Two great answers have been given to the question of where knowledge comes from, and Descartes and Hume are their sharpest champions. For the rationalist Descartes, the ultimate source is reason: the mind can reach truths on its own that no amount of sensory experience could establish. For the empiricist Hume, the ultimate source is experience: strip away what the senses have given us, and nothing is left to think with. The distance between these two positions organises much of modern philosophy.
Descartes begins by doubting everything he possibly can — the senses, the body, even mathematics — in search of something certain. He finds it not in the world but in the mind: the very act of doubting proves that he, the thinker, exists. From this bedrock of pure reason he tries to rebuild knowledge on secure foundations. Along the way he holds that some ideas are innate, built into the mind rather than learned, and that reason, not the fallible senses, is the road to certainty.
Hume takes the opposite starting point and follows it fearlessly. If we are honest, he argues, every idea we have can be traced back to some impression from experience — there is nothing in the mind that did not first come through the senses. And when he tests our most confident beliefs against this standard, they wobble. We assume the future will resemble the past, but he shows this can't be proven. We speak of causes, but we never actually observe a necessary connection — only one event, then another. Even the self, when he looks inward, dissolves into a shifting bundle of perceptions.
Certainty vs skepticismWhat reason can and can't do.
The result is a striking reversal of mood. Descartes uses doubt as a tool to reach certainty and ends up rebuilding a confident structure of knowledge. Hume uses experience as a standard and ends up dismantling much of what we took for granted, arriving at a bracing skepticism. Yet Hume is no defeatist: he thinks nature and habit make everyday life perfectly workable even when reason can't justify our beliefs. We go on expecting the sun to rise — and rightly — not because we've proven it, but because that's how the mind is built.
This debate did not stay put. It set the stage for Kant, who tried to show that both sides had hold of part of the truth — that the mind actively shapes experience, so knowledge depends on reason and the senses working together. As you weigh the two, notice your own instinct: when you want to be sure of something, do you retreat into reasoning it through from first principles, like Descartes, or do you demand to see the evidence of experience, like Hume? That reflex is the living heart of rationalism and empiricism.