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Illustrated: 24 Thought Experiments in Pictures

According to Stanford Encyclopedia of Philosophy, thought experiments are devices of the imagination used to investigate the nature of things. They are used for diverse reasons in a variety of areas, including economics, history, mathematics, philosophy, and the sciences, especially physics.

Most often thought experiments are communicated in narrative form, frequently with diagrams.

Thought experiments should be distinguished from thinking about experiments, from merely imagining any experiments to be conducted outside the imagination, and from psychological experiments with thoughts. They should also be distinguished from counterfactual reasoning in general, as they seem to require an experimental element, which seems to explain the impression that something is experienced in a thought experiment.

In other words, though many call any counter-factual or hypothetical situation a thought experiment, this seems too encompassing. It seems right to demand that they also be visualized (or perhaps smelled, tasted, heard, touched); there should be something experimental about a thought experiment.

1.    Bellum omnium contra omnes (The war of all against all)
— Thomas Hobbes, 1642, De Cive and Leviathan
This thought experiment places people in a pre-social condition, and theorises what would happen in such a condition. According to Hobbes, the outcome is that people choose to enter a social contract, giving up some of their liberties in order to enjoy peace. This thought experiment is a test for the legitimation of a state in fulfilling its role as ‘sovereign’ to guarantee social order, and for comparing different types of states on that basis. (Hobbes added life in the state of nature was ‘solitary, poor, nasty, brutish and short.’)

2.    Big Book (ethics)
— Ludwig Wittgenstein, 1929, Lecture on Ethics
“No statement of fact can ever be, or imply, a judgment of absolute value. Suppose one of you were an omniscient person and therefore knew all the movements of all the bodies in the world dead or alive and that you also knew all the states of mind of all human beings that ever lived, and suppose you wrote all you knew in a big book, then this book would contain the whole description of the world; and what I want to say is, that this book would contain nothing that we would call an ethical judgment or anything that would logically imply such a judgment.”

3.    Brain in a Vat (epistemology, philosophy of mind)
—    Various proponents
Imagine that a mad scientist has removed your brain, and placed it into a vat of liquid to keep it alive and active. The scientist has also connected your brain to a powerful computer, which sends neurological signals to the brain in the way the brain normally receives them. Thus, the computer is able to send your brain data to fool you into believing that you are still walking around in your body.

4.    Brainstorm Machine
— Daniel Dennett, 1997, Quining Qualia
It is not possible to intersubjectively compare any two individuals’ personal experiences, or qualia, even with perfect technology.

5.    Buridan’s Ass
(Named after Jean Buridan c. 1300 – 1358)
It refers to a hypothetical situation wherein an ass that is equally hungry and thirsty is placed precisely midway between a stack of hay and a pail of water. Since the paradox assumes the ass will always go to whichever is closer, it will die of both hunger and thirst since it cannot make any rational decision to choose one over the other.

6.    Changing Places (reflexive monism, philosophy of mind)
—    Max Velmans, 2000, Understanding Consciousness
The experiment was designed to demonstrate the difficulties in distinguishing phenomenologically between a first-person experience of an event (a subjective experience of an object) and a third-person experience of the same (that is, the observation of such an experience in a subject).

7.    China Brain (physicalism, philosophy of mind)
—    Lawrence Davis 1974, Ned Block 1978, Daniel Dennett 1991
What would happen if each member of the Chinese nation were asked to simulate the action of one neuron in the brain, using telephones or walkie-talkies to simulate the axons and dendrites that connect neurons? Would this arrangement have a mind or consciousness in the same way that brains do?

8.    Chinese Room (philosophy of mind, artificial intelligence, cognitive science)
—    John Searle, 1980, Minds, Brains and Programs
If you can carry on an intelligent conversation with an unknown partner, does this imply that the unknown partner understands the conversation, has a mind and experiences consciousness?

9.    Experience Machine (ethics)
—    Robert Nozick, 1974, Anarchy, State and Utopia
It is one of the best known attempts to refute ethical hedonism, and does so by imagining a choice between everyday reality and an apparently preferable simulated reality. If the primary thesis of hedonism is that ‘pleasure is the good’, then any component of life that is not pleasurable does nothing directly to increase one’s well-being. This is a view held by many value theorists, but most famously by some classical utilitarian. Nozick attacks the thesis by means of a thought experiment. If he can show that there is something other than pleasure that has value and thereby increases our well-being, then hedonism is defeated.

10.    Floating Man (Avicenna’s Falling Man)
—    Ibn Sīnā (Avicenna, c. 980 – 16 August 1037)
The soul is a substance and humans cannot doubt their own consciousness, even in a situation that prevents all sensory data input. We cannot know empirically what it is that makes living beings qua living beings alive necessarily. We possess this soul because of our intellect, at a level much higher than plants and animals.

11.    Gettier Problem (epistemology)
—    Edmund Gettier, 1963, Is Justified True Belief Knowledge?
This is a philosophical question about whether a piece of information that happens to be true but that someone believes for invalid reasons, such as a faulty premise, counts as knowledge.

12.    How Many Men? (Taxation as theft)
—    Murray N. Rothbard, 1982, The Ethics of Liberty
Just as no one is morally required to answer a robber truthfully when he asks if there are any valuables in one’s house, so no one can be morally required to answer truthfully similar questions asked by the State, e.g., when filling out income tax returns.

13.    Inverted Spectrum
—    Various proponents
There is an apparent possibility of two people sharing their colour vocabulary and discriminations, although the colours one sees — one’s qualia — are systematically different from the colours the other person see.

14.    Mary’s Room / Knowledge Argument (philosophy of mind)
—    Frank Jackson, 1982, Epiphenomenal Qualia
This is a thought experiment that attempts to establish that there are non-physical properties and attainable knowledge that can be discovered only through conscious experience. It attempts to refute the theory that all knowledge is physical knowledge.

15.    Original Position (politics)
—    John Rawls, 1971, A Theory of Justice
This is a thought experiment to replace the imagery of a savage state of nature of prior political philosophers like Thomas Hobbes. It is designed to be a fair and impartial point of view that is to be adopted in our reasoning about fundamental principles of justice. In taking up this point of view, we are to imagine ourselves in the position of free and equal persons who jointly agree upon and commit themselves to principles of social and political justice.

16.    Philosophical Zombie (philosophy of mind, artificial intelligence, cognitive science)
—  Robert Kirk 1975, Daniel Dennett 1991, David Chalmers 1996
The notion of a philosophical zombie is used mainly in thought experiments intended to support arguments (often called ‘zombie arguments’) against forms of physicalism such as materialism, behaviourism and functionalism. Physicalism is the idea that all aspects of human nature can be explained by physical means: specifically, all aspects of human nature and perception can be explained from a neurobiological standpoint. Some philosophers, like David Chalmers, argue that since a zombie is defined as physiologically indistinguishable from human beings, even its logical possibility would be a sound refutation of physicalism. However, physicalists like Daniel Dennett counter that Chalmers’s physiological zombies are logically incoherent and thus impossible.

17.    Plank of Carneades
—    Carneades of Cyrene, c. 214 – 129 BC
In this thought experiment, there are two shipwrecked sailors, A and B. They both see a plank that can only support one of them and both of them swim towards it. Sailor A gets to the plank first. Sailor B, who is going to drown, pushes A off and away from the plank and, thus, proximately, causes A to drown. Sailor B gets on the plank and is later saved by a rescue team. The Plank of Carneades poses the question of whether Sailor B can be tried for murder because if B had to kill A in order to live, then it would arguably be in self-defense.

18.    Prisoner’s Dilemma
— Merrill Flood and Melvin Dresher, 1950
This is a canonical example of a game analysed in game theory that shows why two purely ‘rational’ individuals might not cooperate, even if it appears that it is in their best interests to do so.

19.    Ship of Theseus, The (concept of identity)
—    Plutarch, c. 1CE, Life of Theseus
The Ship of Theseus, also known as Theseus’ paradox, is a thought experiment that raises the question of whether an object which has had all its components replaced remains fundamentally the same object.

20.    Survival Lottery, The (ethics)
—    John Harris, 1975, The Survival Lottery
The basis of the survival lottery idea is to ask people to imagine if organ donation were expected to save more individuals than it would kill. Hypothetically, all individuals are assigned a number and drawn out of lottery when a donation is needed, and are expected to give up their lives to allow two or more people to live.

21.    Ticking Time Bomb Scenario (ethics)
— Jean Lartéguy, c. 1960s, Les Centurions
This is a thought experiment that has been used in the ethics debate over whether torture can ever be justified. The scenario can be formulated as follows: Suppose that a perpetrator of an imminent terrorist attack, that will kill many people, is in the hands of the authorities and that he will disclose the information needed to prevent the attack only if he is tortured. Should he be tortured?

22.    Trolley Problem
—    Philippa Foot in 1967, The Problem of Abortion and the Doctrine of the Double Effect in Virtues and Vices
A trolley is hurtling down a track towards five people. You are on a bridge under which it will pass, and you can stop it by dropping a heavy weight in front of it. As it happens, there is a very fat man next to you — your only way to stop the trolley is to push him over the bridge and onto the track, killing him to save five. Should you proceed?

23.    Utility Monster (ethics)
— Robert Nozick, 1974, Anarchy, State and Utopia
Nozick created his second thought experiment of the ‘utility monster’ to show that average utilitarianism could lead to a situation where the needs of the vast majority were sacrificed for one individual.

24.    Violinist, The (ethics)
—    Judith Jarvis Thomson, 1971, A Defense of Abortion
In ‘A Defense of Abortion’, Thomson grants for the sake of argument that the fetus has a right to life, but defends the permissibility of abortion by appeal to a thought experiment:
“You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist’s circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. [If he is unplugged from you now, he will die; but] in nine months he will have recovered from his ailment, and can safely be unplugged from you.”


Text sources
•    Lecture on Ethics, 1997, Ludwig Wittgenstein, ‘Moral Discourse and Practice: Some Philosophical Approaches’
•    Daniel Dennet, 1997, ‘Quining Qualia’

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