‘Idealism’ certainly is one of the most dazzling concepts in philosophy, evoking strong emotions but remaining rather obscure. In their book Idealism in Modern Philosophy, Paul Guyer and Rolf-Peter Horstmann examine the various conceptions, significance, and history of the notion from the early rationalism of Descartes, Spinoza, and Leibniz to the analytical philosophy of the twentieth century in Sellars and McDowell. The authors thus focus on idealism in epistemology and metaphysics, while its broader significance in ethics and political philosophy is largely, if reasonably, overlooked. Accordingly, idealism is understood ‘to be a metaphysical doctrine, namely that everything that exists is in some way mental’ (p. 3), which means that the authors take idealism generally to be a monistic ontological position (and in so doing, follow Wolff’s characterisation of it, of which more below). Nonetheless, this approach allows the authors to consider the transformation and shifts that the interpretation of the concept of idealism undergoes over the course of its modern history, and the authors succeed in presenting important stages in the history of modern philosophy under the systematic approach of idealism, which is a major merit of this study.

The book is divided into eleven chapters. After the Introduction, in which Wolff’s decisive definition is given (p. 1), Chapter 2 examines idealism in early modern rationalism (Descartes, Spinoza, Leibniz, Malebranche, Baumgarten, pp. 15–24). Wolff introduces ‘idealism’ as a ‘classificatory term’ (p. 13) in the preface to the second edition of his Vernünfftige Gedancken von Gott, der Welt und der Seele des Menschen, auch allen Dingen überhaupt, the so-called Deutsche Metaphysik.Footnote ¹ There, he distinguishes between idealism and materialism as alternatives within ontological monism, that is, philosophical positions that admit only one class of fundamental entities (in contrast to dualism, which assumes two classes of fundamental entities). Among idealist positions, Wolff further distinguishes egoism and pluralism, where the difference depends upon whether one admits more than one real (i.e. spiritual) entity, beyond oneself. Baumgarten follows Wolff’s terminology (see Baumgarten (Reference Baumgarten1739: §402); however, for Baumgarten, simple substances (monads) can also be corporeal, making his view a sort of monad-dualism. Indeed, Baumgarten even presents an argument against idealism (which is still clearly understood ontologically) that is based on Leibniz’s principle of plenitude, namely, that a universe that contains not only more substances but several kinds of substances is more perfect (p. 23). Baumgarten’s position would become the dominant one in Germany: as the authors summarise, ‘dualism rather than idealism became the default position of the German successors to Leibniz’ (ibid.).

Chapter 3 deals with idealism in early modern British philosophy and especially in Hobbes, Locke, Berkeley, Collier, and Hume (pp. 25–46). The authors rightly trace the roots of British empiricism to Francis Bacon’s programme of the Instauratio Magna of the sciences (p. 26), in which they locate the source of the resulting hostility to metaphysics within this philosophical current. So, Hobbes and Locke want to remain agnostic with regard to the question of the nature of the ultimate substance because of their critical attitude towards all metaphysics. But for Berkeley, what was a mere ‘agnosticism with respect to the ultimate constitution of substances and things’ on Locke’s part (p. 33) becomes a ‘full-blown idealism’, which he calls ‘immaterialism’ (p. 34). For Berkeley, the alternative to idealism is not only materialism, but scepticism, as the authors summarise: ‘Idealism, one could say, is the only tenable basis for a realistic stance for Berkeley, but it leads to a realism about minds, human and divine, rather than of what he always calls material substance’ (p. 37).

Against this background, in Chapter 4, the authors turn to Kant, who famously reconciles these approaches in his critical transcendental philosophy (pp. 47–69). Kant is the first philosopher in Germany to affirmatively take up the label ‘idealism’ as it is introduced by Wolff and establish it as a defensible philosophical position. Naturally, Kant does not simply adopt the term from Wolff without any amendment, as he proceeds to dub his philosophical system of the first Critique transcendental idealism (cf. A 490f). Kant’s position is complex, as the authors rightly emphasise. They are aware of the fact that Kant’s transcendental idealism does not match their definition of idealism and that Kant’s position can only be characterised as a ‘real idealism’ (p. 63) that must be rationally believed in when his moral philosophy is taken into account. Therefore, they start with an overview on the first pages, and then they present and evaluate, with impressive concision, Kant’s arguments for transcendental idealism – first, the direct proof in the aesthetic (pp. 52–4) and then the Refutation of Idealism in B (p. 54) and the second, indirect proof in the Antinomy (p. 55) – and afterwards, they turn to Kant’s moral philosophy. However, the authors overlook the fact that through his system of transcendental idealism, Kant intends to eliminate the doubtfulness of the reality of the external world, a point emphasised in the Introduction to the B edition by the famous reference to this doubtfulness as a ‘scandal of philosophy’ (cf. B39).

The ontological and epistemological arguments of the German idealists who critically followed Kant are the subject of Chapter 5 (pp. 70–109). These philosophers are united in their rejection of transcendental idealism, and they do so, according to the authors, ‘by arguing that there is no real opposition between a subject-independent world that is present to us in the mode of “givenness” and “being” and a world that is thought subject-dependently insofar as it is formed by conceptual tools or other “ingredients of thought” that derive from subjective activity’ (p. 75). The authors reformulate this insight with the instructive, concise, and apt formula that ‘Kant’s successors removed his restriction of a necessary isomorphism between knowledge and the known to the case of appearances, and took it to be a general relation between thought and being’ (ibid.).

In Chapter 6, the authors turn to Schopenhauer’s and Nietzsche’s criticisms of idealism, though the authors note that these two thinkers’ positions – rather surprisingly in Nietzsche’s case – nevertheless exhibit idealistic traits (pp. 111–24). Schopenhauer does not deny the ultimate reality of the subject but merely, in contrast to the positions of German idealism, characterise that ultimate reality as irrational. As the authors explain, Schopenhauer ‘begins from an epistemological premise, namely that we can know ultimate reality through knowing ourselves, and reaches an ontological conclusion, that ultimate reality must be like ourselves, but in opposition to Kant and the other German idealists he assumes that our own nature is essentially non-rational and therefore that the ultimate character of reality, although it is in a certain sense like the mental, is also fundamentally non-rational’ (p. 114). In the case of Nietzsche, the authors emphasise Nietzsche’s perspectivism, according to which ‘the world each of us is experiencing is the product of an interpretation forced on us by some unconscious overriding drive (Trieb) that is the formative mark of the individual character of each of us’, which might be seen as a position close to idealism (cf. pp. 117f).

Chapter 7 deals with the little-noted British and American idealism of the nineteenth century (Green, Bradley, and McTaggart and Royce and Peirce). The interim conclusion here is that, in contrast with Berkeley, for whom the spirit at issue takes the form of God, the infinite mind, for idealists like Green and Royce, ‘the supra-individual mind is not always identified with God, but plays the same role’ (p. 142). Chapter 8 considers the rejection of British idealism by Moore and Russell as an important root of analytic philosophy, especially insofar as they put forward evidence-theoretical objections to idealism (understood as a metaphysical thesis) (cf. pp. 150–58). This yields the provocative suggestion (which the authors unfortunately do not engage with in much detail) that the epistemological compulsion to idealism (especially in the case of Russell) results from the introduction of the distinction between objects and sense data: ‘the very idea of an object as a construction guarantees the endorsement of idealism’ (p. 157). Chapter 9 briefly outlines and highlights the second ‘German answer to idealism’, namely, on the part of the neo-Kantian philosophers Windelbrand, Cohen, and Cassirer (pp. 159–73). Chapter 10 is devoted to more recent discussion in the twentieth century, particularly of those like Sellars and McDowell who claim expressis verbis to take inspiration from Kant and Hegel, respectively (pp. 174–202). Chapter 11 draws a brief conclusion.

It should be noted that the structure and content of the book are largely taken (in some cases, word for word) from their entry ‘Idealism’ in the Stanford Encyclopedia of Philosophy. The authors themselves refer to this source in their Introduction (p. 7), though it would have been helpful for the reader if they had marked the adopted passages as such in the text, and indicated more expressly what specific portions constitute a ‘revision and expansion’ (ibid.) of the original material. A cursory comparison gave the impression that the presentation of the philosophical positions dealt with in the book is more concisely tailored to the question of epistemological and ontological arguments in favour of idealism and that it has been supplemented partly by important systematic references, which, however, often remain very brief and thus unfortunately contribute little that is illuminating. For example, in the second chapter, the discussion of Descartes and Spinoza, which examines the extent to which their positions can be linked to epistemological reasons for idealism, is only slightly modified, as it is supplemented by a remark that these rationalist thinkers follow the traditional adaequatio understanding of truth in their epistemology and therefore cannot be linked to Kant’s Copernican alternative (pp. 17f). Although, this is surely right, further explanation would undoubtedly be helpful. Further to this, the entire section on Schopenhauer’s position simply repeats the original article (cf. pp. 110–14). Another concern relates to the secondary literature canvassed in the book: Not only is there little in the way of critical engagement with the extensive secondary literature (which nonetheless does make the book more accessible to the reader), but it is noticeable that what literature is discussed is largely limited to the anglophone context. It is, finally, also to be regretted that the comments on the philosophy of life as an alternative to idealism (Dilthey, Heidegger, Merleau-Ponty) are very brief (cf. p. 182). Overall, however, the authors succeed in demonstrating the relevance of idealism throughout modern philosophy and in retracing its development.