A Philosophical Essay on Probabilities (French: Essai philosophique sur les probabilités) is an 1814 work by Pierre-Simon Laplace presenting a wide-ranging account of the meaning of probability and the uses of the calculus of probabilities across the natural and moral sciences. The essay became one of the best-known introductions to Laplace's approach to probability and statistical inference, and it is frequently cited in later discussions of the classical probability tradition associated with Laplace.
The work is also known for a celebrated statement of determinismâÂÂlater dubbed Laplace's demonâÂÂwhich imagines an intellect that could predict the future from complete knowledge of the present and the laws of nature.
Laplace published the Essai in French in 1814, with early editions appearing in Paris through Courcier. The essay was closely connected to Laplace's larger technical treatise, the Théorie analytique des probabilités (1812): later summaries and editions describe the Essai as originating as an introductory presentation of ideas developed in the treatise and related teaching in Paris in the mid-1790s.
An English translation by Frederick Wilson Truscott and Frederick Lincoln Emory was published in 1902 (New York: John Wiley & Sons; London: Chapman & Hall). Modern scholarly editions have also appeared, including a late-20th-century translation by Andrew I. Dale based on a revised French edition.
The essay is divided into two parts. Part I introduces the concept of probability and general principles of the calculus of probabilities; Part II surveys applications, ranging from games of chance to questions in physics, social decision-making, law, and demography.
Part I develops Laplace's broad philosophical framing of probability, including:
Part II applies probabilistic reasoning to diverse domains, including:
Later philosophical surveys often use Laplace as a central reference point for the classical interpretation of probability, which assigns probabilities by symmetry when outcomes are treated as "equally possible." In this tradition, probability is defined by the ratio of favorable cases to the total number of cases under an assumption of symmetry or neutrality of evidence.
Laplace's discussion of determinism in the Essai became famous in the philosophy of science. A widely cited formulation imagines an intelligence for whom (given complete knowledge and sufficient computational power) "nothing would be uncertain and the future, as the past, would be present to [its] eyes." Later writers commonly refer to this hypothetical intelligence as Laplace's demon and use it as a touchstone in debates about predictability, determinism, and the limits of scientific forecasting.
The essay is frequently discussed in modern overviews of probability's conceptual foundations because it treats probability not only as a mathematical calculus but also as a general tool for reasoning with incomplete information, spanning scientific, legal, and practical decision contexts.
The Essai has been widely reprinted and has remained part of the historical canon of probability and statistics, in part because it combines philosophical interpretation with concrete examples of probabilistic reasoning across science and society. Its early statement of the classical interpretation of probability and its association with Laplacean determinism have made it a recurring reference in philosophical and historical discussions of probability.