During this period the source of true knowledge was scripture and revelation, and philosphers were primarily concerned with interpretation rather than discovery. Neither rationality nor experiment could provide the support needed to defy the authority of the church.
Descartes was the first well known philosopher to have escaped from the tyrrany of this ecclesiastical epistemology. He is known not only as a scientist and mathematician, but also as a philosopher adopting the radical methods described in this Discourse on Method.
Descartes adopted the method of systematic doubt, in which he first doubted all that possibly could be doubted and then attempted to rebuild on those indubitable foundations which remained.
Descarte's doubts are more convincing than his subsequent reconstruction. He was a mathematician, and consequently valued highly the certainty obtained in ascertaining mathematical results by purely rational means. He did not appear to have much awareness of the limits of rationality.
Locke first of all claims that all our ideas come from our senses, a thoroughgoing empiricist position contrasting Descarte's rationalism.
Berkeley, showing a greater awareness of the difficulties of establishing knowledge in this way takes up an idealist position. If knowledge is to originate exclusively from ideas, then knowledge can only be of ideas, and the "external world" must in reality be just a complex of ideas.
Hume's philosophy shows the most complete understanding of the relative capabilities of rational, or deductive processes, and empirical methods. He demonstrates a good grasp of what cannot be established deductively, an important step in the evolution of the necessary/contingent and analytic/synthetic dichotomies.