The doctrine that reality is in some way mental, related to the contents and structure of the mind. The term derives from idea rather than ideal and is more related to metaphysics than ethics, contrasting with realism and also with materialism. Plato's Theory of Forms could be said to be a type of idealism since for him, his Forms or Ideas existed at a higher level of reality than the sensible world. Yet scholars reject Plato as a true idealist since his Ideas were not merely contents of the human mind but existed as entities in their own right. The Ideas were real, what we perceive mere shadows of this reality.

The most extreme form of idealism holds that matter does not exist except in the form of ideas in the mind or as manifestations of the mind: "To be is to be perceived". (Berkeley) The mind in question may be an individual mind, minds in general or, as Berkeley supposed, the mind of God. Kant rejected this extreme form of idealism and distinguished between things-in-themselves (noumena), which exist but which we cannot know fully, and things as we come to know them (phenomena). The latter depend on the structure of the mind, are products of the human consciousness and hence, in Kant's terms, are "transcendentally ideal". We can only know what our mind is capable of knowing, so our reality is in a sense a construct of the mind.

Hegel's Absolute Idealism, explains all of reality in terms of an Absolute Mind or Spirit which reveals itself to our finite minds in every area of human knowledge. Absolute Mind is seen as a unified totality of all rational truth organising all areas of knowledge and experience into a coherent whole. Full understanding of reality will involve uncovering the underlying rational structure, the totality of Mind.