In Western civilization, Idealism is the philosophy which maintains that the ultimate nature of reality is based upon ideas, values and essences and that the external, or real world is inseparable from consciousness, perception, mind, intellect and reason in the sense of rigorous science.
In certain idealistic philosophies the ideal is said to involve direct and immediate knowledge of subjective mental ideas, or images. Idealism is often opposed to realism in which the real is said to have an absolute existence prior to, and independent of, knowledge and consciousness. Epistemological idealists such as Immanuel Kant might insist that the only things which can be directly known for certain are ideas.
Some forms of idealism, like that of Rene Descartes, are often contrasted with materialism. Some idealists, like Baruch Spinoza, are monist as opposed to dualist.
A broad enough definition of idealism could include most religious viewpoints. The belief that personal beings such as Gods, angels and spirits preceded the existence of insentient matter seems to suggest that an experiencing subject is a necessary reality. Also, the existence of an omniscient God suggests, regardless of the actual nature of matter, that all of nature is the object of at least one consciousness.
Materialism sees no incoherence in a scenario of there being a cosmos where no sentient subject ever develops. A wholly unknown universe where neither any subject, nor any object of a subject’s experience ever exists. Historically, Mechanistic Materialism has been the favorite viewpoint of Atheist philosophers. Still, idealistic viewpoints that have not included God, supernatural beings, or a post-mortem existence have sometimes been advanced.
Many religious philosophies are indeed specifically idealist. Some Hindu denominations view regarding the nature of Brahman, souls, and the world as idealistic, and some have favored a form of substance dualism. Early Buddhism was not subjective idealistic. Some have misinterpreted the Yogācāra school of Mahayana Buddhism that developed the consciousness only approach as a form of metaphysical idealism, but this is incorrect. Yogācāra thinkers did not focus on consciousness to assert it as ultimately real. Yogācāra claims consciousness is only conventionally real since it arises from moment to moment due to fluctuating causes and conditions.
Some Christian theologians have held idealist views. Substance dualism has been the more common view of Christian authors, especially with the strong influence of the philosophy of Aristotle among the Scholastics.
Several modern religious movements, for example the organizations within the New Thought Movement and the Unity Church, may be said to have a particularly idealist orientation.
The theology of Christian Science includes a form of subjective idealism: it teaches that all that exists is God and God’s ideas; that the world as it appears to the senses is a distortion of the underlying spiritual reality.
A Course in Miracles, a spiritual self study course published in 1976, represents an explicitly idealist, pure nondualistic thought system. In the course, only God and His Creation, which is Spirit and has nothing to do with the world, are real. The physical universe is an illusion and does not exist. The Course compares the world of perception with a dream. It arises from the projection of the dreamer. The purpose of the perceptual world is to ensure our separate, individual existence apart from God but avoid the responsibility and project the blame onto others. As we learn to give the world another purpose and recognize our perceptual errors, we also learn to look past them as a way to awaken gradually from the dream and finally remember our true Identity in God. The Course’s nondualistic metaphysics is similar to the Advaita Vedanta school of Hindu philosophy.