God, freedom and immortality cannot be assumed for practical use unless speculative reason gives up its pretension to insight beyond experience

The passage

Speculative reason can reach insight beyond experience only by applying principles of possible experience beyond it; so applied, they turn their object into appearance and declare all practical extension of reason impossible; so the practical assumption of God, freedom and immortality requires that speculative reason be deprived of that pretension.

The argument

1. P1To reach insights beyond experience, speculative reason must apply principles that reach only to objects of possible experience
2. P2Principles of possible experience applied beyond it turn their object into appearance and declare all practical extension of reason impossible
3. P3Assumption. To assume God, freedom and immortality for the practical use of reason is a practical extension of pure reason, so they cannot be so assumed while all practical extension of pure reason is declared impossible. The map's bridge between AA 3:18.33–35 and 3:19.04–05; Kant does not state it.
4. Infer.hypothetical syllogism, then contraposition
5. CGod, freedom and immortality cannot be assumed for practical use unless speculative reason is deprived of its pretension to insight beyond experience

One sentence at B xxix–xxx (AA 3:18.33–19.05). The main clause is the conclusion: I cannot assume God, freedom and immortality for practical use "wenn ich nicht der speculativen Vernunft zugleich ihre Anmaßung überschwenglicher Einsichten benehme" (3:18.35–37). The "weil" clause gives two steps: to reach such insights speculative reason "muß" use principles that reach only to objects of possible experience, applying them to what cannot be an object of experience (3:18.37–19.03), premise 1; so applied they "wirklich dieses jederzeit in Erscheinung verwandeln und so alle praktische Erweiterung der reinen Vernunft für unmöglich erklären" (3:19.03–05), premise 2. The conclusion is the contrapositive of the chain: if practical extension is not to be declared impossible, the pretension must go.

The form reads "wenn ich nicht ... benehme" as stating a necessary condition: the assumption is possible only if the pretension is removed, a → ¬k. To reach it from the two textual premises the form needs a bridge, premise 3, which is the map's and not Kant's: that assuming God, freedom and immortality for practical use is a practical extension of pure reason, so that it cannot stand while all such extension is declared impossible. It is stated as an assumption so that a reader can see exactly where the text stops and the reconstruction begins. The gloss of k, "pursues its pretension", also carries a small step: Kant speaks of what speculative reason must do "um zu diesen zu gelangen" (3:18.37), and the map equates having the pretension with attempting to reach the insights. What is not in the form is the practical premise that the assumption is necessary ("des nothwendigen praktischen Gebrauchs", 3:18.34–35); with it, the next sentence follows, "Ich mußte also das Wissen aufheben" (3:19.05–06), which the map holds as a claim depending on this conclusion rather than as part of this step. The form also does not say why principles so applied "transform" their object into appearance; Kant asserts it here and argues it in the Analytic, to which the preface refers (3:16.30–17.02).