In The Principles of Human Knowledge, George Berkeley presents his theory of idealism, arguing that the material world does not exist independently of the mind. Berkeley contends that all that exists are ideas perceived by the mind and that objects only exist as they are perceived by individuals or by God. His famous dictum ""esse is percipi"" (to be is to be perceived) challenges traditional materialist views, proposing instead that reality is fundamentally mental. The work questions the nature of perception and the existence of the external world, offering a revolutionary perspective on metaphysics and epistemology that influenced later philosophical thought.