The Principles, published when the author was only twenty-six, is the most systematic of all of Berkeley's expositions of his theory of knowledge: it was the direct outgrowth of the Essay Towards a New Theory of Vision (1709), which sought to ban-ish the metaphysical abstractions of Absolute Space and Extension from philosophy, and was itself mainly concerned with the aboli-tion of Abstract Matter and of the ontological and theological corollaries of that concept. The Dialogues treat of substantially the same subjects but are more familiar and elegant in form and are devoted in the main to...