclesiastical mentors behind and arriving in Paris at the age of twenty, Laplace was soon taken under the wing of d'Alembert and found a junior but well-paid teaching post at the Ecole militaire. With the all-important entry to the Académie des sciences achieved (by his election as an adjoint) in 1773, there was no holding someone of his ability and single-minded ambition. His collaboration with Lavoisier in the study of heat followed in the later 1770s, and by the eve of the Revolution he was living comfortably, serving the Académie on committees and frequenting the Parisian scientific elite (though never, significantly, the fashionable salons or political gatherings). This was career-making at its most dedicated and effective.

Hahn's biographical focus also pays dividends for the tempestuous period in which Laplace dextrously navigated the choppy waters of French life between 1789 and the return of the Bourbon line in 1815. By 1796 Laplace was irreversibly a major public figure, a respected popularizer (as the author of the Exposition du système du monde) and a participant in the restructuring of France's scientific institutions, notably the newly founded Institut national des sciences et des arts, after the depredations of the Terror. And on he went, under Bonaparte, to ministerial office (briefly as minister of the interior) and membership of the Senate, with a salary of 72,000 francs that allowed him, with Berthollet, to sustain the brilliant ‘society’ (more properly a circle of promising young scientists) at Arcueil, on the southern outskirts of Paris. Only in the last decade of his life, with some of his most cherished scientific beliefs coming under attack, did his star begin to wane, and even then only slowly.

So what kind of Laplace are we left with? Hahn presents him as ‘steadfast’ in his philosophy, single-minded in his science and resilient in his personal life. One person's steadfastness, of course, can be another's obduracy, but Hahn's Laplace, though cautious in everything he did, does not emerge as the inflexible conservative that he is commonly thought to have been. A starting point of political neutrality, especially evident in his early years, made it easy for him to sway with the changes of regime, and in religion he moved from youthful Catholic conformity to Enlightenment scepticism and finally back to a partial reconciliation with Christianity, tempered by the difficulty of reconciling the notion of supernatural divine intervention with that of a law-bound universe in the Laplacean manner. Not surprisingly, contemporaries sometimes found it hard to determine where the true Laplace lay. This was not someone who inspired affection, and he remained intensely private, travelling little (never abroad and never south of the Loire) and revealing little of himself, apparently even to friends. Such a man was no easy target, and we must be grateful that, in Roger Hahn, he has found the accomplished, tenacious biographer that he deserves.