If reason and all our knowledge is ultimately derived from experience, as Hume seems to argue, why does that mean metaphysics can't exist? I was reading Kant's *Prolegomena* and in the introduction he summarizes Hume's conclusions on the impossibility of *a priori* knowledge. Kant then says Hume "was saying in effect that metaphysics couldn't possible exist." I'm not sure how we get from the conclusion that all our knowledge is based on experience to the conclusion that metaphysics can't exist. This seems to be fairly obvious to Kant so I think there's a huge point I'm missing here. 