The reckoning: reason vs experience
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Six thinkers, two roads, one wreckage. The rationalists built cathedrals of pure reason; the empiricists demolished them plank by plank — and then found the ground giving way beneath their own feet. Time to add up the score.
We began this course with a map: rationalists (Descartes, Spinoza, Leibniz) trusting reason, empiricists (Locke, Berkeley, Hume) trusting experience. Having now walked both roads to their ends, we can do the reckoning — not just who said what, but what each side genuinely secured, what it overreached on, and why a single shared assumption dragged the whole era toward skepticism.
The rationalist ledger — the credits. The rationalists were right that some knowledge is necessary, universal, and a priori — mathematics is not a report on what we happen to have seen. They saw that pure reason can grasp truths the senses could never certify (that the angles of every triangle sum to two right angles), and they took metaphysics seriously enough to demand demonstration, not mere opinion. Descartes’s cogito, Spinoza’s rigorous more geometrico, Leibniz’s Principle of Sufficient Reason — each is an attempt to make thought answerable to proof.
The rationalist ledger — the debits. But the price of that ambition was extravagant metaphysics that no experience could check. Descartes needed God to guarantee his clear and distinct ideas — and then had no coherent account of how an unextended res cogitans (thinking thing) could push around an extended res extensa (extended thing): the mind–body interaction problem. Spinoza’s answer — one infinite substance, Deus sive Natura (Ethics I) — dissolved the problem by dissolving the individual. Leibniz’s answer — windowless monads in pre-established harmony (Monadology §7, §51) — saved the individual at the cost of a universe of perfectly synchronised solitary confinements. Grand, unfalsifiable, and mutually incompatible: three geniuses reasoning rigorously to three irreconcilable worlds.
The empiricist ledger — the credits. The empiricists landed a genuine blow. Locke’s attack on innate ideas (Essay I) and his tabula rasa (blank slate) forced a hard question the rationalists had waved away: where did that idea actually come from? If every concept must trace to experience, then a whole class of grand metaphysical claims — a demonstrated God, the necessity of causal bonds, the inner nature of substance — suddenly owe a debt they cannot pay. Empiricism is epistemic honesty: it insists that ideas be cashed out in the experiences that gave rise to them.
The empiricist ledger — the debits. Yet empiricism carried a hidden charge that detonated late: the way of ideas. Locke held that the mind perceives not the world directly but its own ideas — mental representations. The senses deliver a copy; the mind reads the copy. This is representative realism: real bodies out there cause ideas in here, and we know the world only through the intermediary. It sounds like common sense. It is, in fact, a trap.
What is the 'way of ideas', and why does it become a liability for empiricism?
Watch the slide. The way of ideas is a slope, and the three empiricists stand at three points on it. Locke keeps the external material world but can only infer it behind the veil — representative realism. Berkeley notices that an unperceivable material substance, forever behind the screen, is an idea with no content: so he cuts it away. Esse est percipi — to be is to be perceived (Principles §3): there are only minds and ideas, and the world is real but mental. Hume takes the final step: apply the same knife to everything, including the mind that was doing the perceiving.
Where the slide ends: Hume. Turn the empiricist demand — from what impression does this idea derive? — on the load-bearing beams of ordinary thought, and they snap. Causation: we see one event follow another, again and again, but we never have an impression of the necessary connection between them; so our idea of causal power is a projection of custom, not a perception of a real tie in the objects (Enquiry VII). Induction: no reasoning, demonstrative or probable, can justify moving from observed to unobserved without assuming the very uniformity in question (Enquiry IV). The external world and the self fare no better — each is an idea with no steady impression behind it.
The common threads. Notice what unites all six across the divide. First, nearly all accept the representational theory of mind — that we deal immediately with ideas/perceptions, not with things — and it is this shared assumption, not rationalism or empiricism as such, that opens the veil of perception. Second, all wrestle with the mind–body problem bequeathed by Descartes’s dualism. Third, all are driven by the era’s defining hunger: the search for secure foundations, a bedrock certainty on which to rebuild knowledge after the collapse of the old scholastic-Aristotelian world-picture.
Why did strict empiricism, followed honestly, tend to slide toward skepticism by the time of Hume?
The upshot — a crisis, not a conclusion. The early-modern age did two things at once. It dissolved the old world-picture — teleological, Aristotelian, God-saturated — and cleared the ground for modern science and the sovereign individual mind. But in Hume it also seemed to undermine the very science it began from: if we have no rational warrant for causation or induction, then Newtonian physics rests on custom, not reason. Reason had eaten itself. The question the whole era leaves on the table is stark: can we have both — a world we genuinely know, and an honest account of the mind’s limits? Answering it required someone new.