 The relation between God and the highest good is the basis of Kant’s main argument for belief in God. (See Wood 1970.) The argument, most clearly articulated in the Critique of Practical Reason, goes like this (CPrR 5:110–14, 124–46). The moral law issues categorical demands through each agent’s own reason. If the moral law sets forth an end for us to promote, we must promote it. For our promotion of this end to be rational, the end must be one that we can rationally view as possible for us to promote through our own agency. The moral law gives rise to the highest good (i.e., virtue and conditioned, proportionate happiness), as practical reason’s ultimate end. Thus, we must promote it. Yet it is not clear that we are capable of effecting the highest good, given the limitations of our agency and the conditions and workings of the natural world. If the realization of the highest good is not possible, however, then we cannot rationally promote it as an end; indeed, we cannot even make sense of it as an end. But if we do not promote the highest good, we flout the moral law. Now, suppose that there were a supreme, eternal, all powerful, all knowing ruler of the universe (i.e., God); he could bring to completion our imperfect strivings for the highest good. Thus, if we postulate the existence of God, we can view the highest good as possible, and can rationally follow morality’s command to promote it. There is no reason not to postulate God’s existence, since his non-existence cannot be proven, and since belief in him does not conflict with any necessary beliefs (CPrR 5:135–43). In addition, since belief in God is necessary for seeing the highest good as possible, we must believe in God (CPrR 5:125–26, 142–46). Thus, belief in God is grounded in the necessity of seeing the highest good as possible, and so for the rational compliance with morality. Thus, Kant says that belief in God is morally necessary, and that morality leads ineluctably to religion (CPrR 5:125). We do not, however, have a duty to believe in God. Postulating God’s existence is a theoretical exercise of reason, which cannot be morally commanded (CPrR 5:125, 144). Rather, our duty to promote the highest good gives rise to “a need of reason” to assume the possibility of the highest good. From this need emerges the necessity of postulating God’s existence.