Achenwall’s textbook on ‘ius naturae’, first published in 1750 and subsequently often reprinted, was certainly one of the most powerful lecture-compendia in the second half of the 18th century in Germany, not at least because of its momentous reception by Kant. If one takes a closer look, it turns out - according to the thesis of the present contribution - not to be a finished product from the beginning, but a typical ‘work in progress’. Its incessant process of growth across the various editions is to be demonstrated and followed up here by means of the guiding concept of ‘obligatio’, a key term of natural law in general during the time of the Enlightenment. What will be in detail analysed is the definition, the classification of obligation and its role as the highest principle. As a result of this investigation we see a change from an ethic of perfection, originally marked by Wolff, to an ever stronger emphasis on the will of God.