Epistemological idealism accepts the existence of something independent of the mind but maintains that everything that is known about this mind-independent reality is the result of the mind’s creative and formative functions. Because the mind is the subject’s tool for understanding reality, all of the subject’s perceptions and understandings will be constrained by the mind’s structure. On this view, when the structure of the mind is explored, it is possible that one may not be exploring the most basic truths of the universe at all.

German philosopher Immanuel Kant is arguably the most reputable proponent of epistemological idealism. Kant distinguished between phenomena (“things-as-they-appear”) and noumena (“things-in-themselves”). Here he emphasized a distinction between what subjects (human beings) can experience (the natural, observable world) and what subjects cannot (“supersensible” objects such as God and the soul). In its most simple form, it says that subjects have cognition of appearances but not of things in themselves. By “thing in itself,” Kant meant a thing standing outside any relation to our cognitive powers, but still a thing that one receives a representation of. Kant’s metaphysics of the mind is complex, but suffice it is to say that he believed that a subject’s experience of objects was dependent on a system of mental processes and faculties within the subject. These mental processes and faculties work together to allow a subject to intuit, perceive, and experience objects within the world which means, in Kant’s view, the subject’s perception and understanding of reality are constrained by the mind’s structure.

The subjective idealist maintains that nothing is real other than one’s subjective consciousness and its contents. In other words, when one looks at the world, what she is seeing is a world that has been created by her mind; nothing exists except through a perceiving mind. Perception is reality and what one knows of the world is merely contingent on the perceiver. Bishop George Berkeley (1685-1752), often called the “Father of Idealism”, is the major proponent of this philosophy who argued that knowledge is based on perception and that there is no real, knowable object behind that perception. Berkeley writes that,

“It is evident to any one who takes a survey of the objects of human knowledge, that they are either ideas actually imprinted on the senses, or else such as are perceived by attending to the passions and operations of the mind, or lastly ideas formed by help of memory and imagination, either compounding, dividing, or barely representing those originally perceived in the aforesaid ways.”

Reality consists exclusively of minds and their ideas, a view which Berkeley summarized in his motto “esse est percipi” (“To be is to be perceived”). As an immaterialist, Berkley believed objects like trees and rocks are not material entities but bundles or collections of ideas. A tree, for example, is a combination of visual ideas (of color and visual shape) and tangible ideas (ideas of taste, smell, and so on). For Berkley, all we ever encounter are ideas, never anything material, which means that we are not warranted to believe that matter exists.

Berkeley further argued that God, an infinite mental substance, is the immediate cause of all of our perceptions. Positing God was Berkeley’s attempt to offset a challenge. For example, if a mind stopped perceiving a tree (perhaps by going to sleep or shutting one’s eyes), would that mean that the tree ceased to exist given that the tree is mind-dependent and that the mind stops perceiving it? And would the tree come back into existence if one were to open his eyes again? Or, to use another example, if there were no minds fifty million years ago, does that mean that mountains never existed because there were no minds to perceive them? For Berkley, God solves this challenge because God keeps all things, including trees and mountains, in his infinite mind. This is what makes the tree or mountain continue to exist even if no one perceives it.

Criticisms of Idealism

A subjective idealist like Berkeley posits that only ideas or mind are real. This view, known as immaterialism, is in direct opposition to materialistic views that view matter or atoms as the basic constituents of reality. Here matter exists objectively and independently of the mind. This is, of course, the opposite to what subjective idealism maintains, which is that the basic element of reality is mind or spirit existing apart from matter. Many think that materialism is the more commonsensical view because of experience of a world that seems to exist independently “out there”. This argument was put forth by G. E. Moore (1873-1958) in his essay A Defence of Common Sense (1925) and again in his Proof of an External World (1939). Moore challenges skepticism by raising his right hand and saying “Here is one hand.” He then raises his left hand saying “And here is another”. From this, he concludes that there are at least two external objects in the world and that he can therefore know that the external world exists. 

Some have criticized subjective idealism of occasionalism in that it artificially brings in God to explain away issues it presents itself with. As we noted, Berkeley brings in God, who is defined as an infinite mind, to offset the challenge that without a mind perceiving reality this would seem to suggest reality goes out of existence. To solve this, Berkeley posits God as that who keeps everything in existence when unperceived. The critic might argue that materialistic views are far more commonsensical in that it posits what seems obvious to our experience (that the world exists independently of the mind) and does not require to artificially bring God into the equation to solve problems. Importantly, the critic here does not need to disbelieve in God or religion; instead, he just rejects bringing God into the picture to solve the idealist dilemma.  

Critics have argued that idealism also leads to absurdity in solipsism. Solipsism is the view that nothing exists outside of oneself: there is only one’s own consciousness and mental existence apart from which there is no external world. The world believed to be “out there” is simply a product of the mind. Of course, this is an extreme skepticism that very few philosophers today are willing to embrace.