Principles of Human KnowledgeGeorge Berkeley

Berkeley's Principles advances a thesis so counterintuitive that it has startled readers since 1710: there is no such thing as material substance. What we call physical objects are collections of ideas perceived by minds. To be is to be perceived (esse est percipi) or to perceive. Nothing exists apart from minds and their ideas.

The argument proceeds from Locke's empiricism and turns it against itself. Locke had distinguished primary qualities (extension, figure, motion) from secondary qualities (color, taste, sound), granting only the former to objects as they are in themselves. Berkeley argues that the distinction cannot be sustained. Extension is no more conceivable apart from perception than color is. We cannot form an idea of matter as something existing unperceived, because every attempt to conceive it produces only another idea in the mind. The notion of a "material substratum" underlying our perceptions is, Berkeley concludes, incoherent.

Berkeley insists this is the cure for skepticism. By denying matter, he removes the veil between mind and reality that Locke's representative theory had introduced. We perceive things directly, not representations of things. God sustains the regularity of experience by perceiving all things continuously, which ensures that objects do not blink in and out of existence when no finite mind attends to them. The Principles forced every subsequent empiricist to confront the question of whether the external world can survive the scrutiny of the very method that claims to ground knowledge in experience.