## 1. Important Characteristics of Thought Experiments



Theorizing about thought experiments usually turns on the details or
the patterns of specific cases. Familiarity with a wide range of
examples is crucial for commentators, and the list is very long (see,
e.g., Stuart et al. 2018, pp. 558-560) We will provide a few
here. One of the most beautiful early instances (found in Lucretius,
*De Rerum Natura* 1.951-987; see Bailey 1950, pp.
58-59) attempts to show that space is infinite: if there is a
purported boundary to the universe, we can toss a spear at it. If the
spear flies through, it isn't a boundary after all; if the spear
bounces back, then there must be something beyond the supposed edge of
space, a cosmic wall that stopped the spear, a wall that is itself in
space. Either way, there is no edge of the universe; thus, space must
be infinite.



This example nicely illustrates many of the most common features of
what it means to engage in the conduct of thought experiments: we
visualize some situation that we have set up in the imagination; we
let it run or we carry out an operation; we see what happens; finally,
we draw a conclusion. The example also illustrates the fallibility of
thought experiments. Since the time of Lucretius, we've learned
how to conceptualize space so that it could be both finite and
unbounded. Imagine a circle, which is a one-dimensional space. As we
move around, there is no edge, but it is nevertheless finite. The
universe might be a three-dimensional version of this topology. It is,
therefore, true that we must try to be mindful of unexpected
limitations due to "physical scale effects" (Klee 2008),
or other such things, when imagining counterfactual scenarios.



![A person is about to throw a spear at a wall of stones floating in the clouds. Above the clouds are two winged figures holding a banner that reads 'Welcome to the edge of the Universe'.](figure1.jpg)

Figure 1. "Welcome to the edge of
the Universe"




Often a real experiment that is meant to be the analogue of a thought
experiment is impossible to be carried out as such due to physical,
technological, ethical, or financial limitations (see, e.g., Sorensen
1992, pp. 200-202); but physical unrealizability needn't
be a defining condition of thought experiments. Rather, the main point
is that we seem able to get a grip on nature just by thinking, and
therein lies the great interest for philosophy. That was the position
of Ernst Mach (see Mach, 1897 and 1905; for a most instructive
assessments of his views see Kuhne 2006, pp. 165-202, and
Sorensen 1992, pp. 51-75). Thought experiments are on a spectrum
of different kinds of experiments. They allow us to tap into a great
store of "instinctive knowledge" picked up from past
experience. We will get back to Mach's theory further down. His
account of thought experiments remains one of the major theories of
how thought experiments work. One of Mach's favourite examples
is due to Simon Stevin (see Mach, 1883, pp. 48-58). When a chain
is draped over a double frictionless plane, as in Fig. 2a, how will it
move? Add some links as in Fig. 2b. Now it is obvious. The initial
setup must have been in static equilibrium. Otherwise, we would have a
perpetual motion machine; and according to our experience-based
"instinctive knowledge," says Mach, this is impossible. We
do not have to perform the experiment in the real world, which we
could not do, anyway, since it would require a perfectly frictionless
plane. Nevertheless the outcome seems compelling.




![A black dashed chain rests on top of the two top sides of a triangle.](figure2a.gif)

(a)




![A black dashed chain rests on top of the two top sides of a triangle; the dashed chain is extended into a loop so that it rests, dangling below the bottom of the triangle.](figure2b.gif)

(b)




Figure 2. "How will it
move?"




Judith Thomson provided one of the most striking and effective thought
experiments in the moral realm (see Thomson 1971). Her example is
aimed at a popular anti-abortion argument that goes something like
this: A fetus is an innocent person. All innocent persons have a right
to life. Abortion results in the death of a fetus. Therefore, abortion
is morally wrong. In her thought experiment, Thomson asks you to
imagine a famous violinist falling into a coma. The society of music
lovers determines from medical records that you and you alone can save
the violinist's life by being hooked up to him for nine months.
The music lovers break into your home while you are asleep and hook
the unconscious (and unknowing, hence innocent) violinist to you. You
may want to unhook him, but you are then faced with the following
argument put forward by the music lovers: The violinist is an innocent
person. All innocent persons have a right to life. Unhooking him will
result in his death. Therefore, unhooking him is morally wrong.
However, the argument, even though it has the same structure as the
anti-abortion argument, does not seem convincing in this case. You
would be very generous to remain attached for nine months, but you are
not morally obligated to do so. The parallel with the abortion case is
evident. Thomson's thought experiment is effective in
distinguishing two concepts that had previously been run together:
"right to life" and "right to what is needed to
sustain life." The fetus and the violinist might each have the
former, but it is not evident that either has the latter. The upshot
is that even if the fetus has a right to life (which Thomson does not
believe but allows for the sake of the argument), it may still be
morally permissible to abort. Those opposed to Thomson's view
have two options. They can either dismiss her thought experiment as a
useless fiction. In fact, thought experiments as a method in ethics
have their critics (see, e.g., Dancy 1985). Alternatively, they can
provide a different version of the same scenario to challenge the
conclusion. It is a very intriguing feature of thought experiments
that they can be "rethought" (see Bokulich 2001). Real
experiments are frequently open to reinterpretation, too. In this
respect there does not seem to be a principled difference between the
two classes of experiments.



Like arguments, thought experiments can be criticized in different
ways. Perhaps the set up is faulty; perhaps the conclusions drawn from
the thought experiment are not justified. Similar criticisms can arise
in real experiments. Counter thought experiments are perhaps another
form of criticism. They do not target the premises or conclusions
involved in a particular thought experiment but question the
phenomenon, i.e. the non-propositional heart of an imagined scenario
(see Brown 2007). For example, Daniel Dennett is convinced that Frank
Jackson's Mary thought experiment is poor evidence to oppose
physicalism in philosophy of mind. In Jackson's version, Mary,
who knows everything physics and the neurosciences can possibly know
about colours but grew up in a colourless environment (seeing only
black, white and grey things), allegedly learns something new when she
sees a red tomato for the first time. Now she knows what it is like to
experience red. This is an argument for qualia as something over and
above the physical. Instead of a red tomato, Dennett, in his version
of the thought experiment, presents Mary with a bright blue banana. In
his version of the story (which seems just as plausible as
Jackson's), Mary balks and says she is being tricked, since she
knows that bananas are yellow, and this, says Mary, is a consequence
of knowing everything physical about colour perception. Mary does not
learn anything new when she sees coloured objects for the first time,
so there is no case against physicalism after all. Jackson's
initial thought experiment was very persuasive, but Dennett's
seems equally so, thus, undermining Jackson's argument, although
there is greater resistence to the conclusion of the latter than the
former! Dennett complains a great deal about the ongoing
"Mariology", as he calls the continuing acceptance of
Jackson's thought experiment as a poweful case against
physicalism.



Clearly, thought experiments are characterized by an intriguing
plasticity, and this raises the interesting question of what it is
that preserves the identity of a thought experiment. Replacing a red
tomato with a blue banana might still leave us with the same thought
experiment--slightly revised. But, at what point do we get
a new thought experiment? This is not merely a question about
conceptual vagueness. It helps to facilitate a discussion of the
intuitively most plausible view about the cognitive efficacy of
thought experiments, according to which this power depends on their
being arguments, in a fairly strict sense of argument. John D. Norton
holds such a view, which will be discussed below. In light of cases
where the discussion of one and the same thought experiment played an
important role in settling a dispute, the following problem arises:
how can one and the same thought experiment support opposing views
about a particular matter if the arguments that correspond to the
different versions of the thought experiment that were entertained by
the disputing parties are significantly different? The dilemma is: we
could say that if there is more than one argument then there is more
than one thought experiment involved in the dispute. But if that is
true then the disputing parties simply talked past each other. One
party presented an argument that the other party ignored while
presenting their own. Alternatively, we can say that one thought
experiment can correspond to many different arguments. But, if that is
true then it becomes unclear in what non-trivial sense thought
experiments are supposed to be identical with arguments (see Bishop
1999, and the response by Norton 2004, 63-64).



The plasticity of thought experiments coheres with another feature of
thought experiments, namely that they seem to have "evidential
significance only historically and locally, i.e., when and where
premises that attribute evidential significance to it [...] are
endorsed" (McAllister 1996, p. 248).


## 2. Taxonomies of Thought Experiments



Many taxonomies can be found in the literature. They are not mutually
exclusive. We will present three of them. The first follows the type
of purpose thought experiments serve. A very rudimentary version of it
can be found in Mach (1897 and 1905). Such a classification makes
sense, because an "imaginary experiment should be judged on its
specific purpose" (Krimsky 1973, p. 331). Thought experiments
are conducted for diverse reasons (see, e.g., DeMay 2006; Sorensen
1992, pp. 7-15), and this in a variety of areas, including
economics (see, e.g., Herfeld 2019; Thoma 2016), education (Helm and
Gilbert 1985; Helm et al. 1985, Klassen 2006; Sriraman 2006; Stonier
1990), history (see, e.g., Maar 2014; Reiss 2009), literature (see,
e.g., Davies 2007; Elgin 2004), mathematics (see, e.g. Brown 1991
[2011], pp. 90-97; Glas 1999), morality (see, e.g., Hauerwas
1996; Wilson 2016), as well as the natural sciences (see Krimsky
1973), the socio-political realm (see, e.g. Roberts 1993: Thaler
2016), and theology (see, e.g., Gregersen 2014; Fehige 2024). Thought
experiments may be used to entertain. This is probably true of short
stories or novels which some argue qualify as thought experiments if
certain conditions apply (see, e.g., Davenport 1983). Some thought
experiments fulfil a specific function within a theory (see Borsboom
et al. 2002). Others are executed because it is impossible to run the
experimental scenario in the real world (see, e.g., Sorensen 1992, pp.
200-202). Sometimes thought experiments help to illustrate and
clarify very abstract states of affairs, thereby accelerating the
process of understanding (see Behmel 2001). Again others serve as
examples in conceptual analysis (see Cohnitz 2006). And, then there
are those that matter in the process of theory discovery (Praem and
Steglich-Peterson 2015). The thought experiments that have
received most of the attention are taken to provide evidence for or
against a theory, putting them on a par with real-world experiments
(see, e.g., Gendler 2004). The different ways to use thought
experiments, of course, do not exclude one another. Most obviously,
for example, a thought experiment can both entertain and make a case
against a theory.



A second taxonomy classifies thought experiments in terms of their
logical structure (see Sorensen 1992, pp. 132-166). The idea is
to divide all thought experiments into two types of "alethic
refuters": "Although there are a number of ways to
classify thought experiments, a refutation format scores the most
points when judged by familiarity, specificity, and simplicity.
According to this scheme, thought experiments aim at overturning
statements by disproving one of their modal consequences. Modalities
are operators that are applied to propositions to yield new
propositions. There are deontic modalities (*permissible,
forbidden*), epistemic modalities (*know, believe*), and
alethic modalities (*possible, necessary*). The alethic
modalities are the best-known and more-basic modality.
Hence, we won't miss anything by concentrating on them"
(Sorensen 1992, p. 135). One type of thought experiment "is
designed to refute a statement by showing that something ruled out as
impossible by that statement is really possible after all"
(Sorensen 1992, p. 135). The most discussed examples in the
metaphilosophical discussion on thought experiments is of such a type,
namely the Gettier scenarios (see Grundmann & Horvarth 2014;
Saint-Germier 2019). They are designed to refute the claim that all
knowledge is justified, true belief. They serve as a "necessity
refuter." The other type collects examples of "possibility
refuters". They don't affirm "the possibility of the
thought experiment's content". Instead, they establish
"copossibilities". A wonderful example is the scenario of
an omnipotent God who faces the task of creating a stone too heavy for
that God to lift. It seems God cannot succeed. The notion of divine
omnipotence causes some headache here.



A third taxonomy (see Brown 1991, chapter 2), which has not gone
unchallenged (see Norton 1993b), is more limited than the first two
insofar as it focuses largely on the class of those thought
experiments that are taken to function in theory choice, which is the
use of thought experiments that has been receiving most of the
attention. According to this taxonomy, the main division is
*constructive vs. destructive* and resembles Karl
Popper's distinction between *apologetic* and
*critical* thought experiments. Popper actually distinguishes
between three types of thought experiments: *heuristic* (to
illustrate a theory), *critical* (against a theory) and
*apologetic* (in favour of a theory) (see Popper 1959). His
case in favour of a critical and against an apologetic use of thought
experiments is very limited. He focuses exclusively on quantum physics
and doesn't really say much to address the primary
epistemological challenge presented by the success of critical thought
experiments.



Among *destructive thought experiments*, the following subtypes
can be identified: the simplest of these is to draw out a
contradiction in a theory, thereby refuting it. The first part of
Galileo's famous falling bodies example does this. It shows that
in Aristotle's account, a composite body (cannon ball and musket
ball attached) would have to fall both faster and slower than the
cannon ball alone. A second subtype is constituted by those thought
experiments that aim to show that the theory in question is in
conflict with other beliefs that we hold. Schrodinger's
well-known cat paradox, for instance, does not show that quantum
theory (at least on some interpretations) is internally inconsistent
(see Schrodinger 1935, p. 812; translation: Trimmer 1980, p.
328): "A cat is penned up in a steel chamber, along with the
following diabolical device (which must be secured against direct
interference by the cat): in a Geiger counter there is a tiny bit of
radioactive substance, so small, that perhaps in the course of one
hour one of the atoms decays, but also, with equal probability,
perhaps none; if it happens, the counter tube discharges and through a
relay releases a hammer which shatters a small flask of hydrocyanic
acid. If one has left this entire system to itself for an hour, one
would say that the cat still lives if meanwhile no atom has decayed.
The first atomic decay would have poisoned it. The q-function of the
entire system would express this by having in it the living and the
dead cat (pardon the expression) mixed or smeared out in equal
parts." This thought experiment shows that quantum theory (as
interpreted by Bohr) is in conflict with some very powerful common
sense beliefs we have about macro-sized objects such as
cats--they cannot be both dead and alive in any sense
whatsoever. The bizarreness of superpositions in the atomic world is
worrisome enough, says Schrodinger, but when it implies that same
bizarreness at an everyday level, it is intolerable. There is a third
subtype of negative thought experiments, namely when, in effect, a
central assumption or premise of the thought experiment itself is
undermined. For example, as we have seen above, Thomson showed with
her thought experiment that "right to life" and
"right to what is needed to sustain life" had been run
together. When distinguished, the argument against abortion is
negatively affected.



A fourth sub-type of negative thought experiments are "counter
thought experiments" (see Brown 2007). Norton very usefully
introduces a related idea:
"thought-experiment/anti-thought-experiment pairs" (see
Norton 2004, pp. 45-49). Above, we have already encountered this
subtype in our discussion of Lucretius' spear-thought
experiment, and with Dennett's reply to Jackson's much
discussed Mary the colour scientist thought experiment. Here we would
like to add one more example, namely Mach's counter thought
experiment against absolute space. In his *Principia
Mathematica*, Newton offers a pair of thought experiments as
evidence for absolute space. One is the bucket thought experiment with
water climbing the wall (see Fig. 3), the other is about a pair of
spheres joined by a cord that maintained its tension in otherwise
empty space (see Fig. 4). The explanation for these phenomena, argues
Newton, is absolute space: the bucket and the joined spheres are
rotating with respect to space itself. In response, Mach modifies the
scenario and argues, contra Newton, that the two spheres would move
toward one another thanks to the tension in the cord, and if we
rotated a very thick, massive ring around a stationary bucket, we
would see the water climb the bucket wall. (For further discussion of
Mach's counter thought experiment to Newton's see
Kuhne 2006, pp. 191-202). In short, the point of
Mach's counter thought experiments is to describe the phenomena
of the thought experiments' scenarios differently, that is, to
declare that different things would happen. Mach's counter
thought experiment undermines our confidence in Newton's thought
experiments. Absolute space might be a plausible explanation of the
phenomena in Newton's thought experiments, but now, in light of
Mach's counter thought experiment, we're not so sure of
the phenomena itself and thus of the idea of absolute space.



![Three successive figures: (I) a pail half-full of water is suspended by a rope tied to its handle; (II) arrows show the pail half-full of water is in motion counterclockwise and the water in the pail is in motion clockwise; (III) the surface of the water in the rotating pail is shown higher at the edge of the pail and lower at the center.](figure3.jpg)

Figure 3. Stages in the bucket
experiment




![Two grey spheres joined by a black line. The line is labeled 'tension in cord'.](figure4.jpg)

Figure 4. Two spheres held by a cord in
otherwise empty space




To be effective, counter thought experiments needn't be very
plausible at all. In a court of law a jury would convict provided
guilt is established "beyond a reasonable doubt." A common
defence strategy is to provide an alternative account of the evidence
that has just enough plausibility to put the prosecution's case
into some measure of doubt. That is sufficient to undermine it. A
counter thought experiment need only do that much to be effective, and
in this sense it operates like a "necessity refuter" in
Sorensen's sense.



In addition to destructive ones, there is a second type, the
*constructive thought experiments*. Unsurprisingly, there are
many ways they could provide positive support for a theory. One of
these is to provide a kind of illustration that makes a theory's
claims clear and evident. In such cases thought experiments serve as a
kind of heuristic aid. A result may already be well established, but
the thought experiment can lead to a very satisfying sense of
understanding. In his *Principia Mathematica*, Newton provides
a wonderful example showing how the moon is kept in its orbit in just
the same way as an object falls to the earth (see Ducheyne 2006, pp.
435-437). He illustrates this by means of a cannon shooting a
cannon ball further and further (see Fig. 5). In the limit, the earth
curves away as fast as the ball falls, with the eventual result being
that the cannon ball will return to the spot where it was fired, and,
if not impeded, will go around again and again. This is what the moon
is doing. We could arrive at the same conclusion through calculation.
But Newton's thought experiment provides that elusive sense of
understanding. It's a wonderful example of the "aha
effect" that is typical of many powerful thought
experiments.



![A globe with a hill at the top and a cannon. Lines show a sequence of cannon-ball tracks; each track is longer until a track goes all the way around the globe. There are two larger concentric circles around the globe and the cannon-ball tracks.](figure5.jpg)

Figure 5. "The shot heard around
the world"




Thomson's violinist showed that abortion could be morally
permissible even when the fetus has a right to life. Similarly,
Einstein's elevator showed that light will bend in a
gravitational field, because according to the principle of
equivalence, there is no difference between such a frame of reference
and one that is accelerating in free space; the laws of physics are
the same in all. Suppose then, an observer is inside an elevator
sealed off from the outside so that the observer cannot tell whether
he is in a gravitational field or accelerating. If it were
accelerating, and if a light beam were to enter one side, then, due to
the elevator's motion, the beam would appear to drop or curve
down as it crossed the elevator. Consequently, it would have to do the
same thing if the elevator was in a gravitational field. Therefore,
gravity 'bends' light.



Maxwell's demon showed that entropy could be decreased: The
second law of thermodynamics implies that heat won't pass from a
cold body to a hot one. In classical thermodynamics this law is quite
strict; but in Maxwell's kinetic theory of heat there is a
probability, though extremely small, of such an event happening. Some
thought this a reductio ad absurdum of Maxwell's theory. To show
how it is possible to violate the second law, Maxwell imagined a tiny
creature who controls a door between two chambers. Fast molecules from
the cold box are let into the hot box, and slow molecules from the hot
are allowed into the cold. Thus, there will be an increase in the
average speed in the hot box and a decrease in the average speed of
molecules in the cold. Since, on Maxwell's theory, heat is just
the average speed of the molecules, there has been a flow of heat from
a cold body to a hot one.



Parfit's splitting persons shows that survival is a more
important notion than identity when considering personhood (for a
critical discussion see Gendler 2002a). We say they "show"
such and such, but, "purport to show" might be better,
since some of these thought experiments are quite contentious. What
they have in common is that they aim to establish something positive.
Unlike destructive thought experiments, they are not trying to
demolish an existing theory, though they may do that in passing. To
repeat an important point: in principle, given the fact that thought
experiments can be rethought (see Bokulich 2001), and that the
evidential significance is dependent on historical and local
accomplishments (see McAllister 1996), it cannot be irrelevant to
identify the intention of the thought experimenter, if one wants to
determine the type of a thought experiment: "An imaginary
experiment should be judged on its specific purpose" (Krimsky
1973, p. 331).


## 3. The History of Thought Experiments



The practice of thought experiments is not an invention of modern
science. That fact may be obscured by the dominance of scientific
examples in the lively discussions about thouht experiments today. The
Pre-Socratics "invented thought experimentation as a cognitive
procedure and practiced it with great dedication and
versatility" (Rescher 2005, p. 2). "There is no ancient
Greek term corresponding to what we nowadays refer to as a thought
experiment, and presumably ancient philosophers did not have our
modern notion of a thought experiment. But there is no doubt that they
did use thought experiments. In fact, they often employed them in ways
similar to those of contemporary philosophers, that is, both for
defending their own theories as well as for refuting the theories of
their opponents " (Ierodiakonou 2018, p. 31). (See also Becker
2018; Diamond 2002, pp. 229-232; Fuhrer 2009; Glas 1999;
Ierodiakonou 2005; Ierodiakonou and Roux (eds.) 2011; Irvine 1991;
Rescher 1991 and 2005, pp. 61-72). The situation is similar with
respect to medieval natural philosophy, although there are further
nuances to be considered (see King 1991). According to Edward Grant,
during the late Middle Ages "the imagination became a formidable
instrument in natural philosophy and theology in ways that would have
astonished ancient Greek natural philosophers, especially
Aristotle" (Grant 2007, p. 201). But this doesn't mean
that we have reason to think of Aristotle as an opponent of the
conduct of thought experiments *tout court*. On the contrary,
"Aristotle uses thought experiments for argumentative persuasion
and in places where, due to the obscure nature of the subject matter
or the counterintuitive nature of the thesis they are meant to
support, insight cannot be readily communicated by appeal to
observational facts" (Corcilius 2018, p. 73). With a few
exceptions that involved problems of motion, "the
scholastics" of the medieval period made no meaningful effort to
transform their hypothetical conclusions into specific knowledge about
the physical world. They did, however, assume that although these
hypothetical conclusions were naturally impossible, God could produce
them supernaturally if he wished. Special attention received also a
class of medieval thought experiments that does not rely on
counterfactuals but depends on theological assumptions to study
matters non-theological, namely those thought experiments involving
angels, whose existence were affirmed at that time (see Perler 2008).
Angels are gone by now (see Clark 1992), but not thought experiments.
While most thought experiments involving angels have Christianity as
their context, there is evidence of the practice of thought
experiments also in the context of Islam and Judaism (see McGinnis
2018; Fisch 2019). In fact, the case has been made "that Ibn
Sina is the first philosopher in the Aristotelian tradition, and thus
perhaps the first in Western philosophy overall, to try to identify
the psychological processes that go into postulating a hypothetical
scenario. Ibn Sina also exhibits an interest in accounting for why,
and to what extent, such psychological acts are thought to carry
weight in our study of nature" (Kukkonen 2014, p. 434).



Ernst Mach is commonly credited with introducing the word
"thought experiment" (*Gadankenexperiment*) and
thereby coining a term for philosophical discussion (recently done,
for instance, by Krauthausen 2015, p. 15). " This view is
incorrect, however! [...] it can be substantiated that it was
used [...] already in 1811" (Witt-Hansen 1976, p. 48; see
also Buzzoni 2008, pp. 14-15; 61-65; Kuhne 2005, pp.
92-224; Moue et al. 2006, p. 63). The conceptual history of
"thought experiment" goes back at least to the Danish
"Tankeexperiment," as it was used by Hans-Christian
Orsted. We can go back even further and find in the work of the
German philosopher-scientist Georg Lichtenberg (1742-1799) a
tacit theory of "experiments with thoughts and ideas."
These experiments help to overcome habits of thought that can inhibit
scientific progress, and make possible an enlightened philosophy (see
Schildknecht 1990, pp. 21; 123-169; Schone 1982).
Lichtenberg's "aphoristic experiments" (see Stern
1963, pp. 112-126) reflect "that Lichtenberg's
scientific preoccupations are the formal and thematic prolegomena to
his work as a literary artist" (Stern 1963, p. 126).
Lichtenberg's reflections on thought experimentation resemble
those of Popper and Thomas S. Kuhn, and it is plausible to think of
him as one important figure of the very first period in the history of
philosophical inquiry into thought experiments (see Fehige and Stuart
2014).



Accordingly, the modern history of the philosophical investigation
into thought experiments can be divided into four stages: in the 18th
and 19th century the awareness of the importance of thought
experiments in philosophy and science emerges. In addition to
Lichtenberg and Hans-Christian Orsted, special mention should
be made of Novalis (see Daiber 2001). The topic reemerges in a more
systematic manner at the beginning of the 20th century with little
relation to the attempts made at the first stage. The stakeholders of
the second stage were Pierre Duhem, Mach, and Alexius Meinong (see
Duhem 1913, pp. 304-311; Mach, 1883, pp. 48-58, 1897 and
1905; Meinong 1907). A third stage, probably due to the rediscovery of
the importance of scientific practice for a proper understanding of
science, followed in the first part of the second half of the 20th
century. Again, the contributions of this stage bear little relation
to the two previous stages. While the third period has seen a number
of noteworthy contributions (Cole 1983; Dancy 1985; Dennett 1985;
Fodor 1964; Helm and Gilbert 1985; Helm et al. 1985; Krimsky 1973;
McMullin 1985; Myers 1986; Poser 1984; Prudovsky 1989; Rehder 1980a,b;
Yourgrau 1962 and 1967), the protagonists of this period were
Alexandre Koyre, Kuhn and Popper. The ongoing philosophical
exploration of thought experiments began in the 1980s, and marks the
fourth stage. Arguably, it has been the most prolific one of all four
stages. With some very important sign-postings in place (Horowitz and
Massey (eds.) 1991; Sorensen 1992; Wilkes 1988), the ongoing
discussion took off in light of a debate between James Robert Brown
and John D. Norton (see for a concise statement of each position Brown
2004 and Norton 2004), which many have found useful to establish a
contrast with their own alternative accounts of thought experiments.
These views "represent the extremes of platonic rationalism and
classic empiricism, respectively" (Moue et al. 2006, p. 69).
They will be described below.


## 4. Current Views on Thought Experiments



At this point it is important to recall the key epistemological
challenge described in the introduction: how can we learn about the
real world through merely thinking about imagined scenarios? This
challenge sits at the center of the discussion about thought
experiments even though we must note that not all of the work
discussed below focuses on it directly. Still, this section describes
six views that can be seen as responding in some way to this
challenge: The Skeptical Objection, The Intuition-Based Account, The
Argument View, Conceptual Constructivism, Experimentalism, and The
Mental-Model Account.


### 4.1 The Skeptical Objection



Of course, particular thought experiments have been contested. But for
the most part, the practice of thought experiments in the sciences has
been cheerfully accepted. Pierre Duhem, the great historian of
physics, is almost alone in what has been understood as an outright
condemnation of scientific thought experiments (see Duhem 1913, pp.
304-311). A thought experiment is no substitute for a real
experiment, he claimed, and should be forbidden in science, including
science education. However, in view of the important role of actual
thought experiments in the history of physics -- from
Galileo's falling bodies, to Newton's bucket, to
Einstein's elevator -- it is unlikely that anyone will feel
or should feel much sympathy for Duhem's strictures. We hasten
to add that Buzzoni (2018) questions the validity of this reading of
Duhem, and argues that already Mach's reception of Duhem's
views suggests a more nuanced reading of Duhem's position.



Philosophers can be as critical as Duhem when it comes to thought
experimenting in their own field (see Peijnenburg; Atkinson 2003;
Thagard 2014; Wilson 2016). At least thought experiments in science,
the skeptic claims, can be tested by physical experiment. However,
this is clearly false, since frictionless planes and universes empty
of all material bodies cannot be produced in any laboratory. True, the
results of philosophical thought experiments cannot be even
approximately tested. But, skeptics say little about why thought
experiments enjoy such popularity in philosophy. We are inclined to
say that skeptics underestimate the importance of thought experiments
for the creative mind in any field. Also, one mustn't forget
that the cognitive power of real world experiments isn't a
self-evident matter either.



Few are outright skeptics, however. Many take a more ambiguous stance.
Soren Haggqvist, for example, has developed a normative
model for philosophical thought experiments (see Haggqvist 1996
and 2009). Surprisingly, none of the commonly accepted philosophical
thought experiments satisfies his model. And the process of
identifying successful thought experiments is only the first step in
addressing the central epistemological challenge posed by thought
experiments. It gets much messier once we begin to ask exactly how
reliable "successful" thought experiments are. Granted,
there is some justice in worrying about the reliability of
philosophical thought experiments (see, e.g., Klee 2008). This might
be true for ethics (see Dancy 1985, Jackson 1992; Wilson 2016),
conceptual analysis (see Fodor 1964), and the philosophy of mind:
"A popular strategy in philosophy is to construct a certain sort
of thought experiment I call intuition pump. [...] Intuition
pumps are often abused, though seldom deliberately" (Dennett
1985, p. 12). The claim by Dennett and others is that thought
experiments too often rest on prejudice and faulty common sense; they
are inherently conservative, while real science will likely result in
highly-counterintuitive outcomes. Dennett believes that thought
experiments rest on naive "folk concepts," which is why
they can be so misguided. It is far from clear that this is a fair
charge. Everything involved in Galileo's thought experiment that
produced the principle of relativity could be called "folk
concepts." If we are inside a ship and perform a number of
experiments, such as walking about, tossing a ball, watching birds fly
about, we could not tell whether we are at rest in port or sailing
over a smooth sea. The upshot is that nature behaves the same either
way; the laws of nature are the same in any inertial frame. This
result is profound and is still with us in Einstein's
relativity, whether it is folk physics or not.



Frequently discussed is the skeptical challenge raised by Kathleen
Wilkes. She expresses a deep suspicion of scenarios such as Derek
Parfit's people splitting like an amoeba (see Parfit 1987;
Gendler 2002a). Wilkes wants philosophy "to use science fact
rather than science fiction or fantasy" (Wilkes 1988, p. 1), and
therefore to refrain from using thought experiments because they are
"both problematic and positively misleading" (Wilkes 1988,
p. 2). She claims that thought experiments about personal identity in
particular often fail to provide the background conditions against
which the experiment is set (see Wilkes 1988, p. 7). She thinks we
would not know what to say if we encountered someone who split like an
amoeba. She insists that a legitimate thought experiment must not
violate the known laws of nature. We do agree with Wilkes that
underdetermination can be a problem. But instead of dismissing thought
experiments in philosophy we should consider it a crucial factor in
assessing the quality of a thought experiment (see Rescher 2005, pp.
9-14). That is to say that the more detailed the imaginary
scenario in the relevant aspects is, the better the thought experiment
(see Brendel 2004, pp. 97-99; Haggqvist 1996, p. 28).



We also agree that the inferences drawn in thought experimenting are
highly problematic if the hypothetical scenario "is inadequately
described" (Wilkes 1988, p. 8). But Wilkes seems to think that
the lack of description is unavoidable, which supposedly amounts to a
reason against philosophical thought experiments on personal identity
because persons are not natural kinds. This makes it impossible to
fill in necessary information to make the thought experiment work
given its unavoidable underdetermination. Wilkes thinks that
"whenever we are examining the ranges of concepts that do not
pick on natural kinds, the problem of deciding what is or what is not
'relevant' to the success of the thought experiment is yet
more problematic than the same question as it arises in science; and,
unlike the scientific problem, it may not even have an answer in
principle" (Wilkes 1988, p. 15). She adds that scientific laws
-- especially those describing biological kinds like human beings
-- "are not disjoint and independent, detachable from one
another [...]. They are interrelated, to varying degrees of
course" (Wilkes 1988, p. 29). This implies, for example, that
"a full psychophysiological account of the processes of human
perception must at some stage link up with part at least of linguistic
ability; for we typically see things under a certain description, and
that description may be a very sophisticated one" (Wilkes 1988,
p. 29). These considerations have her rule out experiments that
challenge the human monopoly of personhood. No thought experiment,
claims Wilkes, is well conceived if it involves non-human animals or
computers as persons. But also those thought experiments can be ruled
out which involve the "fission or fusion of humans"
because it is not theoretically possible. "The total impact of
the sum of laws that group us together as human beings (a natural kind
category) precludes our splitting into two [...] or fusing with
someone else" (Wilkes 1988, p. 36).



One can ascertain here all too well the inherent difficulties in
thinking about personal identity and the limited benefit some thought
experiments might have for what is deemed the proper metaphysics of
personal identity. Nevertheless, good reasons have been given in
favour of the use of thought experiments about personal identity (see
Beck 2006; Kolak 1993; Hershenov 2008). We also feel that the problems
about thought experiments on personal identity reveal more about the
intricate nature of the subject than about the usefulness of
philosophical thought experiments. And, disregarding other
shortcomings in Wilkes' skepticism (for further discussion of
Wilkes' views see Beck 1992; Brooks 1994; Focquaert 2003;
Haggqvist 1996, pp. 27-34), her suggestion that thought
experimental scenarios would have to satisfy current scientific
knowledge about the relevant entities featured in a thought experiment
is highly implausible. We learn a great deal about the world and our
theories when we wonder, for instance, what would have happened after
the big bang if the law of gravity had been an inverse cube law
instead of an inverse square. Would stars have failed to form?
Reasoning about such a scenario is perfectly coherent and very
instructive, even though it violates a law of nature.



To some extent we should share Wilkes's concern that thought
experimenting seems to be constrained only by relevant logical
impossibilities and what seems intuitively acceptable. This is indeed
problematic because intuitions can be highly misleading and relevant
logical impossibilities are fairly ungrounded if they cannot be
supplemented by relevant theoretical impossibilities based on current
science in order to avoid the jump into futile fantasy. But in order
to dismiss thought experimenting as a useful philosophical tool one
has to show that intuition cannot be a source of knowledge and that an
epistemic tool should be useless because there is a serious chance it
can fail. Timothy Williamson has argued that we should forget about
intuition as a cushion in the philosophical armchair (see Williamson
2004a,b, 2008, pp. 179-207, and 2009; see also Schaffer 2017).
The importance of intuitions in philosophy has been neglected in the
past (see Williamson 2004b, p. 109-110), and for too long
intuition didn't receive the attention it deserves (see, e.g.,
DePaul and Ramsey (eds.) 1998). Besides the traditional divide between
empiricists, rationalists and skeptics, it is not only a very
non-uniform use of the word "intuition" that makes it
difficult to assess the progress of the last years of philosophical
inquiry about intuitions. The situation has been complicated by the
contributions of experimental philosophers on intuitions who add
different reasons to question their reliability (see for a careful
critique of those reasons: Ludwig 2007; see also Ludwig 2018).
Generally speaking, the reliability of intuitions has been challenged
on two grounds. One stems from an evolutionary explanation of the
capacity to intuit; another is due to experiments which supposedly
show the cultural relativity or racial and gender sensitivity of
intuitions (see, e.g., Buckwalter and Stich 2010): "...a
substantial list of philosophical intuitions vary across demographic
groups and...they are influenced by a number of *prima
facie* irrelevant factors...Some writers...have urged
that these findings justify a thoroughgoing skepticism about the use
of intuitions as evidence in philosophy...But we think this
conclusion is much too strong..." (Stich & Toba 2018,
p. 379). After all, knowledge without intuitions (if only common sense
assumptions) seems impossible.



The recent discussion of intuitions in epistemology has barely made an
impact on philosophical reflections about thought experiments. As far
as philosophical thought experiments are concerned, this is as it
should be, according to Williamson. In this respect George Bealer can
be cited in support of Williamson, because for Bealer the talk about
philosophical thought experiments reveals a conceptual confusion.
Philosophy, he claims, is about "rational intuitions" and
thought experiments can be only about "physical
intuitions" (see Bealer 1998, pp. 207-208, and 2002, p.
74). To many, this is an implausible claim based on a deeply
problematic "phenomenology of intuitions" resulting in a
strict separation of "rational intuitions" from
"physical intuitions", on such grounds as an alleged
immutability of "rational intuitions". There are good
reasons to believe that thought experiments appeal to intuitions in
order to give us new insights about different realms of investigation,
including philosophy. This kind of positive connection is what
Williamson has in mind when addressing the role of intuitions in
philosophical thought experiments like the famous Gettier cases, which
overnight found acceptance by the philosophical community in their aim
to refute the view that knowledge is justified true belief. While
Williamson expects "armchair methods to play legitimately a more
dominant role in future philosophy" (Williamson 2009, p. 126),
he thinks that "we should stop talking about intuition"
(Williamson 2004b, p. 152). This does not impress proponents of what
we call an intuition-based account of thought experiments, and
probably for good reasons, given the problems in Williamson's
approach (see, e.g., Dohrn 2016; Ichikawa and Jarvis 2009; Schaffer
2017), and the strong empirical evidence in favour of the positive
role that intuitions does play in human cognition (see Myers
2004).


### 4.2 The Intuition-Based Account



What we term the "intuition-based account" of
thought experiments comes in a naturalistic version (see Brendel 2004;
Gendler 2007), and in a Platonic version (see Brown 1991a [2011]). We
begin with a discussion of the latter. Brown holds that in a few
special cases we do go well beyond the old empirical data to acquire a
priori knowledge of nature (see also Koyre 1968). Galileo
showed that all bodies fall at the same speed with a brilliant thought
experiment that started by destroying the then reigning Aristotelian
account. The latter holds that heavy bodies fall faster than light
ones (*H* > *L*). But consider Figure 6, in which a
heavy cannon ball (*H*) and light musket ball (*L*) are
attached together to form a compound object (*H*+*L*);
the latter must fall faster than the cannon ball alone. Yet the
compound object must also fall slower, since the light part will act
as a drag on the heavy part. Now we have a contradiction:
*H*+*L* > *H* and *H* >
*H*+*L*. That's the end of Aristotle's
theory. But there is a bonus, since the right account is now obvious:
they all fall at the same speed (*H* = *L* =
*H*+*L*).



![A human stands on one leg atop the leaning Tower of Pisa. The human's outstretched hand appears to have dropped a small black ball, a large white ball, and a small ball attached to a large ball with a cord. The human has a thought bubble which reads, 'I don't even have to look'.](figure6.jpg)

Figure 6. Galileo: "I don't
even have to look"




Brown claims this is a priori (though still fallible) knowledge of
nature, since there are no new data involved, nor is the conclusion
derived from old data. Moreover, is it some sort of logical truth (for
a technical challenge of this claim see Urbaniak 2012). This account
of thought experiments can be further developed by linking the a
priori epistemology to accounts of laws of nature that hold that laws
are relations among objectively existing abstract entities. It is thus
a form of Platonism, not unlike Platonic accounts of mathematics such
as that urged by Kurt Godel.



The two most often repeated arguments against this sort of Platonism
are: it does not identify criteria to distinguish good from bad
thought experiments, and it violates the principle of ontological
parsimony. These seem weak objections. Perhaps they find widespread
acceptance because Platonism seems to be unfashionable these days (see
Grundmann 2018), given the general popularity of various forms of
naturalism. If intuitions really do the job in a thought experiment,
the first objection is weak because neither rationalists nor
empiricists have a theory about the reliability of intuitions. So the
objection should be that intuitions probably just do not matter in
human cognition. However, there are good reasons to question the truth
of this claim (see Myers 2004). This is not to marginalize the
problems that arise when admitting intuitions as a source of knowledge
and justification, especially in philosophy (see Hitchcock 2012).



As for the second objection, the appeal to Occam's razor is in
general problematic when it is employed to rule out a theory. Whatever
we eliminate by employing the principle of parsimony, we can easily
reintroduce it by an inference to the best explanation (see Meixner
2000). And this is exactly what a Platonist contends his or her
Platonism about thought experimenting to be, while conceding that the
Platonic intuition appears miraculous. But are they really more
miraculous than sense perception, which seems similar in many respects
to Platonic intuition? One might want to say yes, because supposedly
we have no clue at all how Platonic intuition works but we do have
some idea about the nature of sense perception. We know that if an
object is far away it appears smaller in vision, and under certain
light conditions the same object can look quite different. However, is
it really impossible to state similar rules to capture the nature of
Platonic intuition? If you are drunk or lack attention you most
probably will not be very successful in intuiting anything of
philosophical value.



A review of the relevant psychological literature will reveal further
criteria that could be employed to identify good and bad conditions
for Platonic intuition while thought experimenting. Yet, proponents of
the naturalistic version of the intuition-based account wonder
how necessary Platonism is once this move is entertained in defence of
the reliability of intuitions (see Miscevic 2004).
Elke Brendel defines intuitions as mental propositional attitudes
accompanied with a strong feeling of certainty. In her view, we can
tell two stories to make sense of their cognitive power and
plasticity. One story relates to our biological constitution and
evolutionary past. The other is about membership in specialized
communities. Brendel's account raises many questions, but it is
difficult to resist its appeal. A universal set is appealing to anyone
not trained in logic because most things we are familiar with can come
in sets, such as books, tables, and philosophers. A set of all sets
seems intuitively plausible. The intuition disappears once you worked
yourself through the problems arising from the idea of a set of all
sets. Brendel is quick to insist that such relativity of our
intuitions doesn't imply that they are cognitively useless.
Without intuitions, we probably wouldn't have knowledge, and
thought experiments are sometimes the only way to access the
intuitions that guide us in our cognitive lives (see Brendel
2004).


### 4.3 The Argument View



John D. Norton is the most influential advocate of what we call
"the argument view" of thought experiments (see Norton
1991, 1993, 1996, 2004a,b, 2008). Even though the argument view seems
to be a natural option for empiricists, it seems that most empiricists
find Norton's argument view too strong. For this reason, many
participants in the debate about thought experiments place themselves
between the extreme views of Norton and Brown, which function as
useful foils for apparently more moderate outlooks. Perhaps (with
tongue in cheek) they could agree with Bernard Shaw on the virtues of
moderation, when Shaw said of the typical member of the middle class
that he is moderately honest, moderately intelligent, and moderately
faithful to his spouse. Norton claims that any thought experiment is
really a (possibly disguised) argument; it starts with premises
grounded in experience and follows deductive or inductive rules of
inference in arriving at its conclusion. The picturesque features of
any thought experiment which give it an experimental flavour might be
psychologically helpful, but are strictly redundant. Thus, says
Norton, we never go beyond the empirical premises in a way to which
any empiricist would object.



There are three objections that might be offered against Norton.
First, his notion of argument is too vague. However, this might not be
the best objection: arguments can be deductive (which are perfectly
clear) or inductive. If the latter are unclear, the fault is with
induction, not with Norton's argument view. Second, it is argued
that Norton simply begs the question: every real world experiment can
be rephrased as an argument, but nobody would say that real world
experiences are dispensable. The account does not address the
question: where do the premises come from? A thought experiment might
be an essential step in making the Norton-style reconstruction. Third,
a thought experiment that is presented in argument form loses its
typical force. The soft-point in Brown's Platonism is linked to
the strength of Norton's account because Norton claims that any
other view implies a commitment to "asking the oracle."
"Imagine an oracle that claims mysterious powers but never
delivers predictions that could not be learned by simple inferences
from ordinary experience. We would not believe that the oracle had any
mysterious powers. I propose the same verdict for thought experiments
in science" (Norton 1996, pp. 1142-1143). Defenders of
empiricist alternatives deny this dispensability thesis. Brendel
(2018) offers a most comprehensive review of merits and perils of the
argument view.


### 4.4 Conceptual Constructivism



"Conceptual constructivism", as we could call it, is among
the empiricist alternatives to the argument view. The position has
been taken up by Van Dyck (2003) to account especially for
Heisenberg's g-ray microscope; but also by Gendler (1998)
to makes sense of Galileo's falling body thought experiment.
Gendler's proposal was advanced in more general terms by
Camilleri (2014) in order to establish a firm middle ground between
the views of Norton and Brown. Conceptual constructivism was first
proposed by Thomas Kuhn (1964). He employs many of the concepts (but
not the terminology) of his well-known *Structure of Scientific
Revolutions*. On his view a well-conceived thought experiment can
bring on a crisis or at least create an anomaly in the reigning theory
and so contribute to paradigm change. Thought experiments can teach us
something new about the world, even though we have no new empirical
data, by helping us to re-conceptualize the world in a new way.
Accordingly, some have entertained the option of conceptual
constructivism in the form of a Neo-Kantian reading of
Einstein's famous clock in the box thought experiment. Such an
approach is inspired by Michael Friedman's proposal to conceive
of scientific revolutions as times when a Kantian kind of natural
philosophy plays a major role in guiding scientists from one paradigm
to another. The work of Kuhn left us with a puzzle: if scientific
rationality is absolutely dependent on a paradigm, and if during
scientific revolutions one paradigm replaces another, not in degrees
but absolutely, comparable to a "Gestalt" switch, then
this transition from one paradigm to the next cannot be a matter of
scientific rationality. Are scientific revolutions irrational periods
in the history of science? Not necessarily; some kind of natural
philosophy may guide the process. Friedman has a Kantian natural
philosophy in mind; his proposal did not earn wide acceptance, but the
problem remains (see Fisch 2017). Be that as it may, it is true that
thought experiments are a valuable currency in times of scientific
revolution. For example, Lennox (1991) has argued that the revolution
brought about by Charles Darwin in 1859 was made possible by thought
experiments (among other things, of course).


### 4.5 Experimentalism



What we might term "experimentalism" encompasses a wide
range of different approaches which all advance the view that thought
experiments are a "limiting case" of ordinary experiments.
Experimentalism was proposed first by Ernst Mach (1897 and 1905). He
defines experimenting in terms of its basic method of variation and
its capacity to destroy prejudices about nature. According to Mach,
experimenting is innate to higher animals, including humans. The
thought experiment just happens on a higher intellectual level but is
basically still an experiment. At the centre of thought experimenting
is a "Gedankenerfahrung", an experience in thought. Such
an experience is possible because thought experiments draw from
"unwillkurliche Abbildungen von Tatsachen"
(non-arbitrary images of facts) acquired in past experiences of the
world. Some thought experiments are so convincing in their results
that an execution seems unnecessary; others could be conducted in a
real-world experiment, which is the most natural trajectory of a
scientific thought experiment. In any case thought experiments can
result in a revision of belief, thereby demonstrating their
significance for scientific progress. Mach also appreciates the
didactic value of thought experiments: they help us to realize what
can be accomplished in thinking and what cannot.



In the spirit of Mach, Sorensen (1992) has offered an aspiring version
of experimentalism that accounts for thought experiments in science
and philosophy, and tackles many of the central issues of the topic.
Sorensen claims that thought experiments are "a subset of
unexecuted experiments" (1992, p. 213). By their logical nature
they are paradoxes that aim to test modal consequences of
propositions. The origin of our capacity of thought experimentation is
explained in terms of Darwinian evolution (as in Genz 1999, pp.
25-29), though the explanation has been criticized to be only
little more than a 'just so story' that fails, on a
posteriori grounds, to epistemically underwrite that capacity (see
Maffie 1997). Others are more optimistic (see Shepard 2008).



Experimentalism does not have to take a naturalistic turn as it does
in Sorensen's case. In a number of contributions Marco Buzzoni
has defended a Neo-Kantian version of experimentalism (see Buzzoni
2004, 2007, 2008, 2011, 2011b, 2013, 2013b). Buzzoni (2008) argues for
the dialectical unity of thought experiments and real-world
experiments. Thought experiments and real-world experiments are
claimed to be identical on the "technological-operational"
level, and at least in science, one is impossible without the other:
without thought experiments there wouldn't be real-world
experiments because we would not know how to put questions to nature;
without real-world experiments there wouldn't be answers to
these questions or experience from which they could draw. Given the
many scientific thought experiments that cannot be realized in the
real-world, Buzzoni might be conflating thought experiments with
imagined experiments to be carried out in the real-world (see Fehige
2012, 2013b; and Buzzoni 2013b).



Idealizations are common in both real experiments and thought
experiments. So-called Aristotelian idealization might ignore, say,
the colour of a falling object. Galilean idealizations ignore some
physical aspects, such as air friction, to get at the underlying
physics (McMullin 1985). So-called Platonic idelization goes beyond
this and ignores what would be actually seen even in a Galilean
idealization. For instance, a rapidly moving object in special
relativity would not look contracted, but rather would look rotated
(surprisingly, this phenomenon is not well known). This rotation is
ignored as an irrelevant optical phenomenon to yield the correct
thought experiment visualization, which is the well-known Lorentz
contraction (Brown 2013).


### 4.6 The Mental-Model Account



The last of the many accounts that emerged in the discussion about
thought experiments is what could be called the
"mental-model account." It attracts the most
followers (see Andreas 2011; Bishop 1998; Cooper 2005; Gendler 2004;
Palmieri 2003; Nersessian 1992, 1993, 2007; McMullin 1985;
Miscevic 1992, 2007, 2021). When we conduct a thought
experiment, according to champions of this view, we manipulate a
mental model instead of the physical realm: "The general claim
is that in certain problem-solving tasks people reason by constructing
a mental model of the situations, events, and processes that in
dynamic cases can be manipulated through simulation" (Nersessian
2018, p. 319). Like physical models, mental models are
non-propositional in nature. This means first of all "that the
carefully crafted thought-experimental narrative focuses on the
construction of a model of a *kind* of situation and
manipulating that model through simulation affords epistemic access to
certain features of current representations in a way that manipulating
propositional representations using logical rules cannot"
(Nersessian 2018, pp. 319-320). A narrative functions as a kind
of user-manual for building the model, but it isn't identical to
a thought experiment. The biggest problem for the mental-model
account is to explain how something non-propositional like a mental
model can make an impact on the propositional realm, which happens
when a thought experiment causes a revision in beliefs.


## 5. Going Forward



The mental-model approach is one of the most promising of all
the accounts the literature on thought experiments has to offer, and
this for several reasons. First, it does not seem to be much of a
stretch to draw connections to the intuition-based account. In
fact, intuitions maybe the missing link to connect the essentially
non-propositional activities surrounding mental models, on the one
hand, and the propositional aspects of thought experiments, on the
other. After all, thought experiments involve propositional reasoning,
and somehow the non-propositional and propositional aspects of thought
experiments must be linked in any account of thought experiments. This
is urgent insofar as thought experiments are credited with a
meaningful role in theory discovery and theory choice. Second, the
mental-model approach also allows for inclusion of important
elements of experimentalism and the argument-view. Thought
experiments are realized in the mind on mental models, and the method
of variation is employed such that the results of the experiment may
be subject to a careful reconstruction of propositional lines of
reasoning to submit it for careful assessment and critique. Third, the
mental-model approach enables us to bring an aspect into focus
that has been widely neglected in the discussion so far: the bodily
component of (thought) experiments. The exception is the work of the
late David Gooding (1992, 1993, 1994, and 1999). Fourth, in critical
engagement with such naturalistic proposals, those theories of the
body may be put to work that the philosophical school of phenomenology
has produced (see Fehige and Wiltsche 2013). To be welcomed,
therefore, is the entry of phenomenology into the discussion on
thought experiments (see Hopp 2014; Wiltsche 2018). Fifth, the
mental-model account also relates naturally to the most
intriguing discussions about the role of literary fiction in thought
experiments.



Some have placed "literary fiction on the level of thought
experiments" (Swirski 2007, p. 6). There are two readings of
such a claim. According to the first, some literary fiction may be of
cognitive power due to the fact that they are thought experiments. In
other words, we shouldn't outright reject the idea that
literature can be of cognitive value. Dystopian novels such as
Orwell's *1984* and Huxley's *Brave New
World* are obvious examples. According to the second reading, the
power of thought experiments is partially a function of the narrative
that conveys it. The work of Novalis remains relevant for the
exploration of this link between narrative development and thought
experiment: experimental writing and experiments on imagined scenarios
go hand in hand; words and thoughts coincide; mind and matter are
entangled (see Daiber 2001). According to the mental-model
approach, both readings have a valid point. Literary fiction and
narratives of thought experiments can be powerful in establishing
mental models in such a way that we can even learn new things about
the world at times from the fictional elements of them. The common
denominator is the work on mental models each may facilitate. It is in
this context that an appreciation can grow for Catherine Elgin's
theory of exemplification to argue against the "valorization of
truth in epistemology" (2004, p. 113). This is also the place to
consider Andras Kertesz's (2015) work on conceptual metaphor
research in its relevance to the epistemological puzzle that thought
experiments pose.



Finally, sixth, mention could be made of visual reasoning in
mathematics, which often seems closely related to thought experiments.
The standard view of mathematics is that the one and only source of
evidence is a proof, and a proof is a derivation from axioms or first
principles. Let's overlook the problem of where the first
principles come from. A simple example such as the following casts
doubt on the standard view:



> 
> 
> Theorem: *1 + 2 + 3 + ... + n = n2/2 + n/2*
> 
> 
> 
> Proof: See Figure 7.
> 
> 
> 



![A stack of white squares forms the bottom-left half of a five-by-five grid cut diagonally. The five white squares along the diagonal are cut in half and the remainder of each cut square shown in black.](figure7.jpg)

Figure 7. Picture proof.




The proof works like this: Start at the top and work down, letting the
little squares represent numbers, *1 + 2 + 3 + 4 + 5*. The
total number of squares in the picture is equal to this sum. Notice
also that the numbers of squares is equal to a large square with sides
of length 5 that is cut in half along the diagonal, i.e.,
*52/2*, plus the shaded bits that were cut off by
the diagonal cut, i.e., *5/2*. It is plausible to claim that
the diagram is a perfectly good proof of the theorem. One can
"see" complete generality in the picture. Even though it
only illustrates the theorem for *n = 5*, somehow we can see
that it works for every number, all infinitely many of them. The
diagram does not implicitly suggest a "rigorous" verbal or
symbolic proof. The regular proof of this theorem is by mathematical
induction, but the diagram does not correspond to an inductive proof
at all, since the key element in an inductive proof is the passage
from *n* to *n + 1*. The simple moral we could draw from
the example is just this: We can in special cases correctly infer
theorems from pictures, that is, from visualizable situations. There
is an intuition and from this intuition we can grasp the truth of the
theorem (see Brown 1999 [2008]).



Our assessment of the prospects of the mental-model account is
very rough and speculative, though certainly not implausible. Of
course, there are challenges to such a vision of a greater synthesis
of the many different takes on thought experiments under the umbrella
of the mental-model account. For example, some see additional support
arising for the argument-view from computer simulations (see
Beisbart 2012). Others find that "computational modeling is
largely replacing thought experimenting, and the latter will play only
a limited role in future practice of science, especially in the
sciences of complex nonlinear, dynamical phenomena" (see
Chandrasekharan *et al.* 2012, p. 239). But, there are also
proposals such as that by Marcus Schulzke (2014) to think of video
games philosophically as executable thought experiments. Whatever the
merits of this particular proposal, future explorations of the
relationship between computer simulations and thought experiments can
build on outcomes of closer inquiries into it (see Behmel 2001, pp.
98-108; Di Paolo *et al*. 2000; El Skaf and Imbert 2013;
Lenhard 2011; Staudner 1998; Lenhard 2018). The work on the
nature of the importance of scientific understanding (see, e.g.,
Stuart 2018) will inform that exploration as much as the fruits of
continuing efforts to clarify the role of the imagination in thought
experiments (see, e.g., Meynell 2014; Stuart 2017 and 2021).



We conclude with an interesting, but still relatively unexplored issue
that concerns the relative importance of thought experiments in
different disciplines. Physics and philosophy use them extensively.
Chemistry, by contrast, seems to attract less attention in this
respect. Why is this the case? Perhaps it is merely an historical
accident that chemists never developed a culture of doing thought
experiments. Perhaps it is tied to some deep feature of the discipline
itself (see Snooks 2006). Economics and history use thought
experiments, but apparently not anthropology. A good explanation would
likely tell us a lot about the structure of these disciplines.



Related to this is the question of the difference, if any, between
thought experiments in the sciences and those in philosophy. We have
assumed throughout this entry that they are the same kind of thing.
Not everyone sees them this way, so perhaps it should be considered an
open question. On the one hand, philosophy and science seem to many to
be different kinds of activities. That might suggest that thought
experiments would differ in the two areas. On the other hand, there is
a huge difference between thought experiments within a single field,
e.g., Newton's bucket attempts to establish absolute space while
Schrodinger's cat aims to show QM as then understood to be
absurd. Is the difference between them less than the difference
between either of them and Searle's Chinese Room or
Thomson's violinist? The case one way or the other is not
obvious. Of course, there are differences between constructive and
destructive thought experiments, but this is true within any
discipline. Perhaps for now the default attitude ought to be that
there is no categorical difference between scientific and
philosophical thought experiments. This should not be treated as a
dogmatic principle, but rather a stimulus to look deeper for important
subtle contrasts.