George Berkeley’s A Treatise Concerning the Principles of Human Knowledge (1710) presents a form of Metaphysical Idealism which asserts that there are two kinds of reality, idea and spirit. Ideas are real because they can be perceived. Spirit is real because it can have ideas, and because it can perceive them.

Berkeley argues that ideas are derived from physical and mental perceptions, from memory, and from imagination. The existence of an idea depends on its being able to be perceived. An idea does not exist unless it is perceived.

According to Berkeley, "esse est percipi" ("to be is to be perceived"). The existence of an idea cannot be separated from its being perceived. If an idea or object is not perceived, then it does not exist.

The perceiving, active being is referred to by Berkeley as the mind, spirit, soul, or self. The existence of a mind or spirit consists of the ability to have ideas and to perceive them. Spirit, as it perceives ideas, is called the understanding. Spirit, as it produces ideas, or as it mentally operates on them, is called the will (Paragraph 27).

Berkeley argues that there is no substance other than spirit. Substance is not material, but spiritual. Matter neither perceives, nor is perceived. Therefore, matter does not exist. What we describe as matter is only the idea derived from the sensory perception of solidity, extension, form, motion, or other physical properties of an object. But the object only exists if it can perceive or is perceived, and therefore its existence is ideal or spiritual.

Berkeley also argues that abstract ideas do not exist. General ideas of qualities may exist, but these general ideas are only representations of particular ideas.

According to Berkeley, existence consists of the state of actively perceiving or of passively being perceived. If something is not able to perceive or is not able to be perceived, then it does not exist. Everything that can perceive, or that can be perceived, exists. Everything that exists can either perceive or can be perceived.

Berkeley explains that spirit is not itself an idea but that it perceives and produces ideas. Spirit is not itself perceived, but the ideas or effects produced by spirit are perceived. Thus, in order for an idea to exist, there must be a mind or spirit capable of producing or perceiving it. Nothing can exist without a perceiving mind or spirit.

Berkeley also explains that spirit is not known by sensory perception, but that its existence can be known by mental perception. Perceiving beings can perceive the presence of each other.

Berkeley argues that the existence of God can be perceived by human beings. The spirit of human beings is finite, but the spirit of God is infinite.

Berkeley also argues that our own existence as perceiving beings depends on God. He maintains that everything that exists is perceived in the mind of God.

Berkeley contends that in order for anything to exist, it must be capable of being perceived. He argues that this does not make the world less real. Indeed, he contends that our ideas of the world are real, but that the world cannot be proved to have an external reality, and that if we approach the world in this way, then we may be able to define valid principles of human knowledge.

According to Berkeley, there are two kinds of knowledge: 1) knowledge of ideas, and 2) knowledge of spirit. If everything that exists is either perceived or is able to perceive, then these two kinds of knowledge may help us to determine what does exist or does not exist.

Berkeley also explains that if we attribute to an object an existence distinct from the state of its being perceived, then we cannot know whether or not that object really exists. However, if we direct our attention instead to what we can be perceived of that object, then we can attain true knowledge of the nature of its reality.