Idealism is a metaphysical theory about the nature of reality which asserts that the external world does not exist independently of the human mind. Thus, only ideas exist. As an opposite view to materialism, idealism declares that reality exists in our minds and not in the external world.
Plato’s theory of forms is often viewed as a form of idealism. The first true idealist philosopher, however, was George Berkeley (1685–1753). Berkeley claimed that all objects are collections of ideas or sensations. Thus, something only exists if it is being perceived by the mind. His famous statement in this regard was Esse est percipi – “to be is to be perceived.” Berkeley’s position naturally coincided with “immaterialism"—the view that there is no matter in the universe.
Immanuel Kant held to a form of idealism called “transcendental idealism.” This perspective asserts that knowledge of the world is dependent on the conceptual processes of the human mind. Humans, thus, cannot not know things as they are in themselves.
A third form of idealism is “absolute idealism.” This view holds that there is a universal self-consciousness or notion that makes reality what it is. The most famous absolute idealist was Georg Hegel who claimed that all reality is shaped by Geist which means “Spirit” or “Mind.”
What all forms of idealism agree upon is that there is no access to reality apart from the mind and what the mind provides us.