 To see why, consider what would happen if we did not believe in God or immortality, according to Kant. In the Critique of Pure Reason, Kant seems to say that this would leave us without any incentive to be moral, and even that the moral law would be invalid without God and immortality (A813/B841, A468/B496). But Kant later rejects this view (8:139). His mature view is that our reason would be in conflict with itself if we did not believe in God and immortality, because pure practical reason would represent the moral law as authoritative for us and so present us with an incentive that is sufficient to determine our will; but pure theoretical (i.e., speculative) reason would undermine this incentive by declaring morality an empty ideal, since it would not be able to conceive of the highest good as possible (5:121, 143, 471–472, 450–453). In other words, the moral law would remain valid and provide any rational being with sufficient incentive to act from duty, but we would be incapable of acting as rational beings, since “it is a condition of having reason at all […] that its principles and affirmations must not contradict one another” (5:120). The only way to bring speculative and practical reason “into that relation of equality in which reason in general can be used purposively” is to affirm the postulates on the grounds that pure practical reason has primacy over speculative reason. This means, Kant explains, that if the capacity of speculative reason “does not extend to establishing certain propositions affirmatively, although they do not contradict it, as soon as these same propositions belong inseparably to the practical interest of pure reason it must accept them […,] being mindful, however, that these are not its insights but are yet extensions of its use from another, namely a practical perspective” (5:121). The primacy of practical reason is a key element of Kant’s response to the crisis of the Enlightenment, since he holds that reason deserves the sovereign authority entrusted to it by the Enlightenment only on this basis.