So How is Metaphysics Possible?

For Kant, the metaphysics of previous generations is worthless nonsense. But it is at the end of his Treatise that Kant answers the question that he started his book with. His answer is: "critique." Reason can't teach us regarding what lies beyond its grasp but it can help us categorize and classify the different concepts of our faculties of sensibility, understanding, and reason. Instead of looking outward, we should look inward.
For Kant, science is a body of synthetic a priori knowledge. Reason has not power to gain a priori knowledge of things that are outside our experience.ut
For Kant, all the metaphysics that has been conducted up till now has been completely useless.

But even then he says that we are drawn to metaphysics and that we can't leave it alone. He envisions that the previously dogmatic metaphysics should begin to advance the critical philosophy that he envisions with great vigor.

He also challenges anyone who disagrees with his dismissal of all dogmatic metaphysics and he asks them to give one example of metaphysical synthetic a priori judgment that can and has been proven with certainty. It is impossible for this type of judgment to be based on conjecture, because something that is categorized as an "a priori truths" is itself necessary, and thus can't be based on common sense, since we derive our common sense from experience. Common sense can't advance metaphysics as a science.