In the second drafting of the Ideas, Plato has become conscious of the need of explaining physical nature by the Ideas. He did not at first think of explaining the nature of the physical world by his metaphysical reality. It was an afterthought, and arose out of the compulsion of having a systematic theory. His conception of the world of Ideas as the world of true Being ultimately demanded that the world of physical nature should be not merely “other than” but dependent upon the Ideas. The Ideas are unchanging; the phenomena are changing. If the Ideas are the reality of the changing world, in what other sense can they be its reality than as its cause? The _Meno_, _Theætetus_, _Symposium_, and _Phædrus_ do not discuss this problem. The _Sophist_ proposes it, and in the _Phædo_ the thought is first expressed that the Ideas are the causes of physical phenomena appearing as they do appear. But how can the Ideas be causes, when the very conception of them as pure and immaterial realities denies to them all qualities of motion and change? The Platonic theory reached its zenith in its solution of this problem. The Ideas must be conceived as the causes of nature phenomena, and still as not moving nor suffering change. They are teleological causes. They are the realized ends of the phenomenal world. The world of Ideas is the actual goal of perfection for _physical_ nature. The world of Ideas is not only the truth of all knowledge; it is also the perfect teleological cause of all actual change. This thought is developed in the _Philebus_ and the _Republic_, where the Ideas as a whole, and in particular the Idea of the Good,――to which all the other Ideas are means,――stand as the final cause of all occurrence. The physical phenomena stand therefore in a teleological relation to the Idea of the Good. From the Good all things get their meaning. It permeates and explains all. =3. The Relation among the Ideas in the Two Drafts compared.= It was natural that the conception of a pluralism of Ideas should lead Plato to a consideration of the law of their relationship. A systematic theory of a multiplicity of reals involves their orderly relationship. They cannot exist independently in the same world. What is the relationship among the Ideas? In the earlier drafting of his theory Plato was principally attentive to the relations of coördination and subordination among the Ideas; in the possibility of the division of class concepts into genera and species. The relationship that he sought was logical relationship, the relationship that the scientist seeks to find in the classification of plants or rocks. Just what result Plato tried to reach by such a logical classification of his realities, it is difficult to say. He was not successful. His attempt to erect a logically arranged pyramid of conceptions with the most abstract at the apex was not carried out.