In the mid-18th century, Paris became the center of an explosion of philosophic and scientific activity challenging traditional doctrines and dogmas. The philosophic movement was led by Voltaire and Jean-Jacques Rousseau, who argued for a society based upon reason rather than faith and Catholic doctrine, for a new civil order based on natural law, and for science based on experiments and observation. The political philosopher Montesquieu introduced the idea of a separation of powers in a government, a concept which was enthusiastically adopted by the authors of the United States Constitution. While the Philosophes of the French Enlightenment were not revolutionaries, and many were members of the nobility, their ideas played an important part in undermining the legitimacy of the Old Regime and shaping the French Revolution.