# Thought Experiments: How to Test Ideas Without Leaving Your Chair

> A thought experiment tests an idea by imagining a scenario and reasoning it through. See Galileo's and Einstein's famous examples and how to run your own.

Canonical: https://mentarium.io/mental-models/thought-experiment.md

Mental model

6 min read By [Nemo](https://mentarium.io/about) Updated October 30, 2026

> A **thought experiment** is an imagined scenario used to test an idea, follow its consequences or expose a contradiction, without running a physical experiment. Galileo used one to challenge the belief that heavy objects fall faster, and Einstein credited imagining a ride alongside a light beam with starting his path to relativity.

- **Category**: Mental model (reasoning and problem-solving)
- **Origin**: Ancient philosophy; named Gedankenexperiment by Ernst Mach, 19th century
- **Best used for**: Testing a belief or plan before spending money or time on it
- **Related**: First-principles thinking, second-order thinking, inversion

## Ask yourself

- If this belief were true, what else would have to be true?
- What happens if I push this idea to its extreme?
- Can I imagine a case where this rule gives an absurd result?
- If I ran this plan in my head, step by step, where would it break?
- What would a perfectly informed observer see here?

## What is a thought experiment?

A thought experiment is an imagined test. You set up a scenario, apply the rules you believe, and reason through what would happen. If the outcome contradicts itself or common sense, something in your starting assumptions is wrong.

Its power is that it is free and fast. You do not need a lab, a budget or permission. You need a clear setup and the discipline to follow the logic where it leads, even when it leads somewhere you would rather not go. That second part is the hard bit, because the human tendency is to stop reasoning once a conclusion feels comfortable.

For decisions, thought experiments let you test a plan before you pay for it. “What happens in month three if our biggest customer leaves?” is a thought experiment. So is “Would I still take this job if the salary were the same as now?”

## Where it comes from

Philosophers used imagined scenarios long before anyone named the method: Zeno’s paradoxes and Plato’s Ring of Gyges are ancient examples. The German word _Gedankenexperiment_ is usually credited to the physicist Ernst Mach in the 19th century.

Two physics examples show how strong the method can be.

**Galileo’s falling bodies.** Aristotle’s physics held that heavier objects fall faster than lighter ones. In _Two New Sciences_ (1638), Galileo offered an imagined counterexample. Tie a heavy stone to a light one. If heavier things fall faster, the light stone should drag on the heavy one, so the pair falls slower than the heavy stone alone. But the pair is heavier than the heavy stone alone, so it should fall faster. The same theory predicts both outcomes, so the theory is broken. Galileo concluded that, apart from air resistance, they should fall at the same rate.

**Einstein’s light beam.** In his _Autobiographical Notes_, Einstein recalled wondering, at about 16, what he would see if he could travel alongside a beam of light. By the physics of the time, he should see a frozen electromagnetic wave, which seemed impossible. He later described this puzzle as an early seed of special relativity (1905).

Other famous examples include Schrödinger’s cat (1935), Maxwell’s demon in thermodynamics, and the trolley problem introduced by philosopher Philippa Foot in 1967.

## How to use thought experiments

1. **State the belief or plan precisely.** “Our customers care most about price” or “Moving cities will make me happier.” Vague beliefs cannot be tested, even in your head.
2. **Build a concrete scenario.** Pick a specific person, day or situation. “A customer with a budget who finds a competitor 20% cheaper but slower to deliver.” Specifics force the logic to run.
3. **Push to an extreme.** Ask what happens if the variable goes to zero, doubles or becomes infinite. Galileo combined two stones; you can double your price or remove your main feature. Extremes expose hidden assumptions.
4. **Follow the consequences, not your hopes.** Walk through what happens next, and next after that. This is [second-order thinking](https://mentarium.io/mental-models/second-order-thinking) in imagined form.
5. **Look for the contradiction or surprise.** If your scenario produces a result you would reject, find which assumption caused it. That is your finding. Then, where possible, check it against the real world.

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## 3 real-world examples

### Business

Say a founder believes users want more features. A thought experiment: imagine the product with twice as many features. What does the first screen look like? How long does onboarding take? Which support tickets arrive? Running it through makes it obvious that more features would bury the core one. The founder then asks the reverse: what if we removed half? That version is easier to picture working, and it becomes a real test. (A hypothetical example.)

### Personal life

Choosing between two job offers, you can imagine a specific Tuesday in each role a year from now. When do you wake up? Who do you talk to? What are you worried about at 4 p.m.? A concrete imagined day often reveals more than a pros and cons list, because it forces you to picture the parts you would otherwise skip, like the commute or the manager.

### Tech

Engineers use thought experiments constantly, often without calling them that. “What happens if this server receives ten times the usual traffic?” or “What if two users edit the same record at the same instant?” are imagined tests that reveal design flaws before any code is written. Asking how a system behaves under extreme or simultaneous conditions is the engineering version of Galileo’s tied stones.

## Common mistakes

- **Building the scenario to confirm what you already think.** If you choose the setup to produce your preferred answer, you are running [confirmation bias](https://mentarium.io/biases/confirmation-bias), not an experiment.
- **Leaving the scenario vague.** “Imagine things go badly” produces nothing. “Imagine our top client cancels on March 1” produces a plan.
- **Mistaking a thought experiment for evidence about the world.** It can expose contradictions and sharpen questions. It cannot tell you what customers will actually do.
- **Stopping at the first step.** The value usually comes from the second and third consequences, not the first.
- **Dismissing the absurd result.** When a scenario produces something strange, the instinct is to say “that would never happen.” Often the strangeness is the point.

## Thought experiments vs real experiments

|  | Thought experiment | Real experiment |
|---|---|---|
| Cost | Close to zero | Time, money, sometimes risk |
| Speed | Minutes | Days to years |
| What it can show | Contradictions, hidden assumptions, consequences of a belief | How the world actually behaves |
| Main weakness | Limited by what you know and imagine | Expensive, can be poorly designed |
| Best use | Filtering ideas before testing | Confirming the ideas that survive |
| Example | Galileo’s tied stones | Dropping objects and measuring the fall |

The two are partners: thought experiments decide which real experiments are worth running.

## The Mentarium take

The thought experiment is the cheapest tool in this library, and the most skipped. People go straight from idea to action because imagining feels like not doing anything. We think it belongs in the Stress-test step of the [Mentarium Decision Loop](https://mentarium.io/decision-making/decision-making-process), in a specific form: before you commit, imagine the decision made and walk through one concrete day, one month later. Write down the first thing that goes wrong.

Two pairings make it sharper. A [pre-mortem](https://mentarium.io/mental-models/pre-mortem) is a thought experiment with a fixed setup: the plan has failed, now explain why. [First-principles thinking](https://mentarium.io/mental-models/first-principles-thinking) supplies the raw material: once you have broken a problem into basic facts, a thought experiment lets you recombine them and see what follows. Galileo did both at once. He took the basic claim (heavier falls faster), built a concrete case and followed it until it broke.

A practical habit: keep a short list of your favorite setups and reuse them. “Double it,” “remove it,” “do it for ten years,” “do it to everyone,” “explain it to a newcomer” and “imagine the best-informed critic watching.” Each is a lever that pushes an idea into a corner. Most ideas survive one or two levers; the ones that survive all of them are usually worth testing for real. The ones that break save you the cost of finding out the expensive way.

**Counters** [Confirmation Bias](https://mentarium.io/biases/confirmation-bias)

## FAQ

### What is a thought experiment in simple terms?

It is an experiment you run in your head. You set up an imagined situation, apply the rules you believe in, and see what follows. If the result is absurd or contradictory, one of your starting beliefs is probably wrong.

### What is the most famous thought experiment?

Strong candidates are Galileo's falling bodies, Einstein's chase of a light beam, Schrödinger's cat (1935) and the trolley problem introduced by philosopher Philippa Foot in 1967. Each changed how people in its field framed a question.

### Who coined the term thought experiment?

The German word Gedankenexperiment is usually credited to physicist and philosopher Ernst Mach in the 19th century, although the method itself is far older and appears in ancient Greek philosophy.

### Can a thought experiment prove something?

It can show that a set of beliefs contradicts itself, which is a real result. It cannot confirm how the physical world actually behaves; for that you still need observation. Thought experiments are best at eliminating ideas and sharpening questions.

### How do you use thought experiments in everyday decisions?

Imagine the decision already made and walk through what happens next, push your assumption to an extreme, or imagine a version of you with full information. Each version surfaces consequences you did not see when the idea stayed abstract.

## Related models & biases

[Mental model First Principles Thinking First principles thinking is a way of reasoning that breaks a problem down to its most basic facts, the things you know are true without relying on convention or analogy, and then rebuilds a solution from those facts. It replaces "how is this usually done?" with "what is actually required?"](https://mentarium.io/mental-models/first-principles-thinking) [Mental model Second-Order Thinking Second-order thinking is the habit of looking past the immediate result of a decision to the effects that follow from it, and the effects that follow from those. Where first-order thinking asks "what happens next?", second-order thinking keeps asking "and then what?" until the picture includes how people and systems react.](https://mentarium.io/mental-models/second-order-thinking) [Mental model Inversion Inversion is a mental model that approaches a problem from the opposite end: instead of asking how to succeed, you ask what would guarantee failure, then avoid those things. It also means testing a belief by asking what would have to be true for the opposite to be right. Charlie Munger made it famous in business.](https://mentarium.io/mental-models/inversion) [Mental model Pre-Mortem A pre-mortem is a planning exercise in which a team imagines that a project has already failed, then each person writes down the reasons it failed. Developed by psychologist Gary Klein, it surfaces risks people would otherwise keep to themselves and tempers overconfidence before a plan is locked in.](https://mentarium.io/mental-models/pre-mortem) [Cognitive bias Confirmation Bias Confirmation bias is the tendency to search for, interpret and remember information in a way that supports what you already believe, while overlooking or discounting evidence against it. It affects how you ask questions, which sources you trust and what you recall later, and it works mostly without you noticing.](https://mentarium.io/biases/confirmation-bias)

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