What Is A Thought Experiment?

Imagine a barber working in a small village. Everyone in the village who does not shave themselves use the services of this barber for their haircuts. The barber does not shave anyone who shaves themselves. In this case, who shaves the barber?

This barber paradox is a quintessential example of a thought experiment—a way to explore a concept, hypothesis, rule or idea on the basis of extensive thought.

Thought Experiment

A thought experiment also called a Gedanken experiment or Gedankenerfahrung, which considers some hypothesis, rule, or theory for the purpose of thinking about its consequences and causes. Given the nature of the experiment, it may not be viable to perform it physically. When finding empirical evidence becomes impossible, we turn to thought experiments to uncover more complex notions.

In the case of the barber haircutting dilemma, resorting to an experimental setup to find out who shaves him would not be viable or even desirable. After all, the barber is imaginary. Thought experiments are often rhetorical.

The idea of a thought experiment is to promote speculation, logical thinking and sometimes modify prevalent stereotypes. They thrust us outside comfort zone, coercing us to tackle difficult and puzzling questions.


Benefits of Thought Experiments

Thought experiments allow even those who are not well-versed with the technical knowledge of a particular field to build an intuitive understanding through a Gedanken experiment. The goal is to first distill principles into a form that can be comprehended through analysis and reflection. To look from a different perspective, many scholars subsume empirical evidence in their thought experiments.

As opposed to aimless rumination, the benefit of thought experiments lies in their inherent structure. In a systematic manner, they allow us to challenge intellectual standards, cross over the limits of ingrained facts, make logical decisions, stimulate revolutionary ideas, and broaden our realm of knowledge and reference.

Types of Thought Experiment

Thought experiments can be generally classified into seven main types:

1. Prefactual

Prefactual thought experiments literally imply experimentation before the facts. Considering the given scenario, this type of thought experiment speculates on probable future consequences. For example, what will A cause to happen?

2. Counterfactual

Counterfactual thought experiments literally imply ‘contrary-to-fact’ conditions. This type of thought experiment speculates on the possible outcomes of disparate past events. For example, what might have occurred if event B had happened instead of event A?

The study of counterfactual speculation has sparked interest amongst scholars in a wide range of fields, including psychology, economics, law and marketing.

3. Semifactual

Semifactual thought experiments contemplate how a different past could have led to the same present. For example, if C had happened instead of A, would the outcome still be the same?

Semifactual speculations find their application in the healthcare sector, especially in clinical trials.

4. Prediction

This type theorizes future outcomes based on existing available information. For example, If A continues to happen, what will the outcome be in five years?

Prediction type experiments done these days often involve computational models for prediction, including those for climate change or market projection.

5. Hindcasting

Hindcasting involves running a prediction in a reverse direction to validate if the prior forecast was correct. For example, if A occurred, could D have predicted it correctly?’

In 2003, a team of researchers designed an advanced algorithm to ‘train’ a computer to use the data of the surface temperature of the oceans from the last two decades. Then, using the data of the surface temperature of oceans for a period between 1857 to 2003, they conducted a hindcasting experiment. They found that their simulation model predicted every El Nino event correctly for the last 146 years.

6. Retrodiction

Retrodiction, also called postdiction, involves traveling step-by-step backward in time in as many stages as are considered necessary, from the present into the speculated past, to discover the root cause for the occurrence of an event. For example, What led to the event A? How can we prevent it from occurring again?

Retrodiction finds its application in the fields of climatology, financial analysis, and cosmology.

7. Backcasting

Backcasting involves the consideration of a specific future outcome, then working forward from the present to infer its causes. For example, If A happens in one year, what would have caused it?

Backcasting serves as a tool in determining the progress and direction of technology development, considering the specified targets for this purpose.

Thought Experiments in Science

Although science often mandates the need for empirical evidence, many scientists tend to use thought experiments as a ‘proxy’ experiment prior to a real physical experiment. In fact, German physicist Ernst Mach opined that a Gedanken experiment is an essential precondition for a physical experiment.

Ernst Mach

Physicist Ernst Mach (Photo Credit : Kuebi /Wikimedia Commons)

Even Albert Einstein used thought experiments for some of his discoveries. The most popular one related to the beam of light—what would happen if you could catch up to a beam of light as it moved? The answers led him on a different path, which later prompted him to conceptualize the special theory of relativity.

Many biologists use thought experiments to understand biological phenomena more profoundly. Evolutionary biologists often question the present state of organisms, such as why goats are not green, using thought experiments. The question might sound absurd to some, but they are pretty valid. Like, for instance, green fur would have helped goats in camouflaging from predators. Another thought experiment might involve asking: why don’t organisms have wheels instead of limbs? Again, the question might apparently sound bizarre, but it’s serious. We know from our own experience with vehicles that wheels would have been more efficient than limbs for traveling quickly across long distances. In this way, thought experiments help us explore reasons that can’t otherwise be tested empirically.


  1. Stanford University
  2. Massachusetts Institute of Technology
  3. Penn State University
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About the Author:

Hussain Kanchwala is an Electronic Engineer from University of Mumbai. He is a tech aficionado who loves to explicate on wide range of subjects from applied and interdisciplinary sciences like Engineering, Technology, FinTech, Pharmacy, Psychology and Economics.

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