When one thinks of the canonical figures of the German Enlightenment, names such as Kant, Leibniz, Wolff and Mendelssohn often come to mind, and rightfully so. However, there is one philosopher of this period who has not received the degree of scholarly attention due to him, Christian August Crusius (1715–75). As a professor of philosophy and theology at the University of Leipzig, Crusius dedicated his career to developing a philosophical system that J. B. Schneewind has characterized as ‘one of the most original products of the eighteenth century’ (Schneewind Reference Schneewind1998: 446). Crusius was highly critical of the dominant philosophy of his day, the rationalism of Christian Wolff and his disciples, and much of the scholarly work that has been done has focused on either his critique of Wolff or his influence on Kant. While both of these topics are worthy of attention, the editors of this volume correctly argue that this focus is overly narrow and thereby fails to do justice to Crusius’ philosophy in its own right.

The stated aim of the text is to build ‘on existing approaches and at the same time to open up new perspectives for further research into one of the most enigmatic and innovative thinkers of 18th century German philosophy’ (p. 2). With respect to this goal, the text is a great success. The chapters it contains offer a superb survey of some of the most fundamental ideas of Crusian philosophy, appealing to both those who are familiar with his thought and those who are approaching Crusius for the first time. The text covers a diversity of topics and highlights the systematic nature of Crusius’ thought. A further virtue is that it brings together a wide variety of scholars working on Crusius, offering the reader insight into the current state of scholarship and evidencing a renewed international interest in Crusian philosophy. Hence, this volume contributes significantly to our understanding of a key figure of the German Enlightenment.

The text is divided into four sections, each of which focuses upon a particular aspect of Crusius’ philosophy. Section one is on metaphysics and natural philosophy, and contains contributions from Tinca Prunea-Bretonnet, Andree Hahmann, Sonja Schierbaum and Kay Zenker. Section two focuses upon freedom of the will, and here one finds contributions from Ansgar Lyssy, Michael Walschots, Gideon Stiening and Steven Tester. Section three covers various aspects of Crusius’ practical philosophy, with contributions from Sonja Schierbaum, Martin Sticker, Gabriel Rivero, Dominik Recknagel and Jutta Heinz. Lastly, in section four, the focus is on Crusius’ theological views, with contributions from Stefan Klingner, Hans-Peter Nowitzki and Paola Rumore. Given the size and scope of the volume, it would be impossible to do each contribution justice in a brief review, and so I will selectively discuss one chapter from each section in order to offer the reader a taste of the contents of this volume.

From the first section (Metaphysics and Natural Philosophy), Sonja Schierbaum’s paper titled ‘Crusius and Wolff on Mind and (Self-)Consciousness’ considers both the Crusian and the Wolffian accounts of self-consciousness. While there are important similarities between them, Schierbaum argues that their notable differences can be traced back to their distinct conceptions of the mind and its powers. Wolff argues that the mind’s distinguishing power is the understanding, by which the agent can clearly perceive an object and account for the differences among its parts. Although non-human animals have powers of sense perception, imagination and desire, they do not have minds for Wolff because they lack understanding. On the other hand, Crusius offers a broader conception, concluding that both humans and non-human animals have minds. He argues that a mind must have understanding (the power of representation) and will (the power to act according to representations). What is distinctive of the human mind, however, is the power of internal sensation, that is, self-consciousness. Through internal sensation, the agent is conscious of her acts of thinking and willing. Hence, self-consciousness is a higher-order power distinct from the power of representation, which is concerned with objects external to the agent. This points to another important difference between Crusius and Wolff. According to Wolff, the mind qua simple substance only has one power, the power of representation. Like Crusius, Wolff argues that self-consciousness is a higher-order act by which the agent cognizes her own mental activities. Because of his single-power theory of mind, though, Wolff must conclude that self-consciousness is merely the application of the power of representation to one’s internal mental activities. Thus, Schierbaum concludes that the major differences between the Crusian and Wolffian accounts of self-consciousness do not follow from rival conceptions thereof. Instead, they are grounded in a more fundamental disagreement about the mind and its powers.

From the second section (Freedom of the Will), Ansgar Lyssy’s contribution, ‘Crusius on Human Nature – An Interpretation of his Telematologie’, explores Crusius’ conception of the human will and argues that he puts forward a view of human nature that should be understood as a philosophical anthropology. Unlike God’s will, which is simply infinite power, Crusius argues that the human will is a composition of irreducible basic powers. One such power is the power of freedom, by which an agent has the capacity to act in an undetermined, truly spontaneous fashion. Contra Leibniz and Wolff, the will is not determined by what the understanding represents as better or worse, but instead freely chooses to act in accordance with a representation, or refrains therefrom, which, as Lyssy argues, allows Crusius to side-step the objection that the will is arbitrary or merely spontaneous. Lyssy outlines Crusius’ classifications of the will’s drives: animal and human, basic and derivative. The most important of these are the three basic human drives – the drive for one’s own perfection, the drive to unite with objects of perceived perfection and the drive of conscience, by which the agent understands God’s moral law and is motivated to conform to it. Although we share many drives with non-human animals, humans are unique due to their distinctively moral vocation, which is only realized in submission to the moral law and in embracing God’s ends as one’s own. Through obedience, the agent can order her basic drives in accordance with moral prescriptions and thereby foster an internal unity among her drives and derivative desires. Lyssy concludes by arguing that the understanding of human nature that emerges in Crusius’ account of the will should be classified as an early Enlightenment philosophical anthropology.

From the third section, concerning practical philosophy, Martin Sticker’s chapter, ‘Sleeping Conscience – Crusius on Moral Fallibility’, examines moral failure with respect to what Crusius calls the ‘sleeping conscience’. Like Kant, Crusius argues that moral knowledge is universally accessible through the drive of conscience, independent of authority, education and revelation. Although Crusius and Kant agree that conscience is an incorruptible judge that cannot, in and of itself, lead an agent into moral error, unlike Kant, Crusius accounts for how conscience can be weakened such that error results. In this case, some cognitive or conative factor (such as natural moral weakness, ignorance, prejudice, habituation to sin, overwhelming or conflicting passions or preoccupation with other matters) prevents the conscience from exercising the requisite authority over one’s choices, and is therefore a sleeping conscience. Sticker raises the objection that Crusius seems to allow for ‘conscientious immorality’, in which an agent endorses an immoral action as moral and justifies it through appeal to (sleeping) conscience. In response to this objection, Crusius could argue that, although the agent is obligated to act according to her conscience, she is also liable for maintaining a healthy, non-sleeping conscience. It is clear that, if an immoral act results from a state of self-deception for which the agent is responsible, then the agent is morally blameworthy. However, this does not seem to hold in a circumstance in which a sleeping conscience follows from a natural moral weakness for which the agent is not responsible. Moreover, such a weakness could implicate God and endanger Crusius’ egalitarian view of moral knowledge and capacities. While Sticker does not identify an explicit argument in Crusius that addresses this, he concludes the paper by suggesting that one could point to Crusius’ understanding of what is ‘natural’ as leaving the conceptual space to rescue his account of moral failure from these implications.

From the fourth and final section (Theology), Stefan Klingner’s chapter, ‘The Systematic Place of Natural Theology in Crusius’ Work’, discusses Crusius’ conception of natural theology and its place within his system. Natural, unlike revealed, theology treats the existence and nature of God, and God’s relation to human beings, through reason alone. Natural theology can be either theoretical or practical, depending upon the framework (metaphysical or moral) within which one examines these matters. Crusius classifies natural theology as the second of the metaphysical sciences, following ontology and preceding cosmology and pneumatology. Klingner makes the case that Crusius chooses this classificatory scheme because theoretical natural theology establishes philosophical knowledge of God via the analytic method, which Crusius views as requisite for the latter sciences. The primary concern of practical natural theology is accounting for duties as God’s commands. As the highest lawgiver, God is the source of duty and obligation, the validity of which the agent recognizes through conscience and reason. Hence, natural theology is a propaedeutic for cosmology, pneumatology and moral philosophy, justifying its prominent place in Crusius’ system. In the conclusion of the paper, Klingner extends his analysis of natural theology beyond the strictly philosophical realm and discusses its role as mediator between philosophy and theology. As a philosophical discipline, natural theology establishes a foundation for revealed theology, religious faith and – ultimately for the highest human vocation – a virtuous life in conformity with God’s will.

Prior to concluding, I would like to make a few observations about the text as a whole. First, while the topical diversity of this volume is commendable, two of the four sections focus either directly upon Crusius’ practical philosophy or on topics immediately germane to it, such as his account of the will. This makes good sense, given the stress that Crusius himself places upon practical concerns (Schneewind Reference Schneewind1998: 446–7). Nevertheless, my hope is that future texts on Crusius will build upon the foundation that this volume establishes and branch further into his theoretical and logical works, bringing the richness of his system into even sharper focus. Second, the reader should be aware that five (out of seventeen) papers in this volume have been kept in their original German, though this volume has much to offer the reader irrespective of one’s knowledge of German. Lastly, I would like to call the reader’s attention to the fact that, although the translations of primary texts in this volume are excellent overall, one may encounter passages that are slightly awkward linguistically, which is inevitable with translations.

In conclusion, Christian August Crusius (1715–1775): Philosophy between Reason and Revelation makes a significant contribution to Crusius scholarship. Whether one is a seasoned reader of Crusius or is coming to his philosophy for the first time, this volume has a wealth of information and insights to offer. The editors have sought to expand our understanding of Crusian philosophy and to do justice to Crusius as a philosopher in his own right, and it is beyond doubt that they have succeeded. This volume makes a compelling case that Crusius is worthy of careful study and deserves recognition as an important voice in the German Enlightenment.