Greek philosophy, which started with these crude, but brilliant speculations, had developed a wonderful variety and subtlety, when Plato, animated by the same desire to discover the Ground of things, introduced his doctrine of Ideas. He held that bodies are not, in themselves, the true reality; they are manifestations of something else. Reality, for him, is a system of real thoughts which he calls Ideas, and the world of objects gets its reality by participating in them or by copying them. The senses, under such conditions, cleave to the copies, whereas the mind, in thinking by general ideas, apprehends the true reality. These ideas must not be regarded as mere products of the mind, but as real existences, which, when manifested under conditions of time and space, multiply themselves in innumerable objects. In fact, so real are they that without them there would be no objects at all. Schopenhauer adopted this doctrine of Ideas, and brought it into connection with his characteristic theory of Will as the ultimate Ground. The Ideas, for him, represent definite forms of existence, manifested in individual things and beings. There are thus, he said, Ideas of the simple elementary forces of nature, such as gravity and impenetrability; there are Ideas of the different forms of individual things; and there are Ideas of the different species of organic beings, including man. He followed Plato in refusing any true reality to individual objects and separated the Idea from its sensuous form. "By Idea, then" (he writes), "I understand every definite and fixed grade of the objectification of will, so far as it is a thing-in-itself, and therefore has no multiplicity. These grades are related to individual things as their eternal forms or prototypes." Hence, the world known to the senses could be nothing other than mere phenomenal appearance. Now it is manifestly an enormous stride in the direction of Nature Mysticism to recognise in material objects a factor, or element, which is akin to the highest activities of the human mind. But, as already stated, in expounding the view known as Ideal-Realism, the nature-mystic cannot be content to stop here. Nor indeed was Schopenhauer consistent in stopping here. If he had been faithful to his conception of Will as the Ground of all existence, he could not well have denied some degree of reality to objects in their own right. This particular tree, this particular table, this particular cloud--what are they, each in its individual capacity, but objectifications of will?--therefore real! Each individual object is _unique_, and fills a place of its own in the totality of objects--each is related to all the rest in particular and defined manners and degrees--each exhibits a special kind of behaviour in a special environment. Why, then, deny to each individual thing its own grade and degree of reality?