  In his two great works of metaphysics, Berkeley defends idealism by attacking the materialist alternative. What exactly is the doctrine that he’s attacking? Readers should first note that “materialism” is here used to mean “the doctrine that material things exist”. This is in contrast with another use, more standard in contemporary discussions, according to which materialism is the doctrine that only material things exist. Berkeley contends that no material things exist, not just that some immaterial things exist. Thus, he attacks Cartesian and Lockean dualism, not just the considerably less popular (in Berkeley’s time) view, held by Hobbes, that only material things exist. But what exactly is a material thing?  Interestingly, part of Berkeley’s attack on matter is to argue that this question cannot be satisfactorily answered by the materialists, that they cannot characterize their supposed material things.  However, an answer that captures what exactly it is that Berkeley rejects is that material things are mind-independent things or substances. And a mind-independent thing is something whose existence is not dependent on thinking/perceiving things, and thus would exist whether or not any thinking things (minds) existed. Berkeley holds that there are no such mind-independent things, that, in the famous phrase, esse est percipi (aut percipere) — to be is to be perceived (or to perceive).