If the deduction is right, causality applies only to things as objects of experience, not to the same things as things in themselves
If the deduction of the critique's concepts of the understanding is right, then the principle of causality applies only to things taken in the first sense, insofar as they are objects of experience, and the very same things taken in the second meaning, as things in themselves, are not subject to it.
wenn die Deduction ihrer Verstandesbegriffe richtig ist, mithin auch der Grundsatz der Causalität nur auf Dinge im ersten Sinne genommen, nämlich so fern sie Gegenstände der Erfahrung sind, geht, eben dieselbe aber nach der zweiten Bedeutung ihm nicht unterworfen sind
if the deduction of its concepts of the understanding is right, and hence the principle of causality applies only to things taken in the first sense, namely insofar as they are objects of experience, while the very same things in the second meaning are not subject to it
The second »wenn« clause of the sentence at B xxvii–xxviii (AA 3:17.20–24). It is not a free-standing assertion in the text but a condition inside Kant's conditional, and the statement keeps it conditional on the deduction's being right; the restriction of causality is drawn from that by »mithin« (3:17.21). »Im ersten Sinne« and »nach der zweiten Bedeutung« are the two meanings the first clause names, »als Erscheinung oder als Ding an sich selbst« (3:17.19–20), which the statement spells out. The same restriction is stated elsewhere in the map of the understanding's principles in general (c-kant-understanding-principles-only-for-experience, Prolegomena § 57) and of the categories (B 308–309); this node is the sentence that applies it to causality for the sake of the will. It is not a premise of a-kant-freedom-thinkable-under-the-critical-distinction, whose antecedent takes the restriction as given; it is the sentence that supplies it, and the argument's body says so.