The thought experiment
from 450 BCE As old as Zeno; named in German in the nineteenth century
Build a case that cannot be run in the world, run it in the head, and read off what your reaction reveals about the principle under test.
- Active
- from 450 BCE
- Where
- Elea, later Vienna
- Also
- Gedankenexperiment
- Status
- In use
- Tradition
- Analytic tools
What it is
A thought experiment constructs a scenario, usually impossible or impractical, precisely so that one variable can be isolated. The scenario is not a prediction and does not need to be realistic. It needs to be clean.
The evidence it produces is your reaction to it. That is either the method's great economy or its fatal weakness, depending on who you ask, and both readings have serious defenders.
How it works
Identify the principle under test and the variable that muddies it in real cases. Construct a scenario where everything else is held fixed and only that variable is present. Report the reaction honestly, including when it is not the one your theory wants. Ask what would have to be true for the reaction to be correct.
The third step is where honesty is required and frequently absent. A thought experiment designed to produce a particular reaction has proved nothing.
The moves, in order
- Identify the principle and the confounding variable
- Construct a case isolating that variable, however unrealistic
- Run the case and report the reaction
- Ask what must be true for the reaction to be right
- Test the answer against a second, differently constructed case
The procedure
Proposition The policy wording covers the loss.
- The clause excludes damage arising from water escaping from a fixed apparatus. The insurer says the burst hose on the washing machine is a fixed apparatus; the insured says it is not.
- Suppose the identical hose had burst while the machine was plugged in but standing in the middle of the room mid-installation. Both parties agree that would not be covered by the exclusion, because nothing is fixed.
- Now suppose the machine had been bolted to the floor and hard-plumbed. Both agree the exclusion would apply.
- The actual machine is plumbed to a tap and standing under a bench, unbolted.
- The two imagined cases show that what the parties are actually tracking is not fixture but permanence of the water connection, since that is the feature that changed between them.
Therefore The exclusion turns on the water connection rather than on whether the appliance is fastened down, and on that reading it applies.
Neither imagined case ever happened and neither needed to. Their whole function was to vary one feature at a time until it became clear which feature the disagreement actually turned on.
What it is good for
Isolating a variable that never occurs alone in real cases. It is indispensable in ethics, where the real cases are always overdetermined, and in law, where the hypothetical is a standard tool of statutory interpretation.
Where it fails
Intuitions about bizarre cases may simply be unreliable. We have no reason to think reactions honed on ordinary situations track anything when applied to teleporters, brains in vats and runaway trolleys, and experimental philosophy has found that reactions to the standard cases vary with the order they are presented in, with wording, and across cultures.
The second failure is design. Because the author builds the case, the case can be built to produce the reaction wanted, and detecting this is much harder than detecting a rigged real experiment.
Common misconceptions
That an unrealistic case is thereby a bad one. Unrealism is often the point — the case is impossible precisely so that the confounding factor is absent.
Also that the trolley problem is about trolleys. It is about the difference between doing and allowing, and the trolley is a device for removing everything else.
Where it came from
Zeno's paradoxes are the earliest, and Galileo's argument that heavier bodies cannot fall faster — tie two together and ask what happens — is the most elegant ever constructed, since it refutes Aristotle without any observation at all.
What became of it
Central to analytic philosophy and to theoretical physics, and now under sustained empirical scrutiny from experimental philosophers testing whether the intuitions are stable.
Lasting influence
Galileo's falling bodies, Einstein's lift and light beam, Searle's Chinese room, Thomson's violinist, Rawls's original position. Some of the most consequential arguments in the history of thought are of this kind.
Why it still matters
Very high, and increasingly questioned.
Put another way
A wind tunnel. Nothing in it is real and the readings still count.
Texts
Galileo Galilei, Two New Sciences
The falling-bodies argument in the first day is the best thought experiment ever devised and takes a page.
Where to start
Read Galileo's argument. It is short, it is airtight, and it shows what the method can do when the case is properly built.
The surprising bit
Galileo overturned two thousand years of physics with an argument that required no equipment and no observation — just a rope and the demand that Aristotle's position be consistent.
In their own words
If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower
Who held it
Ernst Mach, who named it
Connections
- Relies on
- Modal argument
- Influenced
- The method of cases, The veil of ignorance
- Parallel to
- The method of cases
The verdict
The cheapest instrument in philosophy and the one whose readings are least understood.