Diligent observations of a universe ruled by Newton and Napoleon

Somewhere still, no doubt, university first-years continue to invoke the name of Pierre Simon Laplace in their bleary midnight debates over the role of chance and fate in the universe. For when they do, they have the brief thrill of imagining themselves, if only theoretically, as a Being of Infinite Intelligence who, by knowing the position and trajectory of all the universe's constituent parts, can predict its future as it clacks its way like so many billiard balls into the indefinite reaches of time. All who enjoy such speculations will find a compelling portrait of the man behind this Being in Roger Hahn's trim and accessible biography of Laplace.

At the end of the 18th century, when Paris hosted so many of Europe's scientific lights, Laplace was its most constant star. His was a 50-year career of productive contradictions. He was an ingenious mathematical physicist who always subordinated his theories to empirical confirmation. An innovator in mathematical techniques who is remembered primarily as a synthesiser of Newton's celestial project. A natural philosopher who opened new avenues of research into terrestrial phenomena (heat, chemical affinity, optics, the speed of sound) but whose agenda was largely repudiated in the 19th century. And a savant who eschewed partisanship and dutifully served the dizzying succession of political powers that governed France from the ancien régime to the Restoration, notably as Napoleon's first ephemeral Minister of the Interior and as one of the emperor's docile senators-for-life.

When Laplace arrived from Normandy as young man, scientific Paris was still debating whether God could have willed the laws of the universe to be other than they were. France's pre-eminent mathematician, Jean le Rond d'Alembert, had suggested that there were two sorts of laws, one set based on logical assumptions about the nature of matter and another derived from human observation.

Working in complementary fashion, two young suitors for d'Alembert's patronage, Laplace and Marie-Jean-Antoine-Nicolas de Condorcet, gave this answer its modern classical form. In Laplace's version, the distinction was not between two types of laws, but between ontology and epistemology. The universe, he argued, was composed of matter that behaved according to a set of deterministic laws. It was the task of the human investigator, equipped with only his senses, to resolve, as best he could, his uncertainty about those laws. Thus, Laplace's work in probability theory was not meant to demonstrate the rule of chance in the world, but to circumvent human ignorance about the hidden causes that drive physical phenomena-what Hahn calls proto-positivism.

Laplace's central scientific achievement was to resolve the residual discrepancies that had seemed to elude the Newtonian framework - for instance, in the motion of Jupiter and Saturn - by showing that they conformed to those laws over time. One of his most celebrated conjectures was the nebular hypothesis, suggesting how the solar system might have developed mechanistically out of a spinning disk of dust: the culmination of Enlightened Newtonianism. When Napoleon, his former examination pupil, asked Laplace why God was nowhere mentioned in his famous treatise on the system of the world, Laplace is said to have replied, "Because, Sire, I had no need for that hypothesis."

Previous biographies of Laplace, notably the one co-ordinated by Charles Gillispie, have focused on Laplace's scientific achievements. Hahn's admirable goal is to integrate his science with his personal and public life. To do so, Hahn has painstakingly assembled the manuscript sources that make possible such speculation from the outside - proto-positivistically, as it were. In his lifetime, Laplace's scientific and temperamental constancy was often ascribed to his Norman stolidity. Hahn persuasively ascribes it to his decision, made relatively young, to forsake the cloth and light his path unswervingly by his determinist credo. Despite apparent changes in allegiance, he was no girouette , twisting with the political winds like a weathercock.

Yet the relationship between his credo and his career remains enigmatic, as Hahn himself acknowledges. Throughout his later life, Laplace consistently worked to carve out a public role for science, above the political fray yet dedicated to state service. In striking this technocratic pose - suppressing his personal political views while remaining loyal to political authority and supplying it with analytical tools to solve its engineering problems - Laplace seems to have drawn on his solution to the problem of knowledge: acknowledging that the causes of natural phenomena are hidden from science, while contenting himself with models that track the unfolding of immutable laws. In this, too, he helped model one of the mainstays of the modern settlement for science. This political settlement, too, has also kept many inquirers up late at night.

Ken Alder is professor of history, Northwestern University, Illinois, US, and author of The Measure of All Things: The Seven-Year Odyssey That Transformed the World .