There does not then exist any rational psychology as a doctrine furnishing any addition to our knowledge of ourselves. It is nothing more than a discipline, which sets impassable limits to speculative reason in this region of thought, to prevent it, on the one hand, from throwing itself into the arms of a soulless materialism, and, on the other, from losing itself in the mazes of a baseless spiritualism. It teaches us to consider this refusal of our reason to give any satisfactory answer to questions which reach beyond the limits of this our human life, as a hint to abandon fruitless speculation; and to direct, to a practical use, our knowledge of ourselves—which, although applicable only to objects of experience, receives its principles from a higher source, and regulates its procedure as if our destiny reached far beyond the boundaries of experience and life. From all this it is evident that rational psychology has its origin in a mere misunderstanding. The unity of consciousness, which lies at the basis of the categories, is considered to be an intuition of the subject as an object; and the category of substance is applied to the intuition. But this unity is nothing more than the unity in thought, by which no object is given; to which therefore the category of substance—which always presupposes a given intuition—cannot be applied. Consequently, the subject cannot be cognized. The subject of the categories cannot, therefore, for the very reason that it cogitates these, frame any conception of itself as an object of the categories; for, to cogitate these, it must lay at the foundation its own pure self-consciousness—the very thing that it wishes to explain and describe. In like manner, the subject, in which the representation of time has its basis, cannot determine, for this very reason, its own existence in time. Now, if the latter is impossible, the former, as an attempt to determine itself by means of the categories as a thinking being in general, is no less so.[47]