 The writings on progress of the 18th century drew inspiration from the intellectual achievements of the 16th and 17th centuries. During this time, Europe witnessed an explosion of scientific and mathematical activity.  In the natural sciences, the main fields of investigation were physics and astronomy.  Major figures included Copernicus (1473–1543), Galileo (1564–1642), Kepler (1571–1630), and Newton (1642–1727).  Newton synthesized the work of the previous thinkers to bring the behavior of bodies on earth and bodies in space under a single scientific law, the law of universal gravitation.  This law states that two bodies attract each other in proportion to their masses and in inverse proportion to the square of the distance between them (Palmer 1965, 265–271). 