The concept of "thing in itself" has a central position in the philosophy of Kant. But as most of his important concepts often expose their meaning only gradually in the process of the development of his thought, this concept likewise has somewhat different meanings in his different works, and thus makes its understanding more difficult than other. Most scholars refer to his Critique of Pure Reason in their discussion of this concept. In this work, the distinction between "appearance" and "thing in itself" is a transcendental distinction, and its essential meaning is epistemological. Accordingly, "thing in itself" means the foundation of human knowledge on the one hand and its limits on the other hand. For Kant, this is a "problematic" concept. He also conceives "thing in itself" as the object of a non-sensible (intellectual) intuition. But as he denies this kind of intuition for human beings, he denies our ability to know the "thing in itself". So Kant's conclusion in this work is: the concept of "thing in itself" contains no contradiction, but transcends the bounds of our possible knowledge; and at the same time, it is a necessary idea, in order to account for the possibility of our knowledge. 　　But in Groundwork of the Metaphysics of Morals and Critique of Practical Reason, the concept of "thing in itself" presents yet another meaning. Here he interprets "thing in itself" as a viewpoint, through which an "intellihible" world is disclosed. This is a necessary conclusion of his thesis of the primacy of practical reason over speculative reason. As Prof. Richard Kroner in his Kant's Weltanschauung puts it, "thing in itself" opens a "kingdom of freedom". In this way, the concept of "thing in itself" achieves a more fundamental meaning, namely, its ethical meaning. But in the development of its meaning, Kant's interpretation thereof is neither sufficient nor conclusive, so that it becomes a subject for debatet in the ensuing German philosophy. 　　In his Mind and Human Nature, Prof. Mou Tsung-san at first investigates Kant's moral philosophy, and points out that the sphere of freedom in Kant remains abstract, and lacks reality, since Kant looks upon the freedom of will only as a "postulate". Therefore, Kant can only establish a "metaphysics of morals", but not a "moral metaphysics". In his Intellectual Intuition and Chinese Philosophy, Prof. Mou furthur points our that in order to establish the metaphysics of Confucianism, Taoism, and Buddhism (above all the Confucian moral metaphysics), it is necessary to admit an intellectual intuition for human beings. This implies that "thing in itself" can be represented to the moral mind of human beings, and need not lie beyond the extreme limits of practical philosophy. But this does not contradict Kant's thesis that "thing in itself" can't be an object of human knowledge. In his Appearance and Thing in Itself, An Essay on the Highest Good, 14 Lectures on the Meeting of Chinese and Western Philosophy, and his commentary on Critique of Practical Reason, Prof. Mou examines in detail Kant's distinction between "appearance" and "thing in itself", and shows its insufficiency. From this investigation he comes to the conclusion that only in the metaphysics of Confucianism, Taoism, and Buddhism, can this distinction fully establish its systematic meaning.