There is little consensus on the precise beginning of the Age of Enlightenment; the beginning of the 18th century (1701) or the middle of the 17th century (1650) are often used as epochs. French historians usually place the period, called the Siècle des Lumières (Century of Enlightenments), between 1715 and 1789, from the beginning of the reign of Louis XV until the French Revolution. If taken back to the mid-17th century, the Enlightenment would trace its origins to Descartes' Discourse on Method, published in 1637. In France, many cited the publication of Isaac Newton's Principia Mathematica in 1687. It is argued by several historians and philosophers that the beginning of the Enlightenment is when Descartes shifted the epistemological basis from external authority to internal certainty by his cogito ergo sum published in 1637. As to its end, most scholars use the last years of the century, often choosing the French Revolution of 1789 or the beginning of the Napoleonic Wars (1804–15) as a convenient point in time with which to date the end of the Enlightenment.