The world is my idea is a truth valid for every living creature, though only man can consciously contemplate it. In doing so he attains philosophical wisdom. No truth is more absolutely certain than that all that exists for knowledge, and, therefore, this whole world, is only object in relation to subject, perception of a perceiver--in a word, idea. The world is idea.
This truth is by no means new. It lay by implication in the reflections of Descartes; but Berkeley first distinctly enunciated it, while Kant erred by ignoring it. So ancient is it that it was the fundamental principle of the Indian Vedanta, as Sir William Jones points out. In one aspect, the world is idea; in the other aspect the world is will.
That which knows all things and is known by none is the subject; and for this subject all exists. But the world as idea consists of two essential and inseparable halves. One half is the object, whose from consists of time and space, and, through these, of multiplicity; but the other half is the subject, lying not in space and time, for it subsists whole and undivided in every reflecting being.
Thus, any single individual endowed with the faculty of perception of the object constitutes the whole world of idea as completely as the millions in existence; but let this single individual vanish, and the whole world as idea would disappear. Each of these halves possesses meaning and existence only in and through the other, appearing with and vanishing with it. Where the object begins the subject ends.
From The World as Will and Idea,(1819)