Where does genuine knowledge actually come from? Two competing answers dominated European philosophy from roughly the 1600s through the 1700s. Rationalists, led by figures like Descartes and Leibniz, argued that reason alone — pure logical thought, independent of sensory experience — could establish real, certain knowledge about the world. Empiricists, led by Locke and Hume, argued the opposite: that all genuine knowledge ultimately traces back to sensory experience, and that reason without experience to work on produces nothing but empty, unverifiable speculation.
Descartes' certainty, and Locke's blank slate
Descartes' rationalist project began by trying to doubt everything that could possibly be doubted, searching for something reason alone could establish as absolutely certain — arriving at "I think, therefore I am" as a foundation, then attempting to reason his way from that single certainty toward broader knowledge about the external world, all without relying on the senses, which he considered unreliable and capable of deceiving us. Locke's empiricist response, most fully developed in his "Essay Concerning Human Understanding," argued the mind at birth is a blank slate — tabula rasa — with no innate ideas at all, and that every concept a person ever has, however abstract, ultimately derives from sensory experience, either directly or by the mind combining and reflecting on ideas experience first supplied.
Hume's challenge, and Kant's attempted reconciliation
David Hume pushed empiricism to its most radical conclusion, arguing that even something as basic as our belief in cause and effect can't actually be justified by pure reason or direct observation — we never observe causation itself, only one event reliably following another, and Hume argued our confidence that the future will resemble the past is a habit of the mind, not something reason can prove. This became known as the problem of induction, and it struck rationalists and empiricists alike as a genuine crisis. Immanuel Kant later attempted a reconciliation between the two camps, arguing that while all knowledge begins with experience, the mind itself supplies certain structuring concepts — space, time, causation — that experience is then organised through, meaning neither pure reason alone nor raw sensory experience alone can fully account for human knowledge; both are doing indispensable work together.
What we're still unsure about
The rationalism-versus-empiricism debate isn't a settled question modern philosophy has moved past — it persists in updated forms, including in ongoing debates over whether some human knowledge (like basic grammatical structure, or certain mathematical and logical intuitions) is genuinely innate rather than learned purely from experience, a question modern cognitive science and linguistics engage with directly rather than treating as purely historical. Kant's proposed synthesis is influential but not universally accepted either, and contemporary epistemology contains active descendants of both original positions, meaning the underlying question — how much of what we know is built into the mind versus supplied entirely by experience — remains a live, unresolved area of philosophical and scientific inquiry rather than a dispute that was ever definitively closed.
This sits inside Rationalism vs. Empiricism, one of seven topics in Epistemology, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.