I.

Introduction

Idealism, in philosophy, a theory of reality and of knowledge that attributes to consciousness, or the immaterial mind, a primary role in the constitution of the world. More narrowly, within metaphysics, idealism is the view that all physical objects are mind-dependent and can have no existence apart from a mind that is conscious of them. This view is contrasted with materialism, which maintains that consciousness itself is reducible to purely physical elements and processes—thus, according to the materialistic view, the world is entirely mind-independent, composed only of physical objects and physical interactions. In epistemology, idealism is opposed to realism, the view that mind-independent physical objects exist that can be known through the senses. Metaphysical realism has traditionally led to epistemological skepticism, the doctrine that knowledge of reality is impossible, and has thereby provided an important motivation for theories of idealism, which contend that reality is mind-dependent and that true knowledge of reality is gained by relying upon a spiritual or conscious source.

II.

Plato

In the 5th and 4th centuries bc, Plato postulated the existence of a realm of Ideas that the varied objects of common experience imperfectly reflect. He maintained that these ideal Forms are not only more clearly intelligible but also more real than the transient and essentially illusory objects themselves (see Plato: Theory of Forms).

III.

Berkeley and Kant

Eighteenth-century Irish philosopher George Berkeley speculated that all aspects of everything of which one is conscious are actually reducible to the ideas present in the mind. The observer does not conjure external objects into existence, however; the true ideas of them are caused in the human mind directly by God. Eighteenth-century German philosopher Immanuel Kant greatly refined idealism through his critical inquiry into what he believed to be the limits of possible knowledge. Kant held that all that can be known of things is the way in which they appear in experience; there is no way of knowing what they are substantially in themselves. He also held, however, that the fundamental principles of all science are essentially grounded in the constitution of the mind rather than being derived from the external world.

IV.

Hegel

Nineteenth-century German philosopher Georg Wilhelm Friedrich Hegel disagreed with Kant's theory concerning the inescapable human ignorance of what things are in themselves, instead arguing for the ultimate intelligibility of all existence. Hegel also maintained that the highest achievements of the human spirit (culture, science, religion, and the state) are not the result of naturally determined processes in the mind, but are conceived and sustained by the dialectical activity (see Dialectic) of free, reflective intellect. Further strains of idealistic thought can be found in the works of 19th-century Germans Johann Gottlieb Fichte and F. W. J. Schelling, 19th-century Englishman F. H. Bradley, 19th-century Americans Charles Sanders Peirce and Josiah Royce, and 20th-century Italian Benedetto Croce.

See also Philosophy: Idealism and Skepticism.