Broadly speaking, empiricism is the belief that knowledge can only be obtained through direct sensory experience. Rationalists, as distinct from empiricists, do not deny the importance of experience in attaining knowledge, but assert that reason, logic, and inference can be used to attain knowledge not directly available to the senses. George Berkeley offers Locke’s idea of the substratum as an example of rational inference with regard to the absolute existence of matter.

The book’s glossary defines extension as “the property of occupying space” (95). Rene Descartes used this concept as a means of demonstrating the absolute existence of matter, and in the dialogues, Hylas likewise uses it as a counterargument against Philonous’s immaterialism.

This is Berkeley’s primary philosophical position in the text and is the view espoused by Philonous. Idealism is associated with immaterialism, which holds that matter does not have an absolute existence outside a mind that perceives it. The only things that truly exist are ideas and spirits (or minds). Idealism is one branch of empiricism, a philosophical tradition that holds that true knowledge can be gained only through direct observation. In Berkeley’s religious idealism, the bedrock of existence is the mind of God, which perceives all things.