Kant’s philosophy of natural science has traditionally concentrated on a host of issues including the role of laws of nature and teleological judgements. However, so far, the literature has made virtually no contact with the no less important tradition in Kant’s legal and political philosophy. This article explores one aspect of such connection in relation to the normative foundations of Kant’s notion of cosmopolitan right. I argue that Kant’s argument for cosmopolitan right is based on two main premises: the first (...) is what I call the law of equilibrium, and the second is the premise of physical interaction (commercium in Latin) at work behind what Kant called the original ‘community of land’. The article argues that the relevant notion of community qua commercium should be understood in the context of Kant’s metaphysics of nature as a ‘real community of substances’ governed by a dynamical law of equality of action and reaction. This metaphysical-causal interpretive reading has far-reaching implications for the foundations of cosmopolitan right and its scope of applicability well beyond Kant’s envisaged right to universal hospitality. (shrink)