Portrait of the French astronomer and mathematician Pierre Simon Laplace, 1749-1827. Sponsored by D'Alembert, he became professor of mathematics in Paris. Working with Lavoisier, he demonstrated principles of thermochemistry. His most important work was in astronomy. He theorized that the solar system originated from a cloud of gas (the nebular hypothesis, modified forms of which are still accepted). He studied the motion of the Moon and the planets and published a five-volume survey called Celestial Mechanics. He developed probability theory. He also managed to maintain a state office during a time of violent political changes.