Kant takes it as self-evident that it is in everyone's interests to establish metaphysics as a science that proceeds according to agreed-upon and well- grounded principles. Kant's work proposes to do just that, so he feels he is only open to criticism if he has failed to do so, or if he is mistaken in claiming that metaphysics until now has been unproductive. If someone wishes to make the latter accusation, he challenges them to show him one metaphysical truth that is grounded with certainty and agreed upon by all.
The second, and longest, appendix deals primarily with an unfavorable review of Kant's Critique of Pure Reason published in a journal of metaphysics. The reviewer dismisses Kant's work as unclear and unoriginal metaphysical idealism. He makes no mention of Kant's central project of trying to make metaphysics scientific, and does not once refer to Kant's important category of the synthetic a priori. Though this reviewer clearly misunderstood Kant entirely, Kant takes this opportunity to clarify what he means by "transcendental" or "critical" idealism.
Traditional idealists, like Berkeley, assert that all the objects of experience are illusory, and that only pure concepts contain truth. Berkeley claims we have only a posteriori knowledge of space and all the objects in it, and that we therefore cannot be certain of this knowledge. Kant, on the other hand, asserts that we can know about space and time a priori, and that we can have certainty with regard to our experience. Contrary to Berkeley, Kant asserts that our pure concepts are illusory and that only experience contains truth.
Kant's motive for writing his Critique of Pure Reason was that there is no standard for agreement on judgments in metaphysics. This being the case, he does not feel his reviewer's judgment has any solid basis. He challenges his reviewer to produce one metaphysical synthetic a priori principle that can be used to contradict what Kant has written. Better yet, he offers his reviewer the choice of any one of the eight metaphysical propositions listed in his discussion of cosmological ideas. Kant wagers that he could provide a "proof" of the contrary position that his reviewer would not be able to dislodge. He suggests further that he could then provide a similar "proof" of the reviewer's own position, in order to show what dire straits metaphysics stands in.
If people acknowledge the problems in contemporary metaphysics, and study his work as a possible alternative, Kant is confident that much headway can be made, not only in metaphysics, but in other fields as well. For instance, it could free theology from dogmatism and speculation, as well as from the shallow mysticism that masks itself with dogmatic metaphysics.
Kant's Critique of Pure Reason was met mostly with bewilderment when it was first published in 1781. The Prolegomena, published in 1783, was primarily intended to clarify and simplify what was said in the Critique in order to make it accessible. A second, largely revised, edition of the Critique was published in 1787.