The empiricists — Locke, Berkeley, and Hume — responded to the rationalist project by insisting that knowledge could only come from experience. They rejected the Cartesian and Leibnizian confidence in innate ideas and a priori reasoning. The mind at birth, they argued, is a blank slate (tabula rasa), furnished with ideas only through sensation and reflection. Claims to knowledge must be tested against experience, not derived from definitions or the supposed necessities of reason.
The result was not merely a different set of conclusions but a different conception of what philosophy could legitimately claim. The rationalist dream of a complete, systematic, rationally transparent account of God, nature, and the human condition began to look, from the empiricist perspective, like an elaborate exercise in self-deception.
Locke: The Blank Slate and the Limits of Knowledge
John Locke, in his Essay Concerning Human Understanding (1689), set out the empiricist programme with characteristic modesty and clarity. All ideas, he argued, come from two sources: sensation (the mind’s awareness of external objects through the senses) and reflection (the mind’s awareness of its own operations). There are no innate ideas, no innate principles, no innate moral knowledge. The mind at birth is white paper, void of all characters, upon which experience writes.
Locke distinguished between primary qualities — solidity, extension, figure, motion, number — which really exist in the objects themselves, and secondary qualities — colour, taste, smell, warmth — which exist only in the perceiving subject as the effects that primary qualities produce in a sensitive nervous system. This distinction, which had roots in Galileo and Descartes, became central to the new science and to all subsequent debates about the scope of scientific explanation.
Locke was a cautious philosopher. He did not claim that we have knowledge of the real essences of things, only of their nominal essences — the observable qualities by which we classify them. He did not claim that we have certain knowledge of the external world, only that the testimony of the senses, when properly checked and corroborated, gives us reasonable assurance. His philosophy was a philosophy of limits as much as of foundations: the mind is furnished by experience, and the limits of experience are the limits of knowledge. The new science, with its emphasis on observable regularities and mathematical description, fit comfortably within those limits; the old metaphysical ambition to know the ultimate nature of things did not.
Berkeley: To Be Is to Be Perceived
George Berkeley, an Irish bishop, pushed the empiricist logic to a conclusion that shocked even many of his fellow empiricists. If all we ever know are our own ideas, then the very notion of material substance independent of perception is incoherent. To be is to be perceived — esse est percipi. The tree in the quad exists when no one is looking at it only because it is perceived by God, whose mind sustains the world in continuous existence.
Berkeley’s target was not common sense but the metaphysical materialism that he believed underwrote atheism and scepticism. By eliminating material substance, he thought he had removed the ground for the claim that the mind is a mere epiphenomenon of matter. The world of ordinary objects — tables, chairs, apples — remains exactly as it appears; what disappears is the unperceivable, unknowable “something I know not what” that Locke and the materialists had posited behind the appearances. The tree in the quad exists when no one is looking only because it is perceived by God, whose mind sustains the world in continuous existence.
Berkeley’s arguments were ingenious and difficult to refute on strictly empiricist grounds. They also revealed how fragile the boundary between empiricism and idealism could become.
Hume: The Crisis of Causation and the Self
David Hume, in his Treatise of Human Nature (1739–40) and Enquiry Concerning Human Understanding (1748), applied the same critical method to the most fundamental assumptions of both science and common life. We never observe necessary connections in nature, he argued; we observe only constant conjunctions — one event following another with regularity. The idea of causation is not a discovery of reason or a perception of the world; it is a habit of the mind, a custom formed by repeated experience, projected onto the world.
The same analysis applies to the self. When I look within, Hume wrote, I never catch myself without a perception; I never observe anything but the perceptions themselves. The self is not a substance that owns the perceptions; it is nothing but the bundle or collection of perceptions, related by resemblance, contiguity, and causation — all of which are themselves products of the imagination.
Hume’s scepticism was not merely destructive. He believed that once we understand the true origins of our beliefs — in custom, habit, and the natural workings of the imagination — we can return to ordinary life with a mitigated scepticism that is both intellectually honest and practically useful. The self is nothing but the bundle of perceptions; causation is a habit of expectation. Yet the damage to the rationalist and even the moderate empiricist confidence in the foundations of knowledge was profound. The problem was not merely that previous philosophy had failed to justify its claims. It was that the new science itself — whose success was undeniable — rested on assumptions (causation, induction, the uniformity of nature) that neither rationalism nor empiricism had been able to secure.
The Awakening of Kant
Immanuel Kant later said that Hume had awakened him from his dogmatic slumbers. The problem was not merely that previous philosophy had failed to justify its claims. It was that the new science itself — whose success was undeniable — rested on assumptions (causation, induction, the uniformity of nature, the existence of an external world) that neither rationalism nor empiricism had been able to secure on its own terms.
The empiricists had shown that the rationalist dream of building a complete system of reality from clear and distinct ideas was untenable. But they had not shown how the actual practice of science — which clearly produced genuine knowledge — was possible. Locke had defended a modest realism grounded in experience; Berkeley had dissolved material substance into perception; Hume had shown that even causation and personal identity were not objects of knowledge but products of the imagination. The result was a profound crisis of confidence in the foundations of both science and philosophy.