Why did rationalism start?
While it is difficult to say why, exactly, rationalism emerged in late seventeenth-century Europe, it is worth noting that for many intellectuals, and for Europeans in general, that time saw significant amounts of social, cultural, and intellectual turmoil. On the one hand, the seventeenth century saw enormously destructive conflicts, many of which were at least indirectly related to religious disagreement. The Protestant challenge to Catholicism, in short, was still a source of major contention. The seventeenth century also saw a great deal of intellectual ferment, as thinkers like Galileo, Newton, and others articulated a number of ideas that tended to call time-honored understandings of nature into question. So in light of all this, a philosophical approach that sought to base its conclusions on what could absolutely be said to be true appealed to many. Rationalists like Descartes attempted to found their conclusions on irrefutable truths, using deductive reasoning to work their way from these truths to larger, complex ideas. It was the logical nature of this approach that perhaps won it such appeal on the Continent (though not so much in Britain.)