Thought Experiments in Philosophy

While philosophers regularly use thought experiments, there is little agreement on what they are or why they work. Their use may be as old as philosophy itself. We find examples in Plato (the Ring of Gyges in Republic, the Aviary in Theaetetus) and even earlier, such as Zeno’s Achilles and the tortoise. The well-known puzzle of the ship of Theseus, reexamined by Thomas Hobbes, appears first in Plutarch’s Life of Theseus, in the first century.Footnote ³ Thought experiments have a long and well-established history in philosophy for clarifying ideas, providing counterexamples, establishing new conclusions, and generally doing the work that philosophers do.

Despite their long-standing use in philosophy, it is common in contemporary discussions to view the method of thought experiment in philosophy as derived from its use in the sciences.Footnote ⁴ Roy Sorensen even suggests that thought experiments in philosophy “evolved from ordinary [i.e., scientific] experiments by a process of attenuation.”Footnote ⁵ Accounts of thought experiment as a method typically contextualize it in the work of Ernst Mach, Thomas Kuhn, or both. Many leading accounts of thought experiment in general, including those of James Brown and John Norton, deal primarily with thought experiments in science and then extend the discussion to include philosophical thought experiments almost as an afterthought.Footnote ⁶ Even Brown’s “Platonic” thought experiments are “remarkable,” he notes, because “they provide us with a priori knowledge of nature” (my emphasis).Footnote ⁷

There is no doubt that scientists such as Mach and Kuhn, as well as Galileo and Einstein, have contributed significantly to our contemporary use and understanding of thought experiments, including their philosophical use. But given the long history of thought experiments in philosophy (pre-dating Mach’s discussions of their role in science by hundreds of years), framing their use in philosophy as derivative of their use in the sciences cannot give us the whole story. While contemporary philosophical thought experiments about personal identity, knowledge, and justice may take scientific experiments as inspiration, they are at least as likely to find their roots in their much earlier use by Plato, Lucretius, Plutarch, Hobbes, Locke, and others, especially for canonical cases (like Locke’s prince and cobbler or the ship of Theseus).

Nevertheless, some philosophers of thought experiment consider the true history of the concept to begin with Mach (a claim I will examine in the book). I agree that there must be something significant about the introduction of the term “thought experiment” [Gedankenexperiment in German and the lesser-known Tankeexperiment in Danish], together with more sustained attention to thought experiment as a distinct method. On the other hand, the increased recognition of thought experiments as a method should illuminate rather than negate the long tradition of their use, especially in philosophy – which may differ in important ways from their use in the sciences. This book aims to accommodate both the term’s scientific origins in the early 1800s and the long-standing practice of thought experiments in philosophy. To that end, I examine the earliest known uses of the term “thought experiment,” but I emphasize the philosophical use of the terms “experiment” and “thought experiment” even at this key point in history. The result is a view that is informed by the importance of thought experiments for science but takes account of its plurality of uses and meanings in its earliest beginnings. Since the philosopher Søren Kierkegaard, a contemporary of Hans Christian Ørsted, is one of those to employ the term Tankeexperiment in this time period, a study of this key period in the history of thought experiments necessarily takes us beyond their use in the sciences.

Why Kant?

Contemporary examinations of thought experiment as a method increased markedly in the 1980s and the years after.Footnote ⁸ These debates, as I have noted, trace their historical roots to Mach and take place mainly within philosophy of science. Many scholars remain either unaware of, or quickly dismiss – perhaps some with good reasons due to the scope of their projects – the earlier discussions of thought experiments by Ørsted and Kierkegaard. This book fills in some of that earlier history in Copenhagen in the first half of the nineteenth century.

One reason commonly given for moving quickly past Ørsted is that his work is thought to have had little influence on Mach, and therefore little influence on the development of the concept (see Chapter 3). But as Fehige has recently noted, it is not clear Mach’s view had much influence either. His name appears at the head of lists of those who have contributed to the topic, but there is not much discussion of thought experiments beyond their use in mathematics until Kuhn, around 75 years later.Footnote ⁹ Einstein’s use of the term Gedankenexperiment (which he likely learned from Mach) popularized the term. But that does not mean that Mach’s discussions of thought experiment, or Einstein’s use, influenced philosophers’ understanding of thought experiments. As I have noted, Locke’s prince and cobbler thought experiment is a touchstone in philosophy of personal identity and predates Mach, as does Hume’s variation on Plutarch’s ship of Theseus, along with countless other examples.

Although relationships of historical influence matter, what is most important in such analyses are the new interpretive possibilities they provide. In the case of thought experiments, a glance at history reveals we do not have a single concept (originating with Mach, for example) whose meaning evolves linearly over time. It is not the case, or not simply the case, that a new method in science makes its way over to philosophy. A more plausible account is that changes in science provided a new framework to make sense of a method philosophers were already using effectively, recognizably, and in a manner that would be hard to distinguish from its use in philosophy today. To explain why the term was so readily adopted by philosophers as well as scientists, we need to look back before Mach to earlier uses of the term and why it mattered to those who used it.

This book is both a historical examination of a little-studied time period and an original account of how thought experiments really work in philosophy. It is comprehensive in providing meaningful ways to distinguish different kinds of thought experiments, beyond referring to their subject matter or discipline (e.g., math or ethics). Alongside claims that thought experiments provide understanding (Stuart, Elgin, Wiltsche), logical proofs (Sorensen, Norton), or direct intuition of ideal objects (Brown, Chudnoff, Bengson), I argue that thought experiments provide cognition in Kant’s sense of the term [Erkenntnis]. (I define “cognition” in Kant’s sense in Chapter 1.)

Even though Kant himself did not use the term Gedankenexperiment, the question that Ørsted, Kierkegaard, and Mach all ask is a Kantian one: what makes abstract concepts meaningful? The view I put forward here joins other recent Kantian accounts, including those of Marco Buzzoni, Michael Stuart, Yiftach Fehige, and Michael Friedman.Footnote ¹⁰ The earliest Kantian account of thought experiments that we know of is Ørsted’s own. Kant calls the antinomies “experiments of pure reason” (see Chapter 1 of this book). But when Ørsted describes thought experiments as a distinct method, he begins instead from Kant’s discussions of the geometer’s method of construction – for example, drawing a triangle according to strict mathematical rules. The result is a physical triangle drawn on paper, but what matters about the rigorously constructed triangle is its conformity to the ideal version of itself, as enacted through the strict process of constructing it. Here, thought Kant, was a clue as to how something concrete – such as a set of contingent empirical observations – could retain a connection to the type of necessity held by logical and mathematical forms. In Metaphysical Foundations of Natural Science, Kant asks what basis there can be for treating empirically observed relationships in nature as conforming to laws.Footnote ¹¹ Kant wants to give “sense and meaning” to the a priori forms already established.Footnote ¹² If a priori status comes only from the form of experience and not its matter, we cannot say the laws apply to the world. But physics, as Kant reiterates in the Opus Postumum, is concerned with matter. The very idea of a system of matter, or laws pertaining to matter, is on first look paradoxical for Kant, based on the concepts he has given himself to work with.

We might say this is a uniquely Kantian problem that need not concern most of us, especially given the contemporary consensus in philosophy of science that nature has no a priori laws. For anyone with a more pragmatist approach to science, or certainly for a fictionalist view like Hans Vaihinger’s, the question of the a priori validity of scientific concepts does not arise. While we may not be sympathetic to Kant’s problem, I think his underlying motivation is one that matters more widely. Kant wants to ensure that all abstract concepts, laws, and principles have a connection to experience that is appropriate for the kind of thing they are. Current Kantian accounts of thought experiment solve this problem in different ways. For philosophy of science, Marco Buzzoni argues that the requirement for intuitive fulfillment can be met through operationality. That is, theories – and also thought experiments in the natural sciences – must be empirically testable, even if only in principle. (I discuss Buzzoni’s view in detail in later chapters.) In that way, Buzzoni provides a plausible explanation for how scientific theories relate to empirical observations, as well as a possible role for thought experiments in the sciences.

As Buzzoni notes, however, requiring empirical enactment is not very useful for philosophical thought experiments. As he says, “an actual empirical replication of Searle’s TE, even if it confirmed the empirical intuition on which it is based, would not lead to any interesting philosophical conclusions.”Footnote ¹³ Wiltsche suggests that in narrative contexts (including literary works), we more often aim for “existential understanding” instead of physical understanding.Footnote ¹⁴

So what can a Kantian view offer as an account of thought experiment in philosophy, where there is nothing strictly “operational” other than the act of thinking itself? Buzzoni does not offer us much to go on here, as he wants to maintain that there is no “difference in kind between the particular methods of science and philosophy,” though he acknowledges a difference in subject matter and direction of inquiry.Footnote ¹⁵ He also states that philosophical thought experiments must be performed by each individual, whose task is to “retrace, reconstruct, re-appropriate and evaluate in the first person the steps that led to that conclusion being asserted, or, in other words, the reasons why it should be accepted.”Footnote ¹⁶ In other words, for philosophy, the “performance” of the experiment will have to do with the individual’s own appropriation and re-enactment in thought.

The emphasis on individual appropriation will sound familiar to readers of Kierkegaard. In Kierkegaard’s philosophy, we find explanations of thought experiment that – while presumably not influential for the historical development of the concept – are nonetheless in line with Ørsted’s view and its Kantian beginnings, as well with Buzzoni’s own emphasis on appropriation. And since Kierkegaard was already using the term Tankeexperiment well before Mach (presumably having heard it from Ørsted, his contemporary and friend), it is worth taking a more careful look at the role that Kierkegaard believes thought experiments play in such individual appropriation.

Tankeexperiment in Kierkegaard

Kierkegaard uses the term Tankeexperiment (or Tanke-Experiment) across his authorship and in his journals. He also uses the term “psychological experiment” [psychologiske Experiment], by which he means something more like a thought experiment than psychological experiments in the contemporary sense. The term Experiment itself was new to Danish in the early 1800s, and since Kierkegaard often uses it in a way that implies imagination or fiction, Howard and Edna Hong translate the bare term Experiment as “imaginary construction.” Experiment in this sense, together with its more explicitly mental applications (Tanke-Experiment and psychologiske Experiment), is a recurring method in Kierkegaard’s work as well as a topic of reflection. He contrasts “imaginary construction” [Experiment] as a method with more standard philosophical methods like argumentation and observation. One fictional persona (i.e., pseudonym) Johannes Climacus concludes, “This form won my complete approval” (CUP1, 264/SKS 7, 239).Footnote ¹⁷ While Kierkegaard at times raises problems for imaginary construction as a method, it is also something he relies on throughout his authorship, across different signed and unsigned (pseudonymous) works.

In this book, I will show how Ørsted and Kierkegaard extend Kant’s project of clarifying the kinds of cognition appropriate to different domains. Both Ørsted and Kierkegaard think that thought experiments provide a source of cognitions, in carefully qualified ways. Since this is a method Ørsted attributes to Kant (although Kant did not use the term), it is reasonable to recognize Ørsted as having put forward an early Kantian account of thought experiments.Footnote ¹⁸ Fehige and Stuart propose Lichtenberg and Novalis as continuing that Kantian tradition with respect to thought experiments, though without adopting all of Kant’s philosophy.Footnote ¹⁹ Fehige and Stuart call them both “philosopher-scientists,” which I think is fitting, as they interweave the notion of experimentation in natural science with a broader conception of what Lichtenberg calls “experiments with thoughts and ideas.”Footnote ²⁰

Kierkegaard read Lichtenberg carefully and mentions him in overwhelmingly positive terms. While he does not mention the method of thought experiment explicitly in relation to him, he appreciates Lichtenberg’s distinction between thinking for oneself and parroting inherited knowledge. He finds Lichtenberg’s concerns so much in convergence with his own that he becomes almost rhapsodic in a journal entry: “Thank you, Lichtenberg, Thanks! For saying there is nothing more tiresome than talking with a so-called man of letters in science who hasn’t done any thinking himself yet knows 1000 historic-literary particulars … Thanks for this voice in the wilderness, thanks for this thirst-slaker; like the cry of a wild bird in the stillness of the night, it sets one’s whole fantasy in motion” (KJN 1, 222/DD:29/SKS 17, 231).Footnote ²¹ The points on which Kierkegaard refers to Lichtenberg are on the difference between action and calculation (LD 269/SKS 28, 408), and the importance of what we contribute to experience (“Such works are mirrors: when an ape looks in, no apostle can look out”).Footnote ²²

Although Kierkegaard uses imaginative scenarios regularly and even reflects on their use as a method, histories of the concept of thought experiment do not mention him.Footnote ²³ One likely reason for the omission is that the standard translations of Kierkegaard’s writings by Howard V. Hong and Edna H. Hong translate the Danish word Experiment as “imaginary construction” and Tankeexperiment as “imaginary construction in thought.” This decision was well considered, and it does have some advantages (see Chapter 10 of this book for a full discussion).Footnote ²⁴ I think there are at least three benefits to the unique translation. First, their translation prevents the casual reader from automatically projecting our contemporary use of thought experiments (e.g., Gettier problems and zombies) onto Kierkegaard. As Hong and Hong emphasize, the term Experiment is relatively rare in Danish in the 1800s, and Tanke-Experiment is even more rare (used almost exclusively by Ørsted). We should be open to idiosyncratic uses of Experiment and Tankeexperiment by Kierkegaard, and the unusual English phrase “imaginary construction” alerts readers to that peculiarity. Second, “imaginary construction” is neutral toward whether the mental activity is conducive to genuine thinking or not. In fact, Kierkegaard uses the term ambiguously: sometimes imaginary construction is a way of avoiding an issue, while in other cases (or so I argue) imaginary constructions facilitate genuine cognitions. But we should avoid building any implication of success or effectiveness into Kierkegaard’s term. Third, and particularly fortunate in the context of this project, the term “construction” fortuitously invokes the Kantian context of construction in geometry, to which Ørsted appeals in introducing “thought experiment” as a method. As I will argue in this book, I think Kierkegaard does mean something like Kantian construction by Experiment. The translation of Experiment as “imaginary construction” overall points readers in a useful direction.

However, a significant disadvantage is the loss of continuity with the concept of Tankeexperiment in Ørsted and so to the wider history of thought experiment and experiment as a method. Currently, there is little cross-pollination between work in epistemology that examines the use of thought experiments in philosophy (such as Williamson, Chudnoff, and Bengson) and work focused primarily on their use in math and science (such as Brown, Norton, Nersessian, Elgin). This book restores that connection by showing substantial similarities, beyond the use of a common term, between Ørsted’s appeals to thought experiment to link mathematical reasoning and empirical observation and Kierkegaard’s use in “unscientific” [Uvidenskabelig] contexts. By further situating their work in relation to later developments and applications, I show that Ørsted’s and Kierkegaard’s concept of Tankeexperiment not only remains plausible today but solves some problems in contemporary debates.

Kierkegaard and Ørsted

Given that the term Tankeexperiment was seldom used in the early to mid-1800s, it seems likely that Kierkegaard learned the term from Ørsted.Footnote ²⁵ Ørsted was deeply influenced by Kant. As a student at the University of Copenhagen, it was standard to study science through Kant’s philosophy. Physics and chemistry were offered primarily in the Faculty of Medicine, where he later pursued further studies.Footnote ²⁶ Kant’s philosophy fell out of favor among scientists in the following years, which often made it difficult for Ørsted to explain his reasoning to fellow scientists.Footnote ²⁷ While firmly committed to empirical experimentation and emerging modern scientific practices, Ørsted was also occupied with human questions of beauty, goodness, and the divine. For Ørsted, experiment in science was not just a way of acquiring empirical evidence but a way of filling one’s mind with the best material for thinking and living well. There is a danger that one’s mind can be taken up by “empty abstractions” without “the experiences from which these maxims are taken” (SN, 226/AN 2, 111). A primary function of real experiments, proposed Ørsted, is not to acquire new knowledge for a scientific community but to address the personal uncertainty of individual learners (SN, 348-49/SES 3, 95-96). This is a problem and solution that Ørsted inherits from Kant, as I have emphasized. Thought experiments likewise, Ørsted suggests, help solve the problem of empty concepts. This book shows how Ørsted’s concern about emptiness and fulfillment in an individual’s mental life intersects with Kierkegaard’s insistence on seeing for oneself. Both are rooted, as I will show, in Kant’s goal of cognition, or a synthesis between concepts and sensations.

Reexamining this time in the history of philosophy has implications for how we understand thought experiments today. Thomas Kuhn famously asked, “How, then, relying exclusively upon familiar data, can a thought experiment lead to new knowledge or to new understanding of nature?”Footnote ²⁸ The question of how thought experiments provide knowledge, and specifically knowledge in the natural sciences, continues to structure contemporary debates. The question, in other words, is how thought experiments expand what we know. By contrast, the main question for both Ørsted and Kierkegaard is how we, as thinkers, relate to the conceptual and experiential content we already have.

Kant’s question is bilateral: we need not only to fill in concepts with intuitions but also make our observations meaningful through concepts. Science must not be a “mere sensuous” affair but a way of progressing from sensory experience to rational explanations. The scientist seeks laws in nature “with all the powers of his mind, and he does not remain at the mere sensuous point of view” (SN, 287/AN 2, 190). As I have proposed, and will elaborate in later chapters, the problem of joining concepts with experience is solved for Kant through the activity of cognition [Erkenntnis]. The driving philosophical question for Kant is not how to get more knowledge of one kind or another (whether a priori or a posteriori), but how different kinds of knowledge and different faculties in a human person can be related. I propose that the Kantian problem of cognition – and not the question of getting a priori knowledge of nature – guides Ørsted’s discussions of thought experiment. Since thought experiments are planned by a thought experimenter, they are less vulnerable to chaotic, un-lawlike elements than physical experiments; they nevertheless provide more specific content than definitions or proofs, with the aim of expanding and refining concepts. They offer a possible bridge, therefore, between thinking and sensing.

Cognition may, but does not always, lead to knowledge. This book shows how thought experiments are an apparatus for making thought meaningful. It does not answer the epistemological question of whether or how thought experiments also provide justification. I outline some possibilities in the concluding chapter for how they might, but the epistemological question is not the main focus of the book. Part of the work of the book is to show that Kuhn’s epistemological question is not the only one we can (or should) ask about thought experiments. The question of cognition is, I argue, more fundamental.

Kant and Kierkegaard?

Someone skeptical of or unfamiliar with recent work on Kant and Kierkegaard might doubt that an account of thought experiments can be Kantian and Kierkegaardian. Kant’s philosophy emphasizes reason and universality, after all, while Kierkegaard prioritizes subjectivity and the individual (see, e.g., CUP1, 328/SKS 7, 299).Footnote ²⁹ To my mind, and to a number of other recent scholars, the obvious differences between Kant and Kierkegaard (as well as between Hegel and Kierkegaard, and Kierkegaard and Husserl, etc.) make the remaining similarities all the more interesting.Footnote ³⁰ These similarities are not always points of direct influence but rather of conceptual overlap. Fremstedal lists 14 distinct points of overlap between Kant and Kierkegaard, ranging from those broadly shared within the Augustinian tradition to more idiosyncratically Kantian themes (such as the relation of freedom to anxiety).Footnote ³¹ “In any case,” he concludes, “it seems clear that the framework and meaning of Kierkegaard’s thought are in agreement with those of Kant’s thought,” comparing especially Kant’s doctrine of radical evil and Kierkegaard’s discussions of guilt and sin. Karl Verstrynge agrees that the idea of God as a regulative concept links the two thinkers, despite the differences.Footnote ³² Of special relevance for this book, he also notes that imagination as the “capacity instar omnium” for constituting the self “plays a decisive role in the willing a person has,” and therefore for directing the will toward its regulative goal.Footnote ³³ While differences remain (such as the relation between ethics and religion), so do a number of similarities.Footnote ³⁴

On the broad question of Kant’s influence on Kierkegaard and the extent of their similarities, Kierkegaard’s close connection with the Ørsted brothers confirms that he remained in conversation with at least these two Danish Kantians in the 1840s while doing the bulk of his writing.Footnote ³⁵ We also know that Kierkegaard attended Martensen’s lectures on Kant (see Kierkegaard’s notes, “Lectures on the Introduction to Speculative Dogmatics” in the Winter Semester 1837–1838). In his notes on the ninth lecture, dated December 21 [1837], Kierkegaard mentions “the well-known example of the 100 thalers,” a Kantian example referenced often by Marco Buzzoni.Footnote ³⁶ In the lecture, Martensen (or Kierkegaard in his reflections on the lecture) distinguishes thought from being: for the fictional thalers, “I may certainly think of them, but it does not at all thereby follow that they exist” (KJN 3, 139/SKS 19, 139/NB4:11). The discussion occurs in the context of a proof for God’s existence challenged by Kant. The fact that we can think the concept of God, or that the universe would be unthinkable without God, does not prove that God really exists. Nevertheless, Kant argues we can conceive of the idea of God regulatively, “as an idea that the hum[an] being ought to realize in the whole of his life, but that he could never attain” (KJN 19, 139/SKS 19, 140/NB4:11). Yet Kierkegaard, like Kant, does not treat that limitation as the final word. Like Kant, he wonders about the extent to which regulative ideas become true, or attain existence, just in being thought. In notes on the eighth lecture, Kierkegaard writes: “Yet here K[ant] was basically not idealist enough … to the same degree that I think freedom—and with the same energy that I think it—I am also free” (KJN 3, 137/SKS 19, 137/NB4:10). The implication here is significant for an account of thought experiments in philosophy. Whether the view is Martensen’s or Kierkegaard’s gloss on it, the suggestion is that there are some kinds of ideas that are so essentially inward that merely thinking them in the right way makes them actual. While acknowledging their differences, scholars widely agree that for both Kant and Kierkegaard the idea of God is regulative.Footnote ³⁷ They further agree this regulative concept needs to connect in some further connection with existence, perhaps through the use of symbols.Footnote ³⁸ I consider the role of symbol and analogy as a possible source of intuition with respect to regulative ideas later in the book. Whether or not analogy works in the way Kant hopes, there is evidence that Kant’s and Kierkegaard’s philosophies overlap on key topics. One of the places of significant overlap identified by Kierkegaard scholars is on the concept of regulative ideas and the need for (but difficulty of) fulfilling nonempirical concepts in experience. These points of agreement are the center of the Kantian-Kierkegaardian view of thought experiments I propose in this book.

The Defining Features of Thought Experiment

In framing thought experiments as a solution to Kant’s problem of cognition, some essential characteristics of thought experiments come into clearer focus. First, Ørsted describes all experimentation as a method of variation. He means that experiments begin with a representation of the world as it has been observed and then make changes in order to see how different features matter. Empirical experiments make physical changes and observe the visible results. Thought experiments vary by imagination only. For Ørsted, the ability to introduce new variations that are freely chosen by the experimenter is what makes thought experiments properly “experimental.” The presence or absence of empirical data in the result is not, according to him, definitive of what it means to be an experiment.

Second, because the experimenter’s freedom is an essential component, it matters to Ørsted that the variations in thought experiments are deliberately designed. He reiterates Kant’s distinction between everyday observation and scientific experiment as marked by a shift from passivity to autonomy. If experimentation were primarily a method of acquiring new knowledge (i.e., new data), it would not essentially depend on the experimenter’s freedom. The autonomous element is the synthesis between concepts and observations, such that principles (and for physical experiments, hypotheses) govern what data we bother to collect. So while thought experiments do not result in new sensory data, since they are deliberate Ørsted thinks they exercise the same autonomy as empirical experiments.

The last core feature of thought experiments we can take from Ørsted’s discussions is the focus on cognition, as I have already mentioned. He went as far as to wonder whether all the geometry that is already established according to proofs could be reestablished through thought experiments (SN, 462/GN, 468). Thought experiments would not provide new knowledge in this case but would serve to make that knowledge genuine – that is, actually performed by the individual learner. Such thought experiments would not be redundant, he thought, because they would provide the researcher with a new kind of direct access to the ideas that proofs present only procedurally. Deductive proofs are potentially passed on merely mechanically, as they can be learned by rote. Thought experiments, by contrast, could provide immediate access to the necessity of a principle or relationship. He describes this direct access as a “far brighter and more immediate insight into the actual source of each truth, and a much closer amalgamation between it and natural philosophy would thus be gained than formerly existed” (SN, 462/GN, 468). In other words, thought experiments provide a distinct, more direct reason for thinking that something is true, and in such a way as to reduce the gap between mathematical proofs and physical observations. He does not imply (as I discuss later in more detail) that it is possible to do physics a priori.

In setting forward these key features of Ørsted’s view, the book also highlights their similarities with Mach’s descriptions of experiment and thought experiment. The continuity between the two thinkers’ views is strong enough, I suggest, to make it likely that Mach was familiar with and influenced by Ørsted’s account. If so, then Ørsted’s contributions in fact had a substantial causal influence on our modern understanding of thought experiment as a method, even if most readers of Mach’s essay were unaware of Ørsted’s prior work. However, even if there is no direct historical influence, there is no basis given their similarities to separate Mach’s view definitively from Ørsted’s. Both thinkers characterize thought experiments as (1) variations that are (2) free and (3) genuine as a solution to the Kantian problem of empty abstractions and meaningless sensations.

In clarifying Kierkegaard’s view of thought experiments, I show how he retains these three core features of Ørsted’s view. Like Ørsted, he takes the value of thought experiments to be their ability to identify non-sensory continuities that underlie visible changes. Kierkegaard similarly teaches his readers to track essential wholes that survive outward variations. Such abilities matter when identifying selves, recognizing acts of love, and appreciating the changelessness of the divine. Like Ørsted, Kierkegaard argues that recognizing underlying wholes requires activity on the part of the learner. And just as for Ørsted, the genuineness of that recognition matters. It must be enacted as a synthesis of thought with concrete content, not entertained as merely thought, any more than it can be merely an observation in the sensory world. In these ways, understanding the core features of Ørsted’s Kantian account of thought experiment provides new insights into Kierkegaard’s use of imaginary constructions.

Outline of the Book

The book is divided into three main parts. Part I defines the core Kantian notion of cognition [Erkenntnis], examines similarities between Ørsted’s account and Mach’s, and situates the view within contemporary debates on thought experiment. Recent research includes work in philosophy of science, where the debate between James Brown’s Platonism and John Norton’s empiricism has long set the stage, as well as on the epistemology of intuitions, where leading voices include Timothy Williamson, Ernest Sosa, and Elijah Chudnoff. There is currently little overlap between these two contemporary philosophical subfields. This book shows how Ørsted’s and Kierkegaard’s Kantian accounts avoid problems in both sets of views. Additionally, Part I provides the first focused study of the 1811–1897 time period that Yiftach Fehige has called the “inauguration” of the concept of thought experiment.Footnote ³⁹

For Kant, the most immediate goal of cognition is not certainty but shared attention: cognition enables thinkers to have ideas in common, whether true or false. Chapter 1 of the book introduces core Kantian concepts as well as the main claim of the book: Thought experiments provide a tool for cognition. Chapter 2 situates this main claim in relation to contemporary views of thought experiment. Chapter 3 engages rigorously with Ørsted’s writings on thought experiments. Chapters 4 and 5 bring the details of Ørsted’s Kantian account into contemporary debates, first in philosophy of science (Chapter 4) and then for epistemology (Chapter 5). I show how most views in philosophy of science are committed to empirical approaches, and the work on intellectual intuitions in epistemology is susceptible to Kantian critiques of rationalism. These chapters show how Ørsted’s development of Kant’s philosophy offers a way out of the dilemma posed by empiricism and rationalism in contemporary debates.Footnote ⁴⁰

Part II of the book examines some core Kantian ideas to further elucidate the view outlined by Ørsted. In these chapters, I go beyond what Kant and Ørsted explicitly say about thought experiments to develop an original but historically rooted explanation of what thought experiments do. Chapter 6 outlines Kant’s different proposals for achieving cognition outside sensory perception, including (1) through practical action, (2) by analogy, and (3) through construction. Chapter 7 explores why cognition matters even in cases like math and geometry where we already have knowledge and constructing a model seems superfluous. It also clarifies how the core characteristics of variation, freedom, and genuineness help achieve cognition in thought experiments. The rest of Part II examines implications of the claim that thought experiments are an apparatus for cognition. Chapter 8 sets forward the unique advantages of a Kantian approach, including the ability to distinguish between constitutive and regulative concepts. The distinction matters most for the concepts Kant calls “ideas of reason,” such as the universe as a whole and the idea of the self, or personal identity. Chapter 9 shows how a Kantian approach, as modeled by Ørsted, solves the contemporary problem of bizarre cases: How can strange and implausible cases teach us about reality, even though we lack a familiar bank of experiences on which to draw in evaluating them? Kant’s distinctions between constitutive and regulative concepts and between determining and reflecting judgments strengthen existing Kantian accounts and reorient them toward the problem of cognition. Overall, the chapters in Part II show the continuing value of Kant’s philosophy for understanding thought experiments.

Part III turns more directly to Kierkegaard. Chapter 10 shows how Kierkegaard’s discussions of thought experiment as a method incorporate the Kantian insights outlined in Part II. Like Ørsted, Kierkegaard takes thought experiments to be a method of variation, a type of free and active constitution, and a useful means of facilitating genuine thought. Part III examines several works by Kierkegaard, including The Concept of Irony, Either/Or, and Stages on Life’s Way (Chapter 11), Repetition (Chapter 12), and Stages on Life’s Way (Chapter 13), as well as Concluding Unscientific Postscript, Two Ages, and Works of Love (Chapter 14). These chapters also refer occasionally to Kierkegaard’s journals, where he often emphasizes the need for thought to have guidance but also to wander freely. Kierkegaard writes, “No matter how different knights were from scholastics, they had in common that they went on adventures; for thinking also has a way of going on adventures that is just as stimulating, just as noble, just as heaven-sent as that of the knights” (KJN 2, 25/EE:71/SKS 18, 29, translation modified). Across Kierkegaard’s authorship, we find a confidence that thought can be made more concrete, and that such work is valuable for the individual thinker, as Kant and Ørsted claim.

The Kantian strains in Kierkegaard’s philosophy of thought experiment have so far been overlooked because of a standard association of “imaginary construction” (i.e., Experiment) with aesthetics and creativity. Like Kant, however, Kierkegaard wants to make sure that thinking has an anchor (origin) and trajectory (destination) in the concrete world. He finds such connections in slowing down the workings of mental life and embodying them in the thoughts of different pseudonyms. Voicing this need for visibility, Judge William writes: “I require eyes that see in secret, that do not weary of watching, that see the struggle and see the danger. I require ears that hear the workings of thoughts …” (EO 2, 289/SKS 3, 272). While a divine being more obviously has such a view of inner life, perhaps humans can achieve it through effort, with the help of a reliable method. Fictional scenarios, for Kierkegaard, situate the reader in her own life but also, in the other direction, make the abstract world accessible. Kierkegaard remarks in an early journal, “Abstract concepts are as invisible as a straight line, they are only visible when they are made concrete” (KJN 2, 42/EE:127/SKS 18, 46). While thought experiments do not guarantee such fulfillment, Kierkegaard agrees with Ørsted that thought experiments provide a promising method for connecting the form of thought with the matter of sensory observation.