Idealism

Berkeley's argument begins from a strict form of empiricism: everything we know about physical objects, their color, shape, weight, and texture, comes to us as ideas in perception, and we have no independent access to a matter supposedly lying behind those ideas. If matter cannot even be coherently described except in terms of how it is perceived, he argued, then positing an unperceived material substratum adds nothing and explains nothing; better to say plainly that to exist, for a physical thing, just is to be perceived, with God's constant perception keeping the world in being when no finite mind is looking at it.

Hegel's idealism, developed a century later in Germany, is a different animal: rather than denying an external world, he argued that reality as a whole is the self-development of Spirit, or Geist, an underlying mental process that comes to know itself through nature, human history, art, religion, and philosophy, with each stage of thought giving rise to its own contradiction and a higher synthesis. Both versions share the core commitment that mind is not a late add-on to a pre-existing material universe but is, in some sense, prior to or constitutive of the physical world as we encounter it. Samuel Johnson's famous reply to Berkeley, kicking a stone and declaring that he refuted it thus, remains the most quoted, and least philosophically decisive, objection either version has faced.

Idealism is the position that gives philosophical shape to a very old religious intuition, that consciousness or spirit is more fundamental than the matter it seems to inhabit, and versions of it appear well outside the Western academy that produced Berkeley and Hegel. Plato's theory of Forms already treats the intelligible realm as more real than the physical copies that participate in it, and Neoplatonists after him made mind, or Nous, an emanation prior to matter in the order of reality.

Advaita Vedanta is sometimes classed as a form of idealism, since it treats Brahman, cognized as pure consciousness, as the sole reality behind an apparent material world, though other scholars insist Advaita's monism is not identical to any Western idealist system and resist the label. The position's chief liability, across every version, is explaining why the physical world behaves with such stubborn regularity and resistance to wishing if it is ultimately mind-dependent; materialists take this regularity as decisive evidence that matter is the more basic reality, while idealists answer that regularity is exactly what a rational, ordered mind or Absolute would produce.

Idealism's classic academic rebuttal came from G.E. Moore, whose 1903 essay The Refutation of Idealism attacked the Berkeleian formula esse est percipi, to be is to be perceived, arguing that the act of perceiving a thing and the thing perceived are distinguishable, so a quality's being perceived does not make perception the whole of its reality.