"Although," he says retrospectively, "I am one of those who have worked much at mathematics, I have none the less meditated upon philosophy from my youth up; for it always seemed to me that there was a possibility of establishing something solid in philosophy by clear demonstrations. . . . I perceived, after much meditation, that it is impossible to find the _principles of a real unity_ in matter alone, or in that which is only passive, since it is nothing but a collection or aggregation of parts _ad infinitum_."[155:8] [Sidenote: The Historical Differentiation of the Philosophical Problem.] § 59. Though these types are peculiarly representative, they are by no means exhaustive. There are as many possibilities of emphasis as there are incentives to philosophical reflection. It is not possible to exhaust the aspects of experience which may serve as bases from which such thought may issue, and to which, after its synthetic insight, it may return. But it is evident that such divisions of philosophy represent in their order, and in the sharpness with which they are sundered, the intellectual autobiography of the individual philosopher. There is but one method by which that which is peculiar either to the individual, or to the special position which he adopts, may be eliminated. Though it is impossible to tabulate the empty programme of philosophy, we may name certain special problems that have appeared _in its history_. Since this history comprehends the activities of many individuals, a general validity attaches to it. There has been, moreover, a certain periodicity in the emergence of these problems, so that it may fairly be claimed for them that they indicate inevitable phases in the development of human reflection upon experience. They represent a normal differentiation of interest which the individual mind, in the course of its own thinking, tends to follow. It is true that it can never be said with assurance that any age is utterly blind to any aspect of experience. This is obviously the case with the practical and theoretical interests which have just been distinguished. There is no age that does not have some practical consciousness of the world as a whole, nor any which does not seek more or less earnestly to universalize its science. But though it compel us to deal abstractly with historical epochs, there is abundant compensation in the possibility which this method affords of finding the divisions of philosophy in the manifestation of the living philosophical spirit. [Sidenote: Metaphysics Seeks a Most Fundamental Conception.]