The success with which Kant had applied his method to the theoretical side of philosophy led him on to extend it to the practical. Here also he endeavoured to separate pure _a priori_ from empirical _a posteriori_ knowledge. For this purpose he assumed that just as we know _a priori_ the laws of Space, of Time, and of Causality, so in like manner, or at any rate analogously, we have the moral plumb-line for our conduct given us prior to all experience, and revealed in a Categorical Imperative, an absolute "Ought." But how wide is the difference between this alleged moral law _a priori_, and our theoretical knowledge _a priori_ of Space, Time, and Causality! The latter are nothing but the expression of the forms, _i.e._, the functions of our intellect, whereby alone we are capable of grasping an objective world, and wherein alone it can be mirrored; so that the world (as we know it) is absolutely conditioned by these forms, and all experience =must= invariably and exactly correspond to them--just as everything that I see through a blue glass must appear blue. While the former, the so-called moral law, is something that experience pours ridicule on at every step; indeed, as Kant himself says, it is doubtful whether in practice it has ever really been followed on any single occasion. How completely unlike are the things which are here classed together under the conception of =apriority=! Moreover, Kant overlooked the fact that, according to his own teaching, in theoretical philosophy, it is exactly the =Apriority= of our knowledge of Time, Space, and Causality--independent as this is of experience--that limits it strictly to phaenomena, _i.e._, to the picture of the world as reflected in our consciousness, and makes it entirely invalid as regards the real nature of things, _i.e._, as regards whatever exists independently of our capacity to grasp it. Similarly, when we turn to practical philosophy, his alleged moral law, if it have an _a priori_ origin in ourselves, must also be only phaenomenal, and leave entirely untouched the essential nature of things. Only this conclusion would stand in the sharpest contradiction as much to the facts themselves, as to Kant's view of them. For it is precisely the moral principle in us that he everywhere (_e.g., Kritik der Praktischen Vernunft_, p. 175; R., p. 228) represents as being in the closest connection with the real essence of things, indeed, as directly in contact with it; and in all passages in the _Kritik der Reinen Vernunft,_ where the mysterious Thing in itself comes forward a little more clearly, it shows itself as the =moral principle= in us, as =Will=. But of this he failed to take account.