In philosophy, the term idealism identifies and describes metaphysical perspectives which assert that reality is indistinguishable and inseparable from perception and understanding; that reality is a mental construct closely connected to ideas. Idealist perspectives are in two categories: subjective idealism, which proposes that a material object exists only to the extent that a human being perceives the object; and objective idealism, which proposes the existence of an objective consciousness that exists prior to and independently of human consciousness, thus the existence of the object is independent of human perception.

The philosopher George Berkeley said that the essence of an object is to be perceived. By contrast, Immanuel Kant said that idealism "does not concern the existence of things," but that "our modes of representation" of things such as space and time are not "determinations that belong to things in themselves," but are essential features of the human mind. In the philosophy of "transcendental idealism" Kant proposes that the objects of experience relied upon their existence in the human mind that perceives the objects, and that the nature of the thing-in-itself is external to human experience, and cannot be conceived without the application of categories, which give structure to the human experience of reality.

Epistemologically, idealism is accompanied by philosophical skepticism about the possibility of knowing the existence of any thing that is independent of the human mind. Ontologically, idealism asserts that the existence of things depends upon the human mind; thus, ontological idealism rejects the perspectives of physicalism and dualism, because neither perspective gives ontological priority to the human mind. In contrast to materialism, idealism asserts the primacy of consciousness as the origin and prerequisite of phenomena. Idealism holds that consciousness (the mind) is the origin of the material world.

Indian and Greek philosophers proposed the earliest arguments that the world of experience is grounded in the mind's perception of the physical world. Hindu idealism and Greek neoplatonism gave panentheistic arguments for the existence of an all-pervading consciousness as the true nature, as the true grounding of reality. In contrast, the Yogācāra school, which arose within Mahayana Buddhism in India in the 4th century AD, 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 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.