Descartes laid the foundations of modern science by providing a secure basis for empirical knowledge, and showing that this is the only knowledge we can have of the natural world.
In the most famous philosophical dictum of all time ('I think, therefore I am') he uses the method of doubt - doubting whatever can be doubted - to establish the certainty of his own existence. Whatever else he may doubt, he cannot doubt the existence of the doubter. This certainty allows him to overthrow reliance on revealed truth and install scientific method in its place.
