Without a careful critique guarding the limits of reason even in its empirical use, its restricting principles could themselves become transcendent

The passage

Our principles, which restrict the use of reason to possible experience, could themselves become transcendent if a careful critique did not guard the limits of our reason even with regard to its empirical use and put a stop to its pretensions.

Prolegomena §57: the principles "könnten ... selbst transscendent werden ..., wenn nicht eine sorgfältige Kritik die Grenzen unserer Vernunft auch in Ansehung ihres empirischen Gebrauchs bewachte und ihren Anmaßungen ihr Ziel setzte" (AA 4:351.07–12). The clause is a counterfactual in the subjunctive, and the claim keeps it so: the critique is what stands between the restricting principles and their becoming transcendent, of which Kant gives Hume's Dialogues as the example (4:351.09). The claim is not a premise of the argument on this passage; the premise that the outcome is absurd is held separately, as the map's.