René Descartes (1596 1650) and his search for certainty in an uncertain age needs to be seen in the context of his times - the 2000 year plus Aristotlean tradition against which he was struggling; the turmoil and actual physical danger caused by the Reformation and counter Reformation; a continent still dominated by superstition and medieval rumour.... Regardless of the problems he faced in his 20 year odyssey to produce key works such as Method and Meditations, his purpose never wavered with the resulting books and cogito providing Europe with a definitive shift away from Classical Greece and into a brave new world which had an increasingly scientific and industrial Western Europe as its epicentre...
Immanuel Kant (1724-1804) can be seen as the ultimate embodiment of the Enlightenment movement and its emphasis on the use of reason. His works on practical and pure reason put to rest two all-encompassing philosophical debates concerning rationalism vs empiricism, and the quest to establish proof of God's existence which had dominated European thinking for the best part of 250 years. He attempted to establish a purely Enlightenment system of ethics based solely on the use of reason around the idea of the Categorical Imperative which in itself provided impetus and inspiration for the later German Idealists who themselves came to dominate Continental thinking until the end of the First World War...a real Great Thinker!