In Berkeley’s view of what exists in the world, there are only three things: minds, ideas, and God. Angels are also minds, and another way of dividing up the world is into spirits and ideas. Human beings, angels, and God are spirits. Everything else is ideas. Nothing else is known to exist. But, if only spirits and ideas exist, how can there be a world?

Berkeley thought that what we think of as an external world is just one idea added to our ideas of sense. No idea of sense can exist without being perceived by some mind. Berkeley’s motto was esse est percipi, or, “To be is to be perceived.” The idea of an external world is an isolated idea in itself, but no more than an idea. Furthermore, many of the ideas that we think we have, which support the existence of external reality, are no more than special distinct ideas combined with ideas of sense. For example, the ideas “reality” and “physical matter” are just words to which nothing like an external world corresponds. At best, they are merely additional ideas. This doctrine that reality is just another idea, in Berkeley’s sense, is what made him the philosophical idealist par excellence.