Paul Guyer and Rolf-Peter Horstmann
In philosophy, idealism is a diverse group of metaphysical views which all assert that "reality" is in some way indistinguishable or inseparable from human perception and/or understanding, that it is in some sense mentally constituted, or that it is otherwise closely connected to ideas. In contemporary scholarship, traditional idealist views are generally divided into two groups. Subjective idealism takes as its starting point that objects only exist to the extent that they are perceived by someone. Objective idealism posits the existence of an objective consciousness which exists before and, in some sense, independently of human consciousness, thereby bringing about the existence of objects independently of human minds. In the early modern period, George Berkeley was often considered the paradigmatic idealist, as he asserted that the essence of objects is to be perceived. By contrast, Immanuel Kant, a pioneer of modern idealist thought, held that his version of idealism does “not concern the existence of things”, but asserts only that our “modes of representation” of them, above all space and time, are not “determinations that belong to things in themselves” but essential features of our own minds. Kant called this position “transcendental idealism” (or sometimes “critical idealism"), holding that the objects of experience relied for their existence on the mind, and that the way that things in themselves are outside of our experience cannot be thought without applying the categories which structure all of our experiences. However, since Kant's view affirms the existence of some things independently of experience (namely, "things in themselves"), it is very different from the more traditional idealism of Berkeley.
Epistemologically, idealism is accompanied by skepticism about the possibility of knowing any mind-independent thing. In its ontological commitments, idealism goes further, asserting that all entities rely on the mind for their existence. Ontological idealism thus rejects both physicalist and dualist views as failing to ascribe ontological priority to the mind. In contrast to materialism, idealism asserts the primacy of consciousness as the origin and prerequisite of phenomena. Idealism holds consciousness or mind to be the "origin" of the material world – in the sense that it is a necessary condition for our positing of a material world – and it aims to explain the existing world according to these principles. The earliest extant arguments that the world of experience is grounded in the mental derive from India and Greece. The Hindu idealists in India and the Greek neoplatonists gave panentheistic arguments for an all-pervading consciousness as the ground or true nature of reality. In contrast, the Yogācāra school, which arose within Mahayana Buddhism in India in the 4th century CE, based its "mind-only" idealism to a greater extent on phenomenological analyses of personal experience. This turn toward the subjective anticipated empiricists such as George Berkeley, who revived idealism in 18th-century Europe by employing skeptical arguments against materialism. Beginning with Immanuel Kant, German idealists such as Georg Wilhelm Friedrich Hegel, Johann Gottlieb Fichte, Friedrich Wilhelm Joseph Schelling, and Arthur Schopenhauer dominated 19th-century philosophy. This tradition, which emphasized the mental or "ideal" character of all phenomena, gave birth to idealistic and subjectivist schools ranging from British idealism to phenomenalism to existentialism.
Idealism as a philosophy came under heavy attack in the West at the turn of the 20th century. The most influential critics of both epistemological and ontological idealism were G. E. Moore and Bertrand Russell, but its critics also included the new realists. According to Stanford Encyclopedia of Philosophy, 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. Phenomenology, an influential strain of philosophy since the beginning of the 20th century, also draws on the lessons of idealism. In his Being and Time, Martin Heidegger famously states: "If the term idealism amounts to the recognition that being can never be explained through beings, but, on the contrary, always is the transcendental in its relation to any beings, then the only right possibility of philosophical problematics lies with idealism. In that case, Aristotle was no less an idealist than Kant. If idealism means a reduction of all beings to a subject or a consciousness, distinguished by staying undetermined in its own being, and ultimately is characterised negatively as 'non-thingly', then this idealism is no less methodically naive than the most coarse-grained realism."
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