To admit no things in themselves would make the principles that restrict reason to experience themselves transcendent, which is absurd

The passage

To admit no things in themselves is to take principles of the possibility of experience for universal conditions of things in themselves; that could make the principles restricting reason to experience themselves transcendent; that is absurd; so admitting no things in themselves is absurd.

The argument

1. P1To admit no things in themselves is to take principles of the possibility of experience for universal conditions of things in themselves
2. P2If principles of experience were conditions of things in themselves, the principles restricting reason to experience could themselves become transcendent
3. P3It is absurd that the principles restricting reason to possible experience should themselves become transcendent
4. Infer.hypothetical syllogism, then modus tollens
5. CIt is a still greater absurdity to admit no things in themselves at all

The second paragraph of Prolegomena §57 (AA 4:350.34–351.12). Kant states the verdict first, "eine noch größere Ungereimtheit", and the reason after: denying things in themselves, or taking our experience for the only possible mode of cognition, "mithin" takes principles of the possibility of experience for universal conditions of things in themselves (4:350.34–351.05), premise 1; "demnach" our restricting principles "könnten ... selbst transscendent werden" and pass off the bounds of our reason for bounds of the possibility of things (4:351.06–09), premise 2. Premise 3 is the map's: that a principle which restricts reason to possible experience should itself be transcendent is absurd. Kant does not say so in words; he says it "could" happen, that Hume's Dialogues are an example of it happening, and that only a careful critique guards against it (4:351.09–12). The map reads his verdict "Ungereimtheit" at 4:350.34 as resting on this: what makes denying things in themselves absurd is that it turns the critique's own restricting principles into transcendent claims about things. The conclusion is the sentence's own verdict.

The reconstruction is the map's: the text gives the consequences as the explanation of the verdict, and the form arranges them as a modus tollens. The interpretive choice is wholly in premise 3. A reader who thinks the "Ungereimtheit" is asserted rather than argued here, or that Kant's "könnten" leaves the outcome merely possible rather than unacceptable, disputes premise 3, not the inference.

The conclusion is the counterpart of B xxvi's "Erscheinung ohne etwas ..., was da erscheint" (3:17.06–07): both argue that something must be granted beyond appearances, there from the concept of appearance, here from the restricting principles themselves.