In Kant and Rational Psychology, Corey W. Dyck offers a new interpretation of Kant’s paralogisms of pure reason, a key part of his Critique of Pure Reason (primarily of the first edition version of 1781) that is mainly intended as a critique of ‘rational psychology’, the philosophical doctrine concerned with the concept of soul and what can be inferred from it. Dyck’s endeavour is directed against a renowned narrative according to which the paralogisms are pre-eminently concerned with Descartes and Leibniz, whose theories of mind Dyck dubs ‘narrowly rationalistic’. Against this narrative, Dyck persuasively argues for the significance of German Enlightenment philosophers like Christian Wolff and Alexander Gottlieb Baumgarten who have often been neglected as less sophisticated, minor thinkers. Dyck makes a convincing case that in ignoring these thinkers, we not only ignore a philosophical tradition that Kant’s contemporaries would immediately associate with any talk of rational psychology. Second and more importantly, we would also neglect a tradition that in fact develops a radically new methodology in investigating the soul that differs in many ways from Descartes and Leibniz, and that also elaborates in much more detail on the actual issues Kant discusses in the paralogism chapter. It is true that what Descartes and Leibniz thought about the simplicity, immateriality and in particular the personality of the soul can only be conjectured from the available texts, while Wolff and his followers not only coined the very notion of rational psychology but also wrote extensively on it. Third, Dyck argues that key features of Kant’s discussion of the soul make more sense as responses to this tradition rather than as to ‘narrowly rationalistic’ theories.

The book is divided into seven main chapters. The first chapter deals with Wolff’s conception of psychology as a unified or mixed discipline consisting of the two elements of empirical and rational psychology. In the second chapter, Dyck follows the further development of this discipline from Wolff’s successors such as Baumgarten and Georg Friedrich Meier, as well as his Pietist critics, to Kant’s more immediate contemporaries Moses Mendelssohn, Johann Heinrich Lambert and Johann Nikolaus Tetens. Dyck further shows in this chapter that the pre-critical Kant was endorsing Wolffian psychology as a mixed discipline as well. The third chapter explains the fundamental change of Kant’s view of rational psychology that came with the critical turn, without constituting an entire break from the Wolffian conception. Whereas the third chapter focuses on the first paralogism, that of the substantiality of the soul, the subsequent chapters turn towards the other paralogisms: that of the simplicity of the soul (ch. 4), that of personality (ch. 5) and the fourth one that is devoted to forms of idealism. Chapter 7 deals with the use of reason in guiding the investigation of inner appearances Kant discusses in the Appendix to the Transcendental Dialectic.

What are ‘narrowly rationalistic’ theories of mind, and what exactly is Wolff’s alternative of a mixed psychology? Narrowly rationalistic theories would pursue psychology completely independently of experience (e.g. p. 227). It is unclear how many actual theories would really qualify as narrowly rationalistic, but it seems to be an appropriate representation of how the purported opponent of Kant in the paralogisms is often construed.Footnote ¹ In contrast, Wolff’s ‘broad rationalism’ (not restricted to the mind) aims at a ‘marriage of reason and experience’ (p. 27). Wolffian empirical and rational psychology are designed to converge towards a common end by drawing determinate propositions from experience and finding the ground of others based on them. Wolff’s theory is thus rationalistic only in the sense that the investigation of the soul is not limited to experience. Consequently, Dyck argues that Kant’s most important criticism of rational psychology is not that it takes the soul to be intelligibly given (though it does that as well) but rather that it overlooks the boundaries of experience in employing the categories in their empirical signification with regard to the soul.

Dyck provides a lot of textual evidence that Kant was following the lead of Wolffian psychology up to the Critique of Pure Reason and acknowledges that the publication of the latter included a radical change in his conception of rational psychology, which he now conceives as a pure discipline entirely free of experience. The pure character of the I now prevents us from cognizing it based on experience. At the same time, a crucial point Dyck argues for is that Kant’s doctrine of transcendental illusion is primarily directed against alleged empirical knowledge of the soul; thus Kant’s diagnosis is that (mixed) rational psychology sets off from the illusion that the soul is given in inner experience. What mixed and narrowly rationalistc theories have in common is that they claim to cognize the substantiality, simplicity and persistence of the soul, albeit from different sources. Moreover, Dyck persuasively argues that Kant’s discussion of rational psychology retains a crucial connection to experience that also links it to Wolffian psychology on a deeper level: Kant claims, according to Dyck, that narrowly rationalistic psychology overlooks the fact that pure and indeterminate self-consciousness is in fact only possible by abstracting from the empirical conditions that originally accompany the ‘I think’.

Kant and Rational Psychology is a clearly structured and well-argued book that provides a convincing reading of Kant’s paralogisms against the background of his more or less immediate contemporaries. In addition, it offers an interesting discussion of Wolffian and post-Wolffian rational psychology, which has too often been neglected. Dyck’s way of framing rational psychology as a mixed discipline, distinct from narrowly rationalistic conceptions, also connects to current, more general discussions about alternatives to the classic rationalism-empiricism scheme in carving out Wolff as yet another philosopher who does not easily fit into the scheme. Seen in this light, Wolff appears to be neither a Cartesian nor a Leibnizian (as was often thought), but relatively independent. The particular merit of Dyck’s book in this important respect is to highlight that Wolff is not to be criticized for not fitting tightly into the rationalism-empiricism scheme. This is rather the strength of Wolff’s psychology and subsequent thinkers following him, including, to some extent, Kant.

One could try to further corroborate Dyck’s analysis of Kant’s proximate target in the paralogisms by looking at a few additional, more immediate contemporaries of Kant.Footnote ² Not only Tetens but also philosophers like Ernst Platner, Johann August Eberhard or Johann Christian Lossius put even greater emphasis on the role of experience in establishing the usual tenets of rational psychology, i.e. that the soul is simple, immaterial, persistent etc., by maintaining that the ‘feeling of self’ directly reveals that the soul is different from the body (for instance in Platner Reference Platner1790: 53), that it is immaterial (Lossius Reference Lossius1775: e.g. 164–5) or that it is simple and persistent (Eberhard Reference Eberhard1776: e.g. 23–5). Conversely, Dyck’s analysis of Wolffian mixed psychology can also help to better understand these later theories as radicalizations of the latter, which can otherwise seem to be just based on arbitrary claims about the purported ‘feeling of self’.