Is all of our knowledge based on the evidence of the senses, or is some of it justified by other means? This epistemological question about the foundations of knowledge is what separates Rationalism and Empiricism. According to Rationalism at least some knowledge can be had through reason alone. For rationalists, the paradigm example of knowledge acquired independent of sense experience is mathematics. Once we have the concepts required to understand mathematical propositions (like 2+2=4), no experience is required to be justified in accepting their truth. They seem to be adequately known “through the light of reason.” Empiricism, on the other hand, takes all of our knowledge to be ultimately grounded in sense experience. Descartes was the first significant rationalist philosopher of the modern classical period. He rejects sense experience as a trustworthy source of knowledge early in his Meditations. Following Descartes, a number of other European philosophers develop rationalist philosophical systems. Leibniz and Spinoza are the most notable. Meanwhile, an empiricist tradition gets started in Great Britain. The three major empiricist philosophers are John Locke, Berkeley and David Hume. In this chapter we will focus on Descartes, Spinoza, and Liebniz, and we will take up the empiricists in the next chapter.