The indispensability of the ‘postulate of practical reason with regard to Right’ to Kant's property argument in the Rechtslehre is now widely recognized. However, most commentators continue to focus their attention on the relation between the postulate and the deduction of the concept of intelligible possession. The nature of this relation remains a matter of dispute in part because the precise position of the postulate within chapter one of the Rechtslehre remains undecided. Given this, it is perhaps not surprising that (...) the related question has been neglected, as to why Kant should characterize the postulate of Right as a postulate of practical reason. Yet the fact that he does so is of some significance – especially if one recalls the definition in the Critique of Practical Reason of postulates of practical reason as practically necessary but theoretically indemonstrable propositions. What is of interest about this definition is not just the fact that it designates postulates as practically necessary and as theoretically indemonstrable at the same time – even more intriguing is the intimated relation between practical necessity and theoretical indemonstrability. Kant does not think the postulates' theoretical indemonstrability morally insignificant. To the contrary, their moral significance for us appears to be a function, in part, of their theoretical indemonstrability. (shrink)

Immanuel Kant (1724–1804) argued that moral requirements are based on a standard of rationality he dubbed the “Categorical Imperative” (CI). Immorality thus involves a violation of the CI and is thereby irrational. Other philosophers, such as Locke and Hobbes, had also argued that moral requirements are based on standards of rationality. However, these standards were either desirebased instrumental principles of rationality or based on sui generis rational intuitions. Kant agreed with many of his predecessors that an analysis of practical reason (...) will reveal only the requirement that rational agents must conform to instrumental principles. Yet he argued that conformity to the CI (a non-instrumental principle) and hence to moral requirements themselves, can nevertheless be shown to be essential to rational agency. This argument was based on his striking doctrine that a rational will must be regarded as autonomous, or free in the sense of being the author of the law that binds it. The fundamental principle of morality — the CI — is none other than the law of an autonomous will. Thus, at the heart of Kant's moral philosophy is a conception of reason whose reach in practical affairs goes well beyond that of a Humean ‘slave’ to the passions. Moreover, it is the presence of this self-governing reason in each person that Kant thought offered decisive grounds for viewing each as possessed of equal worth and deserving of equal respect. (shrink)