Kant, "to treat of reason and its pure thinking, a knowledge of which is not very far to seek, considering that it is to be found within myself." But the objective aspect of the problem of a priori knowledge, which is here scarce even incidentally glanced at, is of equal importance, and was so felt by Kant, as we know from the history of the inquiry started in the famous letter to Herz. Accordingly, in the preface to the second edition of the Critique, published in 1787, six years after the first, this aspect of its problem is brought into the foreground. The point of view shifts from the dialectic to the analytic, and emphasis is laid rather on reason's conquest of the phenomenal world than its utter inadequacy to seize upon the suprasensible world, which, it is here gently insinuated, is the only circumstance that keeps the celestial portals open to faith, and so leaves us secure in the practical possession of God, Freedom, and Immortality. Here, therefore, Kant appears, no longer as a judge passing sentence upon human knowledge, but as a builder setting it upon new and immovable foundations. This he does by means of a great constructive principle, in virtue of which he regards himself as the Copernicus of philosophy.
What is the Copernican thought with which Kant would revolutionize metaphysics ? It is akin to that which he tells us had already been at work in mathematics and physics and turned them into established sciences. For it must not be overlooked that, in Kant's view, a revolution had been effected in those sciences similar to the one he contemplated for metaphysics. He brings both the fact of that revolution and the means of its accomplishment into the closest relation with his own achievements. As to mathematics, he holds there was a long period of tentative work, among Egyptians and Greeks, before the discovery of that royal road which has led to the surest of sciences. The change is to be ascribed "to a revolution, produced by the happy thought of a single man," whose name has not been preserved to us. But that revolution consisted in the discovery that no scrutiny of an actual geometrical figure or of its concept could give information regarding its qualities, which, on the other hand, was derived
