Many a philosopher after Plato have expressed their views on idealism in their own manner.
One of the earliest of modern idealists, Leibniz, is usually interpreted as a pluralistic idealist. We find in his theory of windowless monads that monads which are a sort of spiritual entities are considered to be the only realities. The rest of the world including matter is stated to be an idea. Matter does not really exist but it is only the external experience of what is actually spiritual.[1] Leibnitz's view, although different from "solipsism", is a form of "idealism".
British philosopher John Locke, though did not deny existence of objective world, believed that our knowledge of anything is merely our sensations of it and the ideas derived from these sensations. Hence, according to him, "the real essence of substance" was forever unknowable. This form of idealism, which is widely known as empiricism, was further developed by later philosophers.
A somewhat identical view was put forward by George Berkeley (1685-1753), who said "All the chairs of heaven and furniture of earth, in a word, all those bodies which compose the mighty frame of the world, have not any substance without the mind. So long as they are not actually perceived by me, or do not exist in my mind, or that of any other created spirit, they must either have no "existence at all, or else subsist in the mind of some Eternal Spirit."[2]
Thus, Berkeley also considers the universe as a subjective reality. Berkeley's empiricism clearly denies independent existence of the material world, though the universe existing in the mind of Eternal Spirit has been considered as an "objective reality".[3]
Philosophy-an Introduction, By Archie. J. Bahm, p. 184
See-The Mysterious Universe, p. 126.
Sir James Jeans interprets this thus: "It does not matter whether objects exist In my mind, or any other created spirit, or not; their objectivity arises from their subsisting 'in the mind of some Eternal Spirit'." - The Mysterious Universe, p. 127