A Treatise Concerning the Principles of Human Knowledge

In Section 7,Berkeley states that the only thing that actually exists is “Spirit, or that which perceives” (26). Sections 8-21 comprise an argument rejecting the notion of matter. Such a substance exists only as an abstraction in the mind; in the real world, we find only individual things with particular properties and qualities, not a generic “inert, senseless substance” (27) called “matter.”

Berkeley’s argument is that even if the ideas of things cannot exist outside of our mind, the things themselves—the originals of which the ideas are “copies or resemblances” (26)—might exist outside of the mind. Berkeley rejects the notion that ideas have a copy or original in the real world, because “an idea can be like nothing but an idea” (26). Anything that is perceivable is intrinsically an idea.

Supposing that external objects or matter did exist outside of the perceiving mind, the only way we could know this would be “by sense or by reason” (31). This again proves Berkeley’s point: All things are only ideas in the mind since sense and reason furnish us only with ideas of our sensations and reflections. The fact that dreams and hallucinations can physically or mentally affect us proves that the things we perceive need not exist outside of us to be real.