Leibniz' Metaphysik der Modalität. Berlin & Boston: De Gruyter (2016). [book]

This (German) book is based on my dissertation and investigates Leibniz's metaphysics of modality. Here is a brief summary in English:

At the very heart of Leibniz’s system is his claim that our world exists contingently and is only one among infinitely many possible worlds. God created this world because it is the best, but he could have chosen to create another one. Leibniz contrasts his own views on modality with those of Spinoza, who holds the extreme position that all possible things are actualized and exist with metaphysical necessity. For his theistic and rationalist program to succeed, Leibniz needs an account of merely possible things, i.e., non-existent possibilia or essences. He takes them to be ideas in God’s intellect. In my book I defend four theses about the metaphysics of Leibnizian possibilia. First, I argue that there is room for non-actual possibilia in Leibniz’s system only because his metaphysical set-up differs from Spinoza’s in at least one fundamental respect. Whereas Spinoza commits himself to a non-combinatorial conception of attributes, Leibniz’s understanding of attributes is combinatorial. That is, while Spinozistic attributes all individually express the divine substance and thus cannot be recombined to form non-divine substances, in Leibniz’s metaphysical framework exactly this is possible. Only with this thesis in place can Leibniz’s God think of several possible substances by conceiving of several combinations of attributes. Second, I maintain that the essences of (possible and actual) finite things, while directly grounded in the ideas in God’s intellect, are ultimately grounded in God’s essence. Otherwise there would be no sufficient reason for the fact that God thinks about these essences and not others. God thus does not have the contents of his ideas primitively; there is an explanation for why he has the ideas he has rather than others. Leibniz thus claims, similar to Spinoza, that finite things are grounded in the divine substance. The difference between the two thinkers is that Leibniz’s metaphysics allows for ideas of merely possible finite substances, whereas Spinoza’s does not. Third, I argue that the essences of individuals cannot be understood independently of Leibnizian possible worlds: God cannot form the idea of an individual without forming the idea of the world this individual is a member of and vice versa. Fourth, I defend the claim that once we have understood what a possible world is for Leibniz, we can explain why not all possibles are compossible. Only possibilia that can be integrated into one and the same system – i.e., into one and the same possible world – are compossible with one another. On my view, then, the notion of compossibility is elucidated in terms of the notion of a possible world and not the other way around as most commentators assume. This solves an important puzzle within Leibniz’s system since, if all possibles were compossible, they would all exist, given God’s goal to maximize being in the universe. Having examined Leibniz’s metaphysics of possibilia, one question remains: Even if Leibniz manages to establish that not everything possible is actual, does this suffice for avoiding the Spinozistic conclusion that everything exists with necessity? One might worry that it does not suffice, since Leibniz still accepts the existence of an omnibenevolent, omnipotent, and omniscient God, and it seems that such a God must necessarily create the best of all possible worlds. I argue that Leibniz can manage to maintain his theism while avoiding this worry. The cost, however, is that the principle of sufficient reason (PSR) – a cornerstone of Leibniz’s rationalism – only holds contingently. If God can choose a suboptimal world, then he can (given his omnipotence and omniscience) act without a reason, so it is in his power to violate the PSR. I also argue that the contingency of the PSR, despite appearances to the contrary, is reconcilable with the many text where Leibniz seems to rely on a necessary version of this principle.

Causation and Cognition in Early Modern Philosophy. Volume ed. with Dominik Perler. New York: Routledge (2020). [book]

This book re-examines the roles of causation and cognition in early modern philosophy. The standard historical narrative suggests that early modern thinkers abandoned Aristotelian models of formal causation in favor of doctrines that appealed to relations of efficient causation between material objects and cognizers. This narrative has been criticized in recent scholarship from at least two directions. Scholars have emphasized that we should not think of the Aristotelian tradition in such monolithic terms, and that many early modern thinkers did not unequivocally reduce all causation to efficient causation. In line with this general approach, this book features original essays written by leading experts in early modern philosophy. It is organized around five guiding questions: What are the entities involved in causal processes leading to cognition? What type(s) or kind(s) of causality are at stake? Are early modern thinkers confined to efficient causation or do other types of causation play a role? What is God's role in causal processes leading to cognition? How do cognitive causal processes relate to other, non-cognitive causal processes? Is the causal process in the case of human cognition in any way special? How does it relate to processes involved in the case of non-human cognition? The essays explore how fifteen early modern thinkers answered these questions: Francisco Suárez, René Descartes, Louis de la Forge, Géraud de Cordemoy, Nicolas Malebranche, Thomas Hobbes, Baruch de Spinoza, Gottfried Wilhelm Leibniz, Ralph Cudworth, Margaret Cavendish, John Locke, John Sergeant, George Berkeley, David Hume, and Thomas Reid. The volume is unique in that it explores both well-known and understudied historical figures, and in that it emphasizes the intimate relationship between causation and cognition to open up new perspectives on early modern philosophy of mind and metaphysics.


Localizing Violations of the Principle of Sufficient Reason - Leibniz on the Modal Status of the PSR. Forthcoming in Journal of Modern Philosophy.

The Principle of Sufficient Reason (PSR)—the principle that everything has a reason—plays a central role in Leibniz’s philosophical system. It is rather difficult, however, to determine what Leibniz’s attitude towards the modal status of the PSR is. The prevailing view is that Leibniz takes the PSR to be true necessarily. This paper develops a novel interpretation and argues that Leibniz’s PSR is a contingent principle. It also discusses whether a merely contingent PSR can do the metaphysical heavy lifting that Leibniz aims for. The paper shows that, despite appearance to the contrary, this is possible. In a nutshell, the argument is that the only possible PSR-violation Leibniz allows for is God’s creation of a suboptimal world; there is no Leibnizian possible world, though, which intrinsically violates the PSR. Despite its contingency, then, Leibniz’s PSR is modally robust enough to serve as a foundational principle of his metaphysics.

Anne Conway's Metaphysics of Change. Forthcoming in History of Philosophy Quarterly 39.1 (2022).

The Aristotelian account of change—according to which no individual can survive a change of species because an individual’s essence is, at least in part, determined by its species membership—remains popular in the seventeenth century. One important, but often overlooked dissenting voice comes from Anne Conway. Unlike many of her contemporaries, Conway firmly rejects the Aristotelian account of change. She instead endorses the doctrine of Radical Mutability, the view that a creature can belong to different species at different times. A horse, for example, can gradually become a human being and yet remain the same individual. Why, though, is Conway so opposed to the prima facie appealing Aristotelian account of change? This paper argues that she levels two arguments against this account which have been largely neglected so far. First, she argues that there could be no causal interaction between creatures belonging to different species with distinct essences, because cause and effect would be too dissimilar in that case. Second, Conway argues that the Aristotelian model is inconsistent with divine goodness, because it allows for the annihilation of creatures and because it imposes arbitrary restrictions on the capacity of creatures to improve.

Spinoza on the Essences of Singular Things. Forthcoming in Ergo 9 (2022).

Essences play a central role in Spinoza’s philosophy, not only in his metaphysics, but also in his philosophy of mind, his theory of affects, and his political philosophy. Despite their importance, however, it is surprisingly difficult to determine what exactly essences are for Spinoza. On a widespread reading, the essence of X is nothing but the concept of X. This paper argues against this identification of essences and concepts. Spinozistic concepts are maximally inclusive: the concept of X contains everything that is needed to make X conceivable. The essence of X, in contrast, is more limited in scope and does not include everything that is needed to make X conceivable. Thus, Spinoza avoids the ‘overloading’ of essences and the problems that would ensue. The account developed in this paper has a surprising implication, namely that the essences of non-divine, singular things do not suffice to render these things fully conceivable on Spinoza’s view. Thus, Spinoza breaks with a tradition according to which the essence of a thing states ‘what the thing is.’ As a result, his conception of essence is much further removed from traditional Aristotelian accounts, and from other seventeenth-century accounts, than usually acknowledged.

Hume's Deep Anti-Contractarianism. Hume Studies 47.1 (2022): 103-129. [paper]

Hume is an avowed critic of contractarianism. He opposes the idea that a legitimate government is based on an ‘original contract’ or on the consent of those who are governed. Most scholars assume, though, that his criticisms apply only to a limited range of contractarian theories, namely to theories according to which actual contractors reach an actual agreement. Theories on which the agreement in question is understood in hypothetical or counterfactual terms, however, are oftentimes seen as being compatible with Hume’s views. Against such interpretations, this paper shows that Hume rejects all contractarian theories, including hypothetical ones. It argues, first, that Hume employs a so far unacknowledged empiricist debunking strategy against contractarianism; if successful, this strategy undermines all variants of contractarianism. Second, it shows that the Humean conception of the state of nature (a topic that has received virtually no scholarly attention) is incompatible with hypothetical contractarianism. Finally, it argues that Hume rejects contractarianism in part because he anticipates a line of criticism which nowadays is often leveled against so-called ideal theory. On Hume’s view, the agreements reached by highly idealized contractors are of little relevance to the non-ideal individuals in the actual world.

Leibniz and the 'petites réflexions'. Archiv für Geschichte der Philosophie 102.4 (2020): 619-645. [paper]

In this article, I defend the thesis that Leibniz’s rational substances always have higher-order perceptions, even when they are, say, in a dreamless sleep. I argue that without this assumption, Leibniz’s conception of reflection would introduce discontinuities into his philosophy of mind which he cannot allow. This interpretation does not imply, however, that rational beings must be aware of these higher-order states at all times. In fact, these states are often unconscious or ‘small’ and only count as reflections when they become distinct or heightened enough. Reflections thus arise out of ‘petites réflexions’ just as conscious perceptions arise out of petites perceptions. I argue, furthermore, that an analysis of some aspects of Leibniz’s theory of memory shows that he is not only committed to the thesis that rational beings always have higher-order states but that he also accepts it. I conclude by considering whether my interpretation is at odds with Leibniz’s doctrine of transcreation and also whether it has any consequences for which theory of consciousness we should ascribe to Leibniz.

Perception: Reductivism and Relationalism. In: Medieval and Early Modern Epistemology: After Certainty (Volume 17: Proceedings of the Society for Medieval Logic and Metaphysics). Ed. by A. Hall, G. Klima & M. Klein. Newcastle upon Tyne: Cambridge Scholars Publishing (2020): 37-46. [paper]

This discussion piece is about Lecture Three ("The Sensory Domain") from Robert Pasnau's book After Certainty. It focuses on the reconstruction of seventeenth-century theories of perception that this lecture provides, in particular what Pasnau calls the “reductive path” and the “relational path.” On the reductive path, the mechanistic processes leading to perceptions are the objects of these perceptions. On the relational path, our perceptive state are more like signs for the mechanistic processes in the world. I argue that the reductive path is more popular in seventeenth-century rationalism than one might expect. I also argue that there are some hidden problems with the relational path.

Berkeley on Causation, Ideas and Necessary Relations. In: Causation and Cognition in Early Modern Philosophy. Ed. by D. Perler & S. Bender. New York: Routledge (2020): 295-316. [paper]

On Berkeley’s immaterialist ontology, there are only two kinds of created entities: finite spirits and ideas. Ideas are passive, and so there is no genuine idea-idea causation. Finite spirits, by contrast, are truly causally active on Berkeley’s view, in that they can produce ideas through their volitional activity. Some commentators have argued that this account of causation is inconsistent. On their view, the unequal treatment of spirits and ideas is unfounded, for all that can be observed in either case are mere patterns of regularity; Berkeley should therefore adopt a full-blown occasionalism and follow Malebranche in holding that God is the only true cause. Other commentators have argued that Berkeley denies the tenet that causes necessitate their effects – that is, the idea that causation involves necessary connection – and that in this way he can avoid inconsistency. This paper argues that Berkeley can subscribe to the thesis that finite spirits are truly causally active without falling into inconsistency, even if it is granted that Berkeleyan causes necessitate their effects. His differing treatment of spirits and of ideas is well founded, since ideas are transparent in a way our notions of spirits are not.

Introduction: Causation and Cognition in Early Modern Philosophy. Co-authored with Dominik Perler. In: Causation and Cognition in Early Modern Philosophy. Ed. by D. Perler & S. Bender. New York: Routledge (2020): 1-17. [paper]

Early modern philosophers took the phenomena of causation and cognition to be closely related. United in their opposition to Aristotelian accounts of cognition, they developed a wide range of competing theories to explain which causal processes lead to cognitions. Somewhat surprisingly, some early modern authors also made cognition a requirement for causation, on the assumption that every cause needs to cognize its effect. This introductory chapter explores both directions of explanation—from causation to cognition and vice versa—and surveys the various early modern approaches to causation and cognition.

Is Leibniz's Principle of the Identity of Indiscernibles Necessary or Contingent? Philosophers' Imprint 19.42 (2019): 1-20. [paper]

The Principle of the Identity of Indiscernibles (PII)—the principle that no two numerically distinct things are perfectly similar—features prominently in Leibniz’s metaphysics. Despite its centrality to his philosophical system, it is surprisingly difficult to determine what modal status Leibniz ascribes to the PII. On many occasions Leibniz appears to endorse the necessity of the PII. There are a number of passages, however, where Leibniz seems to imply that numerically distinct indiscernibles are possible, which suggests that he subscribes to a merely contingent version of the PII. In this paper I attempt to resolve this apparent inconsistency. I argue that Leibniz consistently takes the PII to be necessary and that this view of his shines through even in his correspondence with Clarke. I also show that competing interpretations, on which Leibniz’s PII is contingent, misread a number of crucial passages from this correspondence.

On Worlds, Laws and Tiles—Leibniz and the Problem of Compossibility. In: Leibniz on Compossibility and Possible Worlds. Ed. by G. Brown & Y. Chiek. Dordrecht: Springer (2016): 65-90. [paper]

Leibniz defends two apparently inconsistent doctrines. On the one hand, he holds that substances are independent entities and that God can, at least in principle, create any possible substance whatsoever no matter what else he creates. On the other hand, Leibniz insists that some possible substances are incompossible with one another and thus cannot coexist. I first discuss three attempts of dealing with this tension in Leibniz’s work that have recently been made in the literature: the logical approach, the lawful approach, and McDonough’s tiling approach. I conclude that none of them solves the problem satisfactorily. I then argue that a modified version of the lawful approach, which also takes into account some of the insights of the two other options, is the most promising strategy. I believe that it is crucial to realize that it is, at least in principle, in God’s power to co-create incompossible substances. I do not take this to imply, however, that such substances can coexist. Since coexistence requires being in the same world, and because not all collections of possible substances are worlds in Leibniz’s sense, I argue that co-creation does not entail coexistence. Thus in order to get clear on Leibniz’s conception of compossibility, I suggest that we must better understand what a possible world is for Leibniz. What does it mean, then, for a collection of possible substances to form a world? I argue, first, that all complete concepts of the relevant possible substances need to have the same laws inscribed in them this is in line with some versions of the lawful approach). In addition, I argue that every complete concept in the relevant collection must also involve the concept of every other substance in the collection. Moreover, I maintain that these two requirements are not reducible to each other and that they are individually necessary and only jointly sufficient for a collection of substances to form a world. Only substances whose complete concepts fulfill both conditions can coexist and thus be compossible.

'Si omnia possibilia existerent…'—Why Leibniz Denies That All Possibles Can Exist. History of Philosophy Quarterly 33.3 (2016): 215-236. [paper]

Leibniz denies Spinoza’s claim that all possible things actually exist. He also denies necessitarianism, Spinoza’s claim that all truths are necessary truths. Both denials seem plausible. What is surprising, however, is Leibniz’s view that the first claim entails the second, i.e., that the existence of all possible things implies necessitarianism. Why think this? Couldn’t it be that, as a matter of contingent fact, all possible things actually exist? There seems to be no incoherency in claiming both that all possible things actually exist and that fewer things could have existed. In this paper, I explain why Leibniz thought that such a position is indeed inconsistent. My thesis is that the Principle of Sufficient Reason plays a decisive role in his reasoning. Leibniz claims that the non-existence of a thing requires a sufficient reason. He also holds that if all possible things can coexist, such a reason cannot be given. Together these claims rule out a scenario in which all possible things actually exist, but only contingently so.

Interpreting Leibniz's Modal Language. In: “Für unser Glück oder das Glück anderer”. Vorträge des X. Internationalen Leibniz-Kongresses. Hannover, 18.–23. Juli 2016. Ed. by W. Li. Hildesheim, Zürich, New York: Georg Olms (2016): 439-449. [volume] [penultimate draft]

Modal expressions are notoriously context-sensitive. This makes interpreting Leibniz's modal language difficult on many occasions. Th goal of this paper is to develop some guidelines which help us to make interpretative decisions about passages containing modal terms in Leibniz’s texts. I suggest the following three guidelines: (i) When interpreting Leibniz’s modal statements involving God, be aware of the distinction between potentia absoluta and potentia ordinata. This might help determine the right context against which such statements should be evaluated. (ii) When evaluating Leibnizian counterfactuals, be aware of the fact that two possible worlds W1 and W2 that may seem very close/similar to each other from a contemporary perspective may be very far away from each other on Leibniz’s view and vice versa. (iii) When Leibniz uses the terms ‘abstract’/‘abstraction’ and ‘fiction’/’fictitious’ in modal contexts, he often talks about something he considers to be metaphysically impossible (but may seem possible nonetheless).

Reflection and Rationality in Leibniz. In: Subjectivity, Selfhood and Agency in the Arabic and Latin Traditions. Ed. by J. Kaukua & T. Ekenberg. Dordrecht: Springer (2016): 263-275. [paper]

Leibniz repeatedly states that there is a very close connection between reflection and rationality. In his view, reflective acts somehow lead to self-consciousness, reason, the knowledge of necessary truths, and even to the moral liability of the respective substances. Whereas it might be relatively easy to see how reflective acts lead to self-consciousness, it is much harder to understand how they are connected to rationality. Why should a substance which is able to produce reflective acts therefore be rational? How can the having of reflective acts be responsible for the substance’s ability to reason correctly and to acquire knowledge of necessary and eternal truths? The aim of my paper is to understand better the required mechanisms and to make thus conceivable Leibniz’s bold claim that reflective acts lead to rationality. In order to accomplish this, I will proceed in three steps. First, I will specify what kind of self-consciousness is, according to Leibniz, produced by reflective acts. A substance must recognize itself as a unitary substance bearing perceptions. Second, I will argue that this type of self- consciousness can be seen as the basis for the ability to form judgments. This is possible because the subject-predicate structure of judgments is mirrored by the ontology of substances and their modifications. Third, I will point out that, together with the idea of identity (which we also acquire by reflection), the combination of judgments allows us to make inferences. This ability, in turn, is sufficient for rationality. Thus, I can explain how reflective acts and rationality are connected with each other.

Von Menschen und Tieren—Leibniz über Apperzeption, Reflexion und conscientia. Zeitschrift für Philosophische Forschung 67.2 (2013): 214-241. [paper]

This German paper investigates what kinds of abilities Leibniz ascribes to non-human animals and how they differ from the abilities he ascribes to humans. The paper attempts to clarify how the notions of perception, apperception, reflection, and conscientia are related for Leibniz. More specifically, the paper develops a new reading of section four of the Principles of Nature and Grace, which is a much-discussed passage in Leibniz scholarship. It argues for two claims: (i) Leibniz distinguishes between a reflective and a non-reflective kind of apperception. (ii) Leibnizian conscientia is not to be confused with (phenomenal) consciousness, because it includes a reflective element, from which it follows that non-human animals do not have conscientia according to Leibniz.


Maria Rosa Antognazza (ed.). The Oxford Handbook of Leibniz. New York: Oxford (2018). In: British Journal for the History of Philosophy 28.1 (2020): 204-207. [review]

The Oxford Handbook of Leibniz is a weighty tome, so I inevitably had to make a selection. This review discusses the papers by Jeffrey McDonough (Freedom and Contingency), Donald Rutherford (Monads), Arnauld Pelletier (The Scientia Generalis and the Encyclopaedia), Justin Smith (Medicine), Hartmut Hecht & Jürgen Gottschalk (The Technology of Mining and other Technical Innovations), Stephan Waldhoff (Proposals for Political, Administrative, Economic, and Social Reform), Gonzalo Rodriguez-Pereyra (The Principles of Contradiction, Sufficient Reason, and Identity of Indiscernibles), and Martha Bolton (Theory of Knowledge).

Michael V. Griffin. Leibniz, God and Necessity. Cambridge: Cambridge University Press (2013). In: Archiv für Geschichte der Philosophie 97.2 (2015): 265-269. [review]

This review of Griffin's fascinating and inspiring book focuses on the necessitarian interpretation of Leibniz the book develops. I raise some problems for this reading, but I conclude that one must be careful not to beg the question against Griffin. I also discuss some other themes from the book, including Leibniz's Striving Doctrine, his theory of compossibility, his theory of divine fore-knowledge, and his account of counterfactuals.

Blog Post

Was machen eigentlich PhilosophiehistorikerInnen (What do historians of philosophy really do?) [blog entry]

In this blog entry, which addresses a broader audience, I wonder what exactly historians of philosophy do and how their work relates to non-historical work in philosophy. In particular, I raise the question why systematic philosophers and historians of philosophy are relatively close to each other. After all, they often publish in the same journals and work at the same departments. This is surprising, given that asking what X is seems to be rather different from asking what some person a few hundred (or thousand) years ago thought what X is. I argue that systematic philosophers and historians of philosophy are so close because their work proceeds in a similar way. To be sure, they have very different starting points. Historians of philosophy may start, for instance, with a number of texts from one and the same author which appear to be in conflict. Systematic philosophers, in contrast, may start with a set of intuitions which seem to contradict one other. The way historians of philosophy and systematic philosophers operate upon those different starting points, however, is very similar. This explains at least in part, I maintain, why they are a lot closer than is the case in many other disciplines.