Any doctrine holding that reality is fundamentally mental in nature. The boundaries of such a doctrine are not firmly drawn: for example, the traditional Christian view that God is a sustaining cause, possessing greater reality than his creation, might just be classified as a form of idealism. Leibniz’s doctrine that the simple substances out of which all else is made are themselves perceiving and appetitive beings (monads), and that space and time are relations among these things, is another early version. Major forms of idealism include subjective idealism, or the position better called immaterialism and associated with Berkeley, according to which to exist is to be perceived, transcendental idealism, and absolute idealism. Idealism is opposed to the naturalistic belief that mind is itself to be exhaustively understood as a product of natural processes. The most common modern manifestation of idealism is the view called linguistic idealism, that we ‘create’ the world we inhabit by employing mind-dependent linguistic and social categories. The difficulty is to give a literal form to this view that does not conflict with the obvious fact that we do not create worlds, but find ourselves in one.