Berkeley thinks that there are only two sorts of things in the world: ideas and the minds that have them. This does not mean, however, that he thinks that all of reality is just a subjective figment of our imaginations. He believes firmly in the existence of a "real world." He just thinks that this real world is made up entirely of ideas.

Specifically, real things are collections of sensations. We can distinguish real things from our other ideas (such as the products of our imagination and memory) because they are more vivid, and they are involuntary. In other words, we can tell which ideas are real things by telling which of our ideas are sensory perceptions.

Berkeley's theory that real things are just collections of sensations is often summed up by the Latin phrase, "Esse is percipi," which means, "their existence is to be perceived." This is just another way of saying that real things are mind-dependent, or that they are ideas.

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