Had speculative reason proved freedom unthinkable, morality would have to give way to the mechanism of nature
Supposing that morality necessarily presupposes freedom in the strictest sense as a property of our will, adducing a priori original practical principles lying in our reason that would be utterly impossible without the presupposition of freedom, but that speculative reason had proved that freedom cannot be thought at all, then the moral presupposition would necessarily have to yield to the one whose contrary contains an obvious contradiction, and freedom, and with it morality, would have to give way to the mechanism of nature.
Gesetzt nun, die Moral setze nothwendig Freiheit (im strengsten Sinne) als Eigenschaft unseres Willens voraus, indem sie praktische in unserer Vernunft liegende, ursprüngliche Grundsätze als Data derselben a priori anführt, die ohne Voraussetzung der Freiheit schlechterdings unmöglich wären, die speculative Vernunft aber hätte bewiesen, daß diese sich gar nicht denken lasse: so muß nothwendig jene Voraussetzung, nämlich die moralische, derjenigen weichen, deren Gegentheil einen offenbaren Widerspruch enthält, folglich Freiheit und mit ihr Sittlichkeit (denn deren Gegentheil enthält keinen Widerspruch, wenn nicht schon Freiheit vorausgesetzt wird) dem Naturmechanism den Platz einräumen.
Suppose now that morality necessarily presupposes freedom (in the strictest sense) as a property of our will, in that it adduces a priori, as data of it, original practical principles lying in our reason which would be utterly impossible without the presupposition of freedom, but that speculative reason had proved that freedom cannot be thought at all: then that presupposition, namely the moral one, must necessarily yield to the one whose contrary contains an obvious contradiction, and consequently freedom, and with it morality (for its contrary contains no contradiction unless freedom is already presupposed), must give way to the mechanism of nature.
The sentence at B xxviii–xxix (AA 3:18.11–21), a supposition in two limbs, »Gesetzt nun, die Moral setze ... voraus, ..., die speculative Vernunft aber hätte bewiesen ...: so muß ...«. The statement keeps both limbs under »supposing«, because Kant does not hold the second: the next sentence (c-kant-same-action-in-another-relation, 3:18.21–30) begins »So aber« and says that morality needs only that freedom not contradict itself, which the sentence before it (c-kant-freedom-thinkable-if-critical-distinction-holds) has granted. The parenthesis gives the reason the moral presupposition would be the one to yield: its contrary »enthält keinen Widerspruch, wenn nicht schon Freiheit vorausgesetzt wird« (3:18.19–20), so that, were freedom unthinkable, morality would be the side with no contradiction on its contrary. The first limb, that morality presupposes freedom in the strictest sense, is here a supposition too and is not stated by the map as Kant's own assertion, and no passage in the map has him assert it. Whether »Data derselben« (3:18.14) refers to freedom or to reason is said on the passage.