Pierre-Simon Laplace (1749-1827) was the son of a peasant farmer. He showed his mathematical ability early and at eighteen he went to Paris to make his way in mathematics. A letter on the principles of mechanics written to d'Alembert gained him a professorship at the École Militaire. His discovery that the attractive force of a mass upon a particle could be obtained directly by differentiating a single potential function laid the mathematical foundation for the analysis of heat, magnetism, and electricity. Because he was apolitical he escaped imprisonment and the guillotine during the Revolution. He was later made a marquis.