Two roads: rationalism and empiricism

Given the problem of the last lesson — how can the mind reach a world beyond its own ideas? — the moderns divide into two broad camps over the source of substantive knowledge. The rationalists (Descartes, Spinoza, Leibniz) put their trust in reason, the pure intellect. The empiricists (Locke, Berkeley, Hume) put their trust in experience, the senses. This is the single most useful map of the whole course.

Rationalism, first. Its core claim: the intellect is the primary source of substantive knowledge about reality — knowledge the senses could never certify. Reason can grasp truths that are necessary (they could not have been otherwise) and universal (they hold without exception), reaching past appearances to reality’s deep structure. The model is mathematics: in geometry you prove eternal truths from self-evident axioms, needing no experiment at all.

Two technical terms anchor the rationalist view. Knowledge is a priori (Latin, “from what comes before”) when it is justified independently of sense experience — as in mathematics and logic. It contrasts with a posteriori knowledge, which depends on experience. The rationalists hold that our deepest knowledge of reality is a priori: reason alone, properly used, can demonstrate substantive truths about God, the self, and the world.

A central rationalist commitment is innate ideas: some concepts and truths are not derived from experience but are built into the structure of the mind from birth — latent, waiting to be awakened by reflection. Descartes counts the ideas of God, of the self as a thinking thing, and the axioms of logic and mathematics among the innate. Experience may be the occasion on which we notice them, but it is not their source.

Now the other road. Empiricism’s core claim: all our ideas, and all substantive knowledge, ultimately derive from sense experience. There is nothing in the intellect that was not first in the senses. The mind at birth is, in Locke’s famous phrase, a tabula rasa — a blank slate, “white paper, void of all characters” (Essay II.i) — and experience writes everything on it. Deny innate ideas, and you must show how experience alone can supply the whole contents of the mind.

So empiricist knowledge of the world is a posteriori and, crucially, limited in reach. If every idea must trace back to an impression of sense, then wherever the rationalists claim to reason their way to conclusions beyond all possible experience — the ultimate nature of substance, the necessity of causal connections, a demonstrated God — the empiricist grows suspicious and asks the era’s most dangerous question: from which experience did that idea come? If none, the idea may be empty.

Three battlegrounds structure the whole debate. First: are there innate ideas? Rationalists say yes; Locke opens his Essay with a sustained attack on them. Second: what is the source of our concepts — the pure intellect, or sensory experience? Third, and deepest: how far can reason reach beyond experience? Can the mind demonstrate substantive truths about reality that no experiment could ever reach — or is that pretension the very thing that needs curing?

Here is the arc of the course, the story the map is telling. First the great rationalist systems are built — Descartes’s dualism, Spinoza’s single substance, Leibniz’s monads — each reasoning boldly to sweeping conclusions about reality. Then the empiricist critique bites: Locke, and then Berkeley, press the question of where our ideas really come from. It culminates in Hume’s scepticism, which turns strict empiricism on causation, the self, and the external world with devastating results — and in doing so, famously, wakes Kant from his “dogmatic slumber” to attempt a grand synthesis of the two roads.