After attending grammar school, Achenwall in 1738 began his university studies in Jena, attending lectures in philosophy, history, mathematics and natural sciences, but also French and Italian. He transferred to Halle in the spring of 1740 (the same year that Christian Wolff returned from his “exile” in Marburg; however, there is no indication that Achenwall studied with Wolff). In Halle, Achenwall attended lectures in jurisprudence by Johann Gottlieb Heineccius and Justus Henning Böhmer and lectures in political science by Johann Peter von Ludewig and Martin Schmeitzel, whom Achenwall later mentions as a particular influence. Importantly, it was in Halle that Achenwall met his life-long friend and colleague Johann Stephan Pütter (1725–1807).

In the spring of 1742 Achenwall transferred to Leipzig where he came into contact with the jurist and historian Johann Jacob Mascov. On Mascov’s recommendation, Achenwall was appointed private tutor of the sons of the Saxon chancellor Karl August von Gersdorff, in whose employment he was from 1743 to 1746. Upon receiving his Magister degree from Leipzig in 1746, Achenwall went to Marburg where he held private lectures on natural law, the law of nations, and political science. With the help of his friend Pütter, Achenwall two years later was appointed to the recently founded university of Göttingen, where he remained for his entire career, teaching both in the faculty of philosophy (from 1748) and in the faculty of law (from 1754). During his 24 year-long tenure at Göttingen, Achenwall lectured on a wide range of subjects: natural law, the law of nations, public law, politics, history of the European states, and – under the heading of ‘statistics’ – comparative political studies. On most of these topics, Achenwall authored textbooks, which he regularly revised and expanded and which became widely used compendia at German universities in the second half of the eighteenth century.

There exists no picture of Achenwall.

Achenwall’s natural law compendium, which he reworked and extended continuously, appeared in five editions during his lifetime and saw two further posthumous reprint editions. The first two editions (1750 and 1753), titled Elementa Iuris Naturae, appeared in co-authorship with Johann Stephan Pütter (who contributed two chapters dealing with state and public law). From the third edition (1755/56) onward, Achenwall is the sole author of the work, now titled Ius Naturae and comprising two volumes. The foundational elements of natural law (action, obligation, imputation, etc.) are expounded in greater detail in the Prolegomena Iuris Naturalis (first edition 1758). Notably, in this work Achenwall develops an account of the hexagonal logical relations of the deontic terms (see § 26 in the 1767 edition; cf. Hruschka 1986). Achenwall’s Ius Naturae saw wide-spread use at German universities in the second half of the eighteenth century. Between 1765 and 1790, it was constantly among the most widely used textbooks (cf. Schröder/Pielemeier 1995). Prominently, Immanuel Kant used the 1763 edition as the compendium for his lectures on natural law. Of these, one set of students’ notes, from the summer term 1784, still exist (the Naturrecht Feyerabend lecture notes). Ludwig Höpfner based his Naturrecht (1780) on Achenwall’s compendium, which it subsequently came to replace in lecture-hall use (cf. Schröder/Pielemeier 1995).

Achenwall distinguishes a broad and a strict sense of natural law. In the broad sense, natural law is the same as moral philosophy, comprising all natural obligations and laws. Natural obligations are those moral obligations that can be known “from philosophical principles”, which we can know by reason alone, without divine revelation. In this broad sense, natural law includes all obligations: perfect and imperfect, external and internal. In the strict sense, natural law includes only “the knowledge of perfect natural laws, or the knowledge of external natural rights and obligations” (Ius Naturae 1763 [2020], § 1). External perfect obligations are those obligations that can be enforced by others through force. The basic principle of strict natural law is the duty to not disturb other persons’ preservation (Elementa 1750, § 213; Ius Naturae 1763 [2020], § 38).

Achenwall argues that natural law is of use in both legal practice and theoretical study. In legal practice, natural law can (and has to) be employed, by itself, to judge actions and to resolve conflicts where no human law applies, e.g., between nations or between rulers and their subjects. Subsidiarily, natural law can be used to judge actions and resolve conflicts in cases where human law does apply, but is insufficient in itself (Ius Naturae 1763 [2020], § 2). As regards to theory, natural law is of use in many disciplines: in jurisprudence, it can be used in dealing with customary international law, as well as in all areas of positive law, including divine law taught by Christian moral theology; moreover, natural law also benefits other branches of practical philosophy (§ 3). In line with this, Achenwall argues in Die Staatsklugheit nach ihren ersten Grundsätzen entworfen that the science of the state (Staatswissenschaft) has two parts: practical politics and natural state law, the latter setting boundaries for political action, delineating which means may rightfully be used to further public welfare and happiness (Staatsklugheit 1761, preface and pp. 2–4). However, the development of a system of natural law that may aid other disciplines requires itself a firm grasp of philosophy, jurisprudence and history as well as “of the various systems of natural law themselves” (Ius Naturae 1763 [2020], § 5; in a sketch of the history of natural law, he singles out the most important authors: 1763, pp. 39–53). Philosophy is of paramount importance since it furnishes natural law with both its foundation and systematic method (§ 5). As can be gathered from the manuscript sources, in his later years Achenwall worked on augmenting natural law with a historical account of the state of nature (Streidl 2003, esp. pp. 112–123).

The central and foundational concept of Achenwall’s system of natural law is the concept of natural obligation. The first edition of the Elementa (1750) presents a rationalist, Wolffian theory of natural obligation that eschews any reference to God. The ground of obligation is our natural striving for self-preservation and perfection, recognizable through experience. “The first natural law of all free actions is: perfect yourself” (§ 110). In the second edition (1753), Achenwall introduces a perfect obligation to adhere to God’s will, which follows from the principle of perfection (§182). In the Prolegomena (1758) and the 4th edition of the Ius Naturae (1758/59) we find a fundamentally revised, voluntaristic concept of obligation that particularly draws on the work of Heinrich and Samuel von Cocceji: God’s will is the sole normative basis of natural obligation: natural law exists and obligates precisely because there is a divine legislator whose will we can recognize (see Schwaiger 2020, Ludwig 2020 and Stiening 2021). Natural law in the broad sense (i.e., moral philosophy) fundamentally requires us to act in accordance with the prescriptions of the divine will which are recognizable by natural reason (Ius Naturae 1763, § 28). Notably, Achenwall’s revised concept of obligation excludes from moral philosophy any precepts of mere prudence – something that the Wolffian notion of obligation failed to do. Moreover, as natural law in the strict sense comprises only perfect and external obligations, it excludes any ethical, i.e. merely moral, obligations: Certain actions may be allowed by natural law insofar as they do not violate another’s rights, yet still be internally, i.e. morally, wrong. Such actions are not legally culpable, hence may not be averted by coercion or sanctioned by state punishment, yet constitute a sin subject to divine punishment.