Idealism holds that reality is fundamentally mental or immaterial. The physical world, on this view, is either a construction of mind or a manifestation of something mind-like. Plato is often called an idealist, though his theory of Forms is more complex. His Forms are eternal, non-physical patterns that particular things participate in. George Berkeley gave idealism its most direct formulation: to be is to be perceived. Material substance does not exist. What we call a table is a collection of ideas in the mind of God and in our own minds.

German idealism, starting with Kant and continuing with Fichte, Schelling, and Hegel, treated the mind as actively constituting the world it experiences. Kant argued that we can never know things as they are in themselves, only as they appear under the forms of our intuition and categories of our understanding. Hegel argued that reality is the self-unfolding of spirit, and that history is the process by which spirit comes to know itself. Idealism declined in the 20th century as analytic philosophy turned toward physicalism, but it has never disappeared. Debates about consciousness, perception, and the nature of reality keep idealist options alive.

Forms of idealism

• Platonic. Reality consists of eternal Forms.
• Berkeleyan. To be is to be perceived.
• Transcendental. The mind structures experience.
• Absolute. Reality is the self-unfolding of spirit.

Idealism is often contrasted with materialism, which holds that only matter exists. The debate between them is one of the oldest in philosophy.