Pierre-Simon, Marquis de Laplace (1749-1827) is often described as the ‘French Newton’, though he lived a century later.His working life took him through the French Revolution, the Napoleonic era – during which he enjoyed various political positions – and afterwards, to the Bourbon Restoration. But his overriding importance was his contribution to science, with a remarkable range of interest and expertise including engineering, mathematics, statistics, physics, astronomy and philosophy. One of his most enduring works is recorded here – A Philosophical Essay on Probabilities (1825). As he explains in his opening introduction: “This philosophical essay is the development of a lecture on probabilities which I delivered in 1795 to the normal schools whither I had been called, by a decree of the national convention, as professor of mathematics with Lagrange. I have recently published upon the same subject a work entitled The Analytical Theory of Probabilities. I present here without the aid of analysis the principles and general results of this theory, applying them to the most important questions of life, which are indeed for the most part only problems of probability.” The Essay comprises 18 chapters, including ‘General Principles of the Calculus of Probabilities’, ‘Application of the Calculus of Probabilities to the Moral Sciences’ and ‘Concerning Tables of Mortality and the Mean Durations of Life, Marriage and Some Associations’. He considers the ratio of the birth of boys to girls; he looks at probabilities behind risk when applied to the (increasingly widespread acceptance of) smallpox inoculation discovered by Edward Jenner; he discusses probabilities in games of chance, the reliability of witnesses in jury trials and the fluctuations of tides. He even applies it to the range of feeling in animals analogous to humans. The Essay is clear and generally accessible, but also surprising in that the personality of Laplace himself shines through – a brilliant polymath and mathematician who nevertheless lived fully engaged in the everyday world. Translated by Frederick Wilson Truscott and Frederick Lincoln Emory.