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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 type of reality or have the greatest claim to being considered "real". Because there are different types of idealism, it is difficult to define the term uniformly.

Indian philosophy contains some of the first defenses of idealism, such as in Vedanta and in Shaiva Pratyabhijña thought. These systems of thought argue for an all-pervading consciousness as the true nature and ground of reality. Idealism is also found in some streams of Mahayana Buddhism, such as in the Yogācāra school, which argued for a "mind-only" (cittamatra) philosophy on an analysis of subjective experience. In the West, idealism traces its roots back to Plato in ancient Greece, who proposed that absolute, unchanging, timeless ideas constitute the highest form of reality: Platonic idealism. This was revived and transformed in the early modern period by Immanuel Kant 's arguments that our knowledge of reality is completely based on mental structures: transcendental idealism.

Epistemologically, idealism is accompanied by a rejection of the possibility of knowing the existence of any thing independent of mind. Ontologically, idealism asserts that the existence of all things depends upon the mind; thus, ontological idealism rejects the perspectives of physicalism and dualism. In contrast to materialism, idealism asserts the primacy of consciousness as the origin and prerequisite of all phenomena.

Idealism came under heavy attack in the West at the turn of the 20th century. The most influential critics were G. E. Moore and Bertrand Russell, but its critics also included the new realists and Marxists. The attacks by Moore and Russell were so influential that even more than 100 years later "any acknowledgment of idealistic tendencies is viewed in the English-speaking world with reservation." However, many aspects and paradigms of idealism did still have a large influence on subsequent philosophy.