Intellectual setting
Berkeley wrote in direct response to Locke's representative theory of perception and the mechanical philosophy of Descartes and Newton, arguing that their notion of mind-independent material substance led inexorably toward skepticism and atheism, and aiming to secure both certain knowledge and religious belief by removing matter from the picture entirely.
Key ideas
Esse est percipi
Berkeley argued that to exist, for anything other than a perceiving mind, is simply to be perceived; there is no mind-independent material substratum underlying sensory qualities, only ideas and the minds that perceive them.
God as guarantor of continuity
Since ordinary objects seem to persist even when no finite mind perceives them, Berkeley argued that God's infinite, continuous perception sustains the existence of the physical world when human minds are not attending to it.
Legacy
Berkeley is regarded as the central idealist figure between Locke and Hume in the British empiricist tradition; his critique of abstract material substance influenced Hume's more thoroughgoing skepticism and later shaped Kant's transcendental idealism and twentieth-century phenomenalism.