"Idealism" in its philosophical sense, is the view that mind and spiritual values are fundamental in the world as a whole. Thus, idealism is opposed to naturalism, that is, to the view that mind and spiritual values have emerged from or are reducible to material things and processes. Philosophical idealism is also opposed to realism and is thus the denial of the commonsense realist view that material things exist independently of being perceived. Some philosophers who have held the idealist view in its antinaturalist form have not opposed commonsense realism, and thus it is possible to be a metaphysical idealist and an epistemological realist. More often, however, arguments against commonsense realism have been used in order to establish metaphysical idealism. The description "subjective idealism" is sometimes used for idealism based on antirealist epistemological arguments, and the description "objective idealism" for idealism that is antinaturalist without being antirealist.

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