"Berkeley’s argument consists of two parts. On the one hand, he argues that we do not perceive material things, but only colours, sounds, etc., and that these are “mental” or “in the mind.” His reasoning is completely cogent as to the first point, but as to the second it suffers from the absence of any definition of the word “mental.” He relies, in fact, upon the received view that everything must be either material or mental, and that nothing is both."