Thought Experiment

Thought experiments enable us to explore often impossible situations and predict their implications and outcomes. It helps to stretch our minds by confronting difficult questions. When empirical evidence is impossible, thought experiments help to think about complex concepts.

The purpose is to encourage speculation, logical thinking and to change paradigms. Thought experiments push us outside our comfort zone by forcing us to confront questions we cannot answer with ease. They reveal that we do not know everything and some things cannot be known.

A classical example of a thought experiment is the 'impossible barber'.

Impossible barber

Imagine a small town with a hard-working barber. The barber shaves everyone in the town who does not shave themselves. He does not shave anyone who shaves themselves. So, who shaves the barber?

The Oxford Reference refers to this as the Barber's Paradox.

A village has a barber in it, who shaves all and only the people who do not shave themselves. Who shaves the barber? If he shaves himself, then he does not, but if he does not shave himself, then he does. The paradox is actually just a proof that there is no such barber, or in other words, that the condition is inconsistent.

Use of thought experiments

Albert Einstein used thought experiments for some of his most important discoveries. The most famous of this thought experiments was on a beam of light, which was made into a brilliant children’s book. What would happen if you could catch up to a beam of light as it moved he asked himself? The answers led him down a different path toward time, which led to the special theory of relativity.

Types of Thought Experiment

Several key types of thought experiment have been identified:

• Prefactual – Involving potential future outcomes. E.g. ‘What will X cause to happen?’
• Counterfactual – Contradicting known facts. E.g. ‘If Y happened instead of X, what would be the outcome?’
• Semi-factual – Contemplating how a different past could have lead to the same present. E.g. ‘If Y had happened instead of X, would the outcome be the same?’
• Prediction– Theorising future outcomes based on existing data. Predictions may involve mental or computational models. E.g. ‘If X continues to happen, what will the outcome be in one year?’
• Hindcasting– Running a prediction in reverse to see if it forecasts an event which has already happened. E.g. ‘X happened, could Y have predicted it?’
• Retrodiction– Moving backwards from an event to discover the root cause. Retrodiction is often used for problem solving and prevention purposes. E.g. ‘What caused X? How can we prevent it from happening again?’
• Backcasting – Considering a specific future outcome, then working forwards from the present to deduce its causes. E.g. ‘If X happens in one year, what would have caused it?’

From: Thought Experiment: How Einstein Solved Difficult Problems - Farnam Street