Major debates in epistemology revolve around the sources of knowledge, such as rationalism versus empiricism.

A central historical and ongoing debate within epistemology concerns the primary source from which we acquire knowledge. Rationalism posits that reason, rather than sensory experience, is the chief source and test of knowledge. Rationalists argue that some knowledge, particularly in areas like mathematics and logic, is 'a priori' – independent of experience – and can be derived purely through intellectual intuition and deduction. Prominent rationalists include René Descartes, Baruch Spinoza, and Gottfried Leibniz, who emphasized innate ideas and logical inference. Conversely, empiricism maintains that sensory experience is the ultimate source of all knowledge. Empiricists argue that all ideas and concepts are derived from sensory input, and knowledge is 'a posteriori' – dependent on experience. Figures like John Locke, George Berkeley, and David Hume are key proponents of empiricism, emphasizing observation, experimentation, and the inductive reasoning built upon them. These two traditions represent fundamental disagreements about the very architecture of human understanding and continue to inform contemporary discussions about evidence, intuition, and scientific method.