This chapter lays out the ways in which Hans Christian Ørsted (1777–1851) influenced the development of the concept of thought experiment. Ernst Mach (1838–1916) is currently more often credited with laying the foundations of contemporary views, and he is sometimes thought to have been little (if at all) influenced by Ørsted. Against these standard accounts, I will show that Ørsted’s and Mach’s descriptions have key features in common. Both thinkers hold that thought experiments: (1) are a method of variation, (2) require the experimenter’s free activity, and (3) are useful in educational contexts for guiding students to arrive at certain conclusions on their own (i.e., to genuinely appropriate new concepts). The process of variation is guided by the search for invariants, some of which do not directly appear in experience. Since it is important that teachers and students be able to bring the same ideal objects to mind, thought experiments play a key role for both Ørsted and Mach in math education. While Ørsted’s emphasis on the role of thought experiments in math has been proposed as a reason why his descriptions are not relevant for contemporary use of thought experiments, I will show how their role in mathematical thinking – stemming from Kant’s descriptions of the method of construction in geometry – are part of a wider account of thought experiments that encompasses their role in the sciences and also philosophy.

This chapter further situates my Kantian account of thought experiments among competing views. I identify problems for contemporary accounts and contrast epistemological questions (How do thought experiments justify?), which guide most of the current scholarship, with Kant’s emphasis on cognition [Erkenntnis] (What makes concepts meaningful?). I note that metaphilosophical questions on the relationship between conceivability and possibility are not relevant for thought experiments if they are an apparatus for cognition, which is neutral toward the truth or actuality of the objects of cognition. Contemporary accounts that begin with Kuhn’s epistemological question differ on what the basis of knowledge might be. Leading approaches appeal to logic, stored knowledge, and intellectual intuition. I will briefly sketch here some of the basic approaches.

This chapter develops and analyzes how thought experiments connect thinking with actuality. Superficially, imaginary constructions are mere possibilities that diverge from actuality. However, Kierkegaard also characterizes thought experiments as a kind of experience, providing concrete, fulfilling content for an otherwise empty concept – that is, providing what Kant calls a “synthesis” between thought and experience. Two Ages shows how the work of synthesis can begin from observations and move toward understanding or from understanding toward fulfillment in experience. In Works of Love, I propose, we find material for a basic taxonomy of thought experiments that distinguishes them by whether the thought experiment offers cognition of (a) objects or (b) concepts and whether it (a) proceeds from existing concepts or (b) guides the reader in gaining new ones. This taxonomy mirrors Kant’s distinctions between constitutive and regulative concepts and determining and reflecting judgments. It also anticipates the proposals of recent rationalist accounts of intuition that thought experiments provide nonsensory presentations.

The book’s introduction explains the history of thought experiments in philosophy. It also describes Hans Christian Ørsted’s interactions with Kierkegaard and his influence on Kierkegaard’s concept of Tankeexperiment. The introduction outlines the ways in which thought experiments make thoughts meaningful by providing immediate presentations.

In two of Kierkegaard’s earliest works, The Concept of Irony and Either/Or, imaginary construction (i.e., thought experiment, or Experiment) is often characterized negatively. However, the three core features of thought experiment shared by Ørsted and Mach also begin to emerge, laying foundations for a more positive view in other works. Kierkegaard’s characterizations of thought experiment indicate that imaginary construction guides mental action. This focus contrasts with the standard emphasis in Kierkegaard scholarship on thought experiment as supplying the concreteness of (empirical) actuality. In The Concept of Irony, Kierkegaard critiques irony as a retreat from reality but also shows it can be used to achieve new kinds of wholeheartedness and unity. In this chapter, I will argue that thought experiments can similarly lead the experimenter away from reality but, like irony, may also be a useful tool for self-development.

This chapter distinguishes the work of thought experiments in exemplifying concepts from their role in aiding cognition of regulative ideas. Contemporary interest in thought experiments as a “method of cases” treats thought experiments as providing instances or exemplars. For regulative ideas as Kant understands them, however, no observable instances or exemplars are possible. Nevertheless, thought experiments can direct attention toward regulative ideas negatively (by distinguishing them from what is observable) or positively by indicating a direction for extrapolation or ongoing inquiry. These positive uses are forms of cognition. The distinction between regulative and constitutive concepts matters for thought experiments that deal with regulative ideas such as the self – for example, for thought experiments about personal identity, where a number of objections to the use of thought experiments have been raised. I argue in this chapter that some of these objections can be answered by distinguishing regulative from constitutive concepts.

Kant thinks it is possible to achieve nonperceptual cognition in three ways: (1) through practical action, (2) by analogy, and (3) through construction. The type of cognition available depends on the kind of object or concept being cognized. The fact that cognition of nonperceptual objects is possible in some cases opens the way for thought experiments to provide cognition in ways that go beyond providing fictional examples and exemplifications. In this chapter, I describe these other possibilities for cognition and show how they are at work in different kinds of thought experiments in philosophy.

This chapter proposes that thought experiments are a cognitive apparatus and situates this view among contemporary accounts of thought experiment. I set forward the project of the book, which is to (1) propose a new account of thought experiments as a method and (2) trace the historical foundations of the term and concept of “thought experiment” from Kant through Ørsted to Kierkegaard. I define “cognition” [Erkenntnis] for Kant as a synthesis of concepts with intuitions and propose that Kierkegaard, like Kant and Ørsted, views thought experiments as useful for achieving cognitions. I introduce the term Tanke-experiment in Kierkegaard and suggest why it has been little emphasized by Kierkegaard scholars and remains widely unacknowledged in contemporary descriptions of the history of thought experiment.

This chapter analyzes Stages on Life’s Way as an extended thought experiment. Though it has some similarities with a literary work of art and is sometimes called a novel, I distinguish extended thought experiment narratives like Stages from literary novels. I will show how Stages, like Repetition, embodies and develops Ørsted’s core elements of variation, active constitution, and the pursuit of genuine thought. I will also contrast Stages as a “psychological experiment” with the field of empirical psychology emerging in the 1800s. Against increasing interest in empirical observation, Kierkegaard’s thought experiments direct attention to what is not outwardly observable.

The conclusion outlines key points in the book. On a Kantian-Kierkegaardian account, thought experiments lead to presentations that justify calling a thought experiment an experience, though not an empirical or sensory one. Without a sensible object, we can nevertheless have givenness, or presence, but not existence. Cognition, for that reason, is not necessarily truthful: although object-directed, cognition on its own does not justify belief in any particular object’s existence. My conclusion that thought experiments provide cognition rather than knowledge echoes Michael Stuart’s claim that thought experiments increase understanding. Cognition is a common basis for knowledge in Kant, but it is not the same thing as knowledge. The conclusion also draws implications for how we understand faith (religious belief) in Kierkegaard and how thought experiments make sense of the complexity of reality.

Rationalist accounts of thought experiment in epistemology offer an alternative to the more predominantly empiricist approaches in philosophy of science. In this chapter, I will pose a Kantian critique of recent rationalist accounts of intellectual intuition. Some epistemologists have recently argued that intellectual intuitions can provide prima facie justification for judgments. In this chapter, I highlight some promising elements of recent rationalist accounts, especially the proposal that there can be nonsensory presentations analogous to empirical perceptions. If they are right, then thought experiments can provide new experiential content even without empirical confirmation. However, I also draw attention to Kant’s objections to the possibility of purely intellectual intuitions.

Kierkegaard and Ørsted were not just contemporaries but personally knew each other. In this chapter, I argue that Kierkegaard probably learned the term Tankeexperiment from Ørsted. This chapter contextualizes Kierkegaard’s use of “imaginary construction” (Experiment) in his work as a whole, including his well-known uses of paradoxes. I will show how the core elements of Ørsted’s account – thought experiment as a method of variation, the need for free and active constitution, and the use of thought experiments for facilitating genuine thought – are echoed in Kierkegaard’s discussions. Along the way, I will describe some decisions on how to translate Experiment and Tankeexperiment that are unfortunate in some ways and fortuitous in others, as I will explain. In these ways, Kierkegaard indirectly takes up Kant’s proposal that “construction” (i.e., Experiment in Danish) is a means of achieving cognition.

Kierkegaard’s book Repetition, along with his descriptions of the book in Concluding Unscientific Postscript, offer a more positive characterization of thought experiments than we find in earlier works. This chapter argues that imaginary construction has a positive aim of identifying underlying continuities. I identify some similarities between Ørsted’s pursuit of invariants and Kierkegaard’s. One new addition in Kierkegaard’s discussions is the role of exceptions. An exception is a case that falls outside a rule without breaking it. Exceptions can neither establish a rule nor refute its necessity, but they can turn attention to the principles and their limits as well as further determine their scope and content. A further similarity between Kierkegaard’s work and Ørsted’s is the fact that variation must be active and free.

This chapter explains why cognition (Erkenntnis) is its own kind of cognitive good, apart from questions of justification. I argue against reducing the work of thought experiments to their epistemological results, such as their potential to provide prima facie justification. As an apparatus for cognition, a thought experiment enacts the three core elements of Ørsted’s Kantian account: (1) it is a tool for variation; (2) it proceeds from concepts, and (3) its goal is the genuine activation or reactivation of mental processes. Cognition has two components: givenness and thought. I will show in this chapter how givenness and thought are both achieved through thought experiments.