That the principles which restrict the use of reason to possible experience should themselves become transcendent, passing off the bounds of our reason for bounds of the possibility of things themselves, is absurd.
This claim is the map's reading of Prolegomena §57, not a sentence of Kant's, and it is held as a node so that it can be disputed as one. What Kant says is that the restricting principles "könnten demnach selbst transscendent werden und die Schranken unsrer Vernunft für Schranken der Möglichkeit der Dinge selbst ausgeben" (AA 4:351.06–09), that Hume's Dialogues "zum Beispiel dienen können" (4:351.09), and that this happens "wenn nicht eine sorgfältige Kritik" guards the limits of reason (4:351.10–12). He calls the position that leads there "eine noch größere Ungereimtheit" (4:350.34). The map takes the absurdity to lie here: a principle whose whole content is that reason is restricted to possible experience cannot, without undoing itself, be a claim about the possibility of things. A reader who holds that Kant merely asserts the verdict, or that "könnten" leaves the outcome possible rather than unacceptable, disputes this claim.