The Enlightenment, also referred to as the Age of Enlightenment, was a philosophical movement that dominated the realm of ideas in 18th-century Europe. It was founded on the principle that reason is the fundamental source of power and legitimacy, and it promoted principles such as liberty, progress, tolerance, fraternity, constitutional governance, and church-state separation. The Enlightenment was defined by a focus on science and reductionism, as well as a growing suspicion of religious rigidity. The Enlightenment's ideals challenged the monarchy and the church, laying the groundwork for the political upheavals of the 18th and 19th centuries. According to French historians, the Age of Enlightenment began in 1715, the year Louis XIV died, and ended in 1789, the year of the French Revolution. According to some contemporary historians, the era begins in the 1620s, with the birth of the scientific revolution. However, during the first decades of the 18th century and the first decades of the 19th century, several national variations of the movement developed.The Englishmen Francis Bacon and Thomas Hobbes, the Frenchman René Descartes, and the prominent natural philosophers of the Scientific Revolution, including Galileo Galilei, Johannes Kepler, and Gottfried Wilhelm Leibniz, were significant 17th-century antecedents of the Enlightenment. Its origins are often ascribed to 1680s England, when Isaac Newton published his "Principia Mathematica" (1686) and John Locke wrote his "Essay Concerning Human Understanding" (1689)—two works that laid the groundwork for the Enlightenment's great advancements in science, mathematics, and philosophy.