Philosophical treatise: A Treatise Concerning the Principles of Human Knowledge

Overview
George Berkeley presents a radical empiricist philosophy that rejects material substance and locates existence in perception. The central slogan is "esse est percipi" , to be is to be perceived , meaning that objects do not exist independently of minds. Berkeley contends that what are commonly called physical objects are only collections of sensations or ideas experienced by perceivers, and that the supposed material substratum is an unnecessary fiction.
The argument proceeds by analyzing how people claim to know things. Sensible qualities such as color, taste, and extension are known only through perception; nothing is ever experienced that could plausibly be a nonperceptual material essence. From this empirical starting point, Berkeley reconstructs ontology so that minds and their ideas replace matter as the ultimate furniture of reality.

Core Argument
Berkeley targets the notion of material substance as posited by philosophers like Descartes and Locke. He challenges Locke's distinction between primary qualities (thought to be intrinsic to objects) and secondary qualities (produced in the mind). Berkeley argues that primary qualities depend on perception just as much as secondary qualities do: extension, motion, and shape are as directly perceivable as color or smell, and cannot be conceived apart from being perceived.
Abstraction plays a crucial role in Berkeley's critique. He argues that the idea of abstract general qualities detached from any particular perception is incoherent. When one tries to imagine "extension" apart from a particular extended thing or "pain" apart from a particular pain, the supposed abstract generality collapses into concrete perceptions. Without a coherent conception of a material substratum, positing matter becomes an idle hypothesis.

Perception, Ideas, and Minds
Berkeley distinguishes between ideas, which are passive and immediately known in perception, and spirits or minds, which are active and capable of perceiving, willing, and thinking. Ideas cannot exist without being perceived, while minds are the perceivers that have ideas. Physical bodies, on this account, are orderly and regular collections of ideas that persist because minds perceive them consistently.
Continuity and causal regularity in the world are explained by the operations of spirits rather than by interactions among material particles. Human minds perceive patterns of ideas that exhibit regularity; these patterns are sustained by a supreme mind. Berkeley thus reinterprets causal language: what we call physical causation is regular succession of ideas as governed by intelligences.

God as Ultimate Perceiver
To answer the worry that objects would cease to exist when no finite mind perceives them, Berkeley invokes God as the perpetual perceiver. God perceives all ideas perfectly and thus guarantees the continued existence and orderliness of the sensible world. Divine perception accounts for the stability, coherence, and law-like behavior of what humans take to be physical objects.
God also functions as the source of regularity and the ground of natural laws. Where matter would be an inert cause, God provides a non-material explanation: God wills the consistent sequence of ideas that humans apprehend as physical events. This theological anchor allows Berkeley to secure objective reality without recourse to matter.

Reception and Influence
Berkeley's immaterialism provoked immediate controversy and enduring debate. Critics accused him of solipsism or of collapsing the external world, while supporters praised his rigorous empiricism and his innovative solution to skepticism about the external world. The Treatise influenced later philosophers who grappled with perception, the limits of abstraction, and the role of mind in constituting reality.
Although few endorse Berkeley's metaphysical conclusions wholesale today, his arguments remain central to discussions about perception, the philosophy of mind, and the metaphysics of objects. The Treatise stands as a provocative challenge to materialist assumptions and a distinctive attempt to reconcile common-sense experience with a strict empiricist principle.