The whole cycle of the philosophy of mind exhausts itself in the study of the four fundamental forms of human activity, the concepts of which we have seen slowly developing through the mazes of Croce's early speculations: the æsthetic, the logic, the economic and the ethic; of the distinction and the unity of æsthetic and logic in the theoretical activity, or knowledge, and of economic and ethic in the practical activity, or action; and finally of the relations between the theoretical and the practical, or knowledge and action. This may be said to be the positive aspect of Croce's philosophy: the negative aspect consists in the criticism and exclusion of any other form of activity from the system of the human spirit, and of that which is not the spirit, or nature, from the system of reality. To the four forms or grades of spiritual activity, correspond four philosophical sciences: Æsthetics, Logic, Economics, and Ethics. Each of them can be said to be the _organum_ of the particular form of activity which it studies; the affirmation of that sphere of consciousness which is proper to it, and of its relations to the other forms. Each of them is therefore related to the others in the same way as the various forms of activity are related to each other. They might be defined as the projection on the plane of logic of the whole system of human activity, that is, of the whole of reality. They derive their intrinsic validity from this perfect coincidence of their several objects with the only conceivable aspects of reality. We shall in this and in the following chapters attempt to fill in with the strictly necessary detail this very ample frame. But we can already point to the idealistic character of such a philosophy resulting from its method, which is that of the testimony of consciousness, as opposed to the naturalistic or psychologic method of indirect observation; from its object, which is the human spirit or mind in the fulness of its determinations; and from the exclusion of any aspect of reality which is not immanent in consciousness, that is, both of the naturally and the supernaturally transcendent. As against another kind of idealism, of which the typical example is Platonic transcendentalism, Croce's idealism is realistic and immanentistic: the task of the philosophy of mind is to discover the immanent logic of reality. But against current realism, which considers mind as the mere spectator and observer of external or natural reality, it asserts the identity of reality and consciousness, which is the basic position of all idealism.