Subjective idealism, or empirical idealism or immaterialism, is a form of philosophical monism that holds that only minds and mental contents exist. It entails and is generally identified or associated with immaterialism, the doctrine that material things do not exist. Subjective idealism rejects dualism, neutral monism, and materialism; it is the contrary of eliminative materialism, the doctrine that all or some classes of mental phenomena (such as emotions, beliefs, or desires) do not exist, but are sheer illusions.

Subjective idealism is a fusion of phenomenalism or empiricism, which confers special status upon the immediately perceived, with idealism, which confers special status upon the mental. Idealism denies the knowability or existence of the non-mental, while phenomenalism serves to restrict the mental to the empirical. Subjective idealism thus identifies its mental reality with the world of ordinary experience, and does not comment on whether this reality is "divine" in some way as pantheism does, nor comment on whether this reality is a fundamentally unified whole as does absolute idealism. This form of idealism is "subjective" not because it denies that there is an objective reality, but because it asserts that this reality is completely dependent upon the minds of the subjects that perceive it.

The earliest thinkers identifiable as subjective idealists were certain members of the Yogācāra school of Indian Buddhism, who reduced the world of experience to a stream of subjective perceptions. Subjective idealism made its mark in Europe in the 18th-century writings of George Berkeley, who argued that the idea of mind-independent reality is incoherent, concluding that the world consists of the minds of humans and of God. Subsequent writers have continuously grappled with Berkeley's skeptical arguments. Immanuel Kant responded by rejecting Berkeley's immaterialism and replacing it with transcendental idealism, which views the mind-independent world as existent but incognizable in itself. Since Kant, true immaterialism has remained a rarity, but is survived by partly overlapping movements such as phenomenalism, subjectivism, and perspectivism.

Hegel rejected Kant's immaterialism, demeaning it to a "reduction of the facts of consciousness to a purely personal world."

Thinkers such as Plato, Plotinus and Augustine of Hippo anticipated idealism's immaterialistic thesis with their views of the inferior or derivative reality of matter. However, these Platonists did not make Berkeley's turn toward subjectivity. Plato helped anticipate these ideas by creating an analogy about people living in a cave which explained his point of view. His view was that there are different types of reality. He explains this with his cave analogy which contains people tied up only seeing shadows their whole life. Once they go outside, they see a completely different reality, but lose sight of the one they saw before. This sets up the idea of Berkley's theory of immaterialism because it shows how people can be exposed to the same world but still see things differently. This introduces the idea of objective versus subjective which is how Berkeley attempts to prove that matter does not exist. Indeed, Plato rationalistically condemned sense-experience, whereas subjective idealism presupposed empiricism and the irreducible reality of sense data. A more subjectivist methodology could be found in the Pyrrhonists' emphasis on the world of appearance, but their skepticism precluded the drawing of any ontological conclusions from the epistemic primacy of phenomena.

Yogacarin thinkers such as the 7th-century epistemologist Dharmakīrti, identified ultimate reality with sense-perception. The most famous proponent of subjective idealism in the West was the 18th-century Irish philosopher George Berkeley, whose popularity eclipsed his contemporary and fellow Anglican philosopher Arthur Collier, who perhaps preceded him in refuting material existence or, as he says a "denial of an external world." Berkeley's term for his theory was immaterialism, according to which the material world does not exist, and the phenomenal world is dependent on humans. Hence the fundamental idea of this philosophical system (as represented by Berkeley or Mach) is that things are complexes of ideas or sensations, and only subjects and objects of perceptions exist. "Esse est percipi," meaning, "to be is to be perceived," is how Berkeley summarized his argument. He believed that things exist if they are understood and seen the same way, writing: "for the Existence of an Idea consists in being perceived". This categorizes everything as objective or subjective. Matter is subjective because everyone perceives matter differently.

Berkeley believed that all material is a construction by the human mind. According to the Stanford Encyclopedia of Philosophy, his argument is: "(1) We perceive ordinary objects (houses, mountains, etc.). (2) We perceive only ideas. Therefore, (3) Ordinary objects are ideas."

Berkeley's claim that matter does not exist is in opposition to the materialists. "If there were external bodies, we couldn’t possibly come to know this; and if there weren’t, we might have the very same reasons to think there were that we have now": "A thinking being might, without the help of external bodies, be affected with the same series of sensations or ideas as you have." Berkeley believed that people cannot know that what they think to be matter is simply a creation in their mind.