The Emergence of Phenomenology Amid Varieties of Rationalism Rationalism and empiricism are typically glossed as opposing theories of knowledge. For rationalism, knowledge is founded in reason; for empiricism, knowledge is founded in sense perception. Yet the rationalist impulse is felt in different areas of philosophy, and it is felt differently in different epochs (see chapter 1).
Plato's metaphysics focused on the ideal forms (eidos, logos) in which all concrete things participate. Our knowledge or appreciation of the forms, witnessed in Plato's dialogues, is approached through Socratic dialectic. In the next generation Aristotle furthered Platonic rationalism by codifying logic (for the first time in human history), both canonizing the discipline and crafting his theory of syllogism. Yet Aristotle also sought a metaphysics of forms that placed them in matter in the natural world. This naturalism invited the turn to empirical methods in the formation of knowledge, though this empiricist impulse remained restrained until the beginnings of modern science in Galileo and Newton.
With the dawn of modern science, Descartes' epistemology fashioned the rationalism we associate with the seventeenth century, followed by Leibniz, Malebranche, and Spinoza. For Descartes, knowledge is developed through the practice of reason very much as in mathematical proof or demonstration. In the pure light of reason, over the course of Descartes' Meditations on First Philosophy (1641), we see the steps of inference that lead from the cogito (I think, therefore I am) ultimately to the metaphysics of mind-body dualism. In Descartes' hands, then, rationalism defines epistemology but in practice grounds his metaphysics. So too the other great early modern rationalists produced magnificent systems of metaphysics, in Leibniz's monadology and Spinoza's monism. The light of reason leads thus through reflection into insight about the fundamental nature of reality. Thus, early modern rationalism moved through epistemology to metaphysics. Soon, modern empiricism arrived in opposition. For Locke, Berkeley, and Hume, knowledge begins with sensory perception; all our ideas of things begin or are formed from ideas of sense. And empiricist epistemology led into idealist metaphysics. For Berkeley, material objects are but bundles of ideas, ultimately sensory ideas; for Hume, they may as well be mere impressions (we cannot be sure of anything else); for Locke, the things themselves lie beneath our ideas but are we know not what.