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Gabrielle Émilie Le Tonnelier de Breteuil, Marquise du Châtelet was a French natural philosopher and mathematician from the early 1730s until her death due to complications during childbirth in 1749. Her most recognized achievement is her translation of and commentary on Isaac Newton's 1687 book Philosophiæ Naturalis Principia Mathematica containing basic laws of physics. The translation, published posthumously in 1756, is still considered the standard French translation. Her commentary includes a contribution to Newtonian mechanics—the postulate of an additional conservation law for total energy, of which kinetic energy of motion is one element. This led her to conceptualize energy, and to derive its quantitative relationships to the mass and velocity of an object. Her philosophical magnum opus, Institutions de Physique, circulated widely, generated heated debates, and was republished and translated into several other languages within two years of its original publication. She participated in the famous vis viva debate, concerning the best way to measure the force of a body and the best means of thinking about conservation principles. Posthumously, her ideas were heavily represented in the most famous text of the French Enlightenment, the Encyclopédie of Denis Diderot and Jean le Rond d'Alembert, first published shortly after du Châtelet's death. She is also known as the intellectual collaborator with and romantic partner of Voltaire. Numerous biographies, books and plays have been written about her life and work in the two centuries since her death. In the early 21st century, her life and ideas have generated renewed interest.