Form of idealism represented primarily by George Berkeley (1685-1753), though his own name for it was immaterialism.
Berkeley distinguished minds or spirits (including both God and finite spirits like us), which are active, from ideas which are their contents and are passive. To be is to perceive, in the case of spirits, or to be perceived, in the case of ideas; ‘perceive’ here really means ‘have as content’, and ‘be perceived’ means ‘be had as content’.
Berkeley was mainly concerned to reject the notion of matter, which he regarded as unknowable and the source of paradoxes, and itself stemming from the doctrine of ‘abstract ideas’, which he made his first target.
The term ‘subjective idealism’, used of Berkeley and also of Immanuel Kant (1724-1804) (see transcendental idealism) by objective idealists, perhaps depends on emphasizing only one side of Berkeley’s view, that to be is to be perceived; and in the case of Kant, his treatment of ideas as dependent on our minds.
G Berkeley, A Treatise concerning the Principles of Human Knowledge (1710)
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, rather than appealing to the unitary world-spirit of pantheism or 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.
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 proves 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.
The first mature articulations of idealism arise in Yogacarin thinkers such as the 7th-century epistemologist Dharmakīrti, who identified ultimate reality with sense-perception. The most famous proponent of subjective idealism in the Western world was the 18th-century Irish philosopher George Berkeley, although Berkeley’s term for his theory was immaterialism. From the point of view of subjective idealism, 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” is Berkeley’s whole argument summarized into a couple words. It means “to be is to be perceived”. This summarized his argument because he based his point around the fact that things exist if they are all understood and seen the same way. As Berkeley wrote: “for the Existence of an Idea consists in being perceived”. This would separate everything as objective and subjective. Matter falls into the subjective category because everyone perceives matter differently, which means matter is not real. This loops back to the core of his argument which says that in order for anything to be real, it must be interpreted the same way by everyone.
Berkeley believes 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 makes such a radical claim that matter does not exist as a reaction to the materialists. He says “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 believes that people cannot know that what they think to be matter is not simply a creation in their mind.
People have contested that premise (2) is false, claiming that people don’t perceive ideas but instead, “distinguishing two sorts of perception” they perceive objects and then have ideas about them, effectively cutting down the equality. This might seem to obviously be the case, but in fact it is contestable. Many psychologists believe that what people actually perceive are tools, impediments, and threats. The famous gorilla psychological study, where people were asked to watch a video and count the number of basketball passes made, showed that people do not actually see everything in front of them, even a gorilla that marches across a high school gym. Similarly, it is believed that human reaction to snakes is faster than it should physically be if it were consciously driven. Therefore, it is not unfair to say that objects go straight to the mind.
Berkeley even pointed out that it is not obvious how motion in the physical world could translate to emotion in the mind. Even the materialists had difficulty explaining this; Locke believed that to explain the transfer from physical object to mental image one must “attribute it wholly to the good pleasure of our Maker.”  Newton’s laws of physics say that all movement comes from the inverse change in another motion, and materialists believe that what humans do is fundamentally move their parts. If so how you explain the correlation between objects existing, and the completely other realm of regular ideas is not obvious. The fact “that the existence of matter does not help to explain the occurrence of our ideas” seems to Berkeley to undermine the reason for believing in matter at all. If the materialists have no way of knowing that matter exists, it seems best to not assume that it exists.
According to Berkeley, an object has real being as long as it is perceived by a mind. God, being omniscient, perceives everything perceivable, thus all real beings exist in the mind of God. However, it is also evident that each of us has free will and understanding upon self-reflection, and our senses and ideas suggest that other people also possess these qualities as well. According to Berkeley there is no material universe, in fact he has absolutely no idea what that could possibly mean. To theorize about a universe that is composed of insensible matter is not a sensible thing to do. This matters because there is absolutely no positive account for a material universe, only speculation about things that are by fiat outside of our minds.
Berkeley’s assessment of immaterialism was criticized by Samuel Johnson, as recorded by James Boswell. Responding to the theory, Dr. Johnson exclaimed “I refute it thus!” while kicking a rock with “mighty force”. This episode is alluded to by Stephen Dedalus in James Joyce’s Ulysses, chapter three. Reflecting on the “ineluctable modality of the visible”, Dedalus conjures the image of Johnson’s refutation and carries it forth in conjunction with Aristotle’s expositions on the nature of the senses as described in Sense and Sensibilia. Aristotle held that while visual perception suffered a compromised authenticity because it passed through the diaphanous liquid of the inner eye before being observed, sound and the experience of hearing were not thus similarly diluted. Dedalus experiments with the concept in the development of his aesthetic ideal.
Subjective idealism is featured prominently in the Norwegian novel Sophie’s World, in which “Sophie’s world” exists in fact only in the pages of a book.
A parable of subjective idealism can be found in Jorge Luis Borges’ short story Tlön, Uqbar, Orbis Tertius, which specifically mentions Berkeley