Idealism in philosophy, also known as philosophical idealism or metaphysical idealism, is the set of metaphysical perspectives asserting that, most fundamentally, reality is equivalent to mind, spirit, or consciousness; that reality is entirely a mental construct; or that ideas are the highest form of reality or have the greatest claim to being considered "real". The radical latter view is often first credited to the Ancient Greek philosopher Plato as part of a theory now known as Platonic idealism. Besides in Western philosophy, idealism also appears in some Indian philosophy, namely in Vedanta, one of the orthodox schools of Hindu philosophy, and in some streams of Buddhism.
Epistemologically, idealism is accompanied by philosophical skepticism about the possibility of knowing the existence of any thing that is independent of the human mind. Ontologically, idealism asserts that the existence of things depends upon the human mind; thus, ontological idealism rejects the perspectives of physicalism and dualism, because neither perspective gives ontological priority to the human mind. In contrast to materialism, idealism asserts the primacy of consciousness as the origin and prerequisite of phenomena.
During the European Enlightenment, certain qualified versions of idealism arose, such as George Berkeley's subjective idealism, which proposed that physical objects exist only to the extent that one perceives them and thus the physical world does not exist outside of a mind. According to Berkeley, who was an Anglican Bishop, a single eternal mind keeps all of physical reality stable, and this is God.
By contrast, Immanuel Kant said that idealism "does not concern the existence of things," but that "our modes of representation" of things such as space and time are not "determinations that belong to things in themselves," but are essential features of the human mind.