EN
Two meanings of the notion of duty are distinguished: the first one refers to practical necessity being an expression of the practical rationality; the second one refers to a normative order of obligations. There were temptations to reduce practical rationality to logic of norms, for example in Kant. Some major difficulties of this kind of analysis were pointed out by V. Descombes. The author explains the impossibility of the reverse reduction that is the reduction of normative notions to one of practical rationality. As a case study the conception of Hobbes is discussed in details. The author argues in favor of an implicit idea presented in Hobbes according to which the rights are conventional human institutions that nevertheless are founded on 'natural' reasoning , that is, as Descombes put it, on 'reasoning which aims the human good'.