As intelligent living organisms, humans possess curiosity, which is a driving force both for scientific research and for reflections on the encountered living conditions and their origin. This leads them to carry out theoretical and experimental scientific research and to make thought experiments. The outcome of these approaches represents important contributions to the worldview, which provides us with orientation knowledge that gives us an important basis for our life activities.
Nature and examples of thought experiments
Thought experiments are fictive scenarios imagined in order to explore the implications of some theory, for example a scientific theory or a philosophical account (, , ). Philosophers widely use thought experiments in order to check whether some philosophical account accords with widely held intuitions, for example, intuitions about the moral acceptability of actions. By contrast, scientists use thought experiments in order to check whether their theories are internally consistent, or whether they agree with everyday experience. An example of the latter kind can be found in Galileo’s dialogues, when he shows that falling objects can still appear as if they were moving in a straight line perpendicular to the surface of the Earth, even if the Earth moves (). He shows by this asking his interlocutors to imagine that they are standing on a moving ship and looking at the ship’s rigging. Even though the ship moves, to the observer moving along with it the rigging would appear as being at rest. In a similar way, when we watch a rock falling from a tower we cannot perceive the tower’s horizontal motion due to the Earth’s orbital motion. And the same is true of the horizontal component of the falling rock’s motion. Therefore, the rock appears to be falling perpendicularly to the surface of the Earth.
In a similar way, the physicist James Clerk Maxwell imagined a microscopic demon who operates a shutter between two containers of gas, letting only the fastest molecules pass. This thought experiment exposed certain limits of the Second Law of Thermodynamics ().
But thought experiments are not only used in the history of science. In the journal Nature Communications we can find an article published 18 September 2018 where two contemporary physicists, Daniela Frauchiger and Renato Renner, describe a thought experiment that is supposed to prove that quantum theory cannot consistently be applied to itself, i.e., to an agent performing a quantum measurement ().
Thought experiments can also be found in other areas of science such as economics (, ) or biology (, ). For example, the British economist Thomas Malthus  famously imagined what would happen if the human population reproduced unrestrictedly. Because this leads to geometrical, i.e., exponential growth while the productivity can only grow arithmetically, starvation, poverty and disease would ensue. This thought experiment is known to have been an important influence on Charles Darwin’s idea of natural selection.
Darwin struggled to provide proof for his principle of natural selection. One line of evidence he provided in the Origin of Species consisted in showing how much artificial selection could change an organism’s heritable traits, for example, in domestic dogs or pigeons. But this was artificial selection and what he needed to prove was the possibility of natural selection. As he had no experimental data or field observations available, he resorted to thought experiments . An example of such a thought experiment can be found in the following citation from the Origin of Species:
Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. I can under such circumstances see no reason to doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected … Now, if any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring. Some of its young would probably inherit the same habits or structure, and by the repetition of this process, a new variety might be formed which would either supplant or coexist with the parent-form of wolf (, 90-91).
Thus, Darwin imagined what would happen under certain conditions in nature, and this was one of his chief arguments for the occurrence of natural selection.
It should also be noted that some contemporary accounts of the origin of life are also thought experiments, for example, the idea that life started out with ribonucleic acids (RNAs) that can catalyze their own synthesis , which came to be known as the “RNA world hypothesis”. Such molecules are known to exist but their presumed role in the origin of life is nothing but a thought experiment.
Of course, it could be argued if all of these examples really constitute thought experiments or if some are better viewed as hypotheses, theories or theoretical models. Indeed, the cases are different. Some of the examples mentioned (Galilei, Maxwell) involve a concrete experiment that is imagined rather than performed. The case of Darwin involves what may be called an imagined natural experiment. The cases of Malthus and the RNA world may also be viewed as just hypotheses. However, it should also be noted that all these cases also have something in common, namely that they imagine possible worlds that are in some respects unlike the world we know. In these imagined worlds, certain suppositions are assumed to be true. Scientists then intuit (or in some cases calculate) what the consequences would be. Indeed, thought experiments have been characterized as “expeditions to possible worlds” (, p. 135). Hypothesizing and model-building may also be characterized in this way. Both thought experiments and model building involve idealizations, even though perhaps to different degrees. Thus, the difference between thought experiments, hypothesis and theoretical models may just be a difference of degrees rather than of kind .
We hope these examples suffice to illustrate the point that, while all scientific knowledge must be ultimately based on empirical data, imagination and thought experiments can nonetheless play a role in scientific research. They help the human mind to chart our scientific knowledge and make sure that it coheres with rest of our knowledge, including our common experience. Indeed, a considerable part of our scientific worldview is based on thought experiments.
Can we consider Creation myths as thought experiments?
Reflections on people’s lives under convenient living conditions are likely to have been made by curious early farmers and their compatriots already a few thousand years ago. Various resulting creation myths took their origin at a number of locations on our planet . Often a divinity was seen to act as creator of living conditions and of living organisms. One of these accounts is included in the Genesis chapter of our Bible. Religious people interpret this kind of insights into our existence as divine revelations. But this does not exclude that one can count creation myths as resulting from a kind of thought experiments based on intelligence.
Creation myths also had an undeniable impact on science, as can be seen in the case of Darwin. A theologian and Anglican priest, he started out as an ardent follower of the English tradition of physico-theology that sought to prove the existence of God by studying the ingenious ways in which organisms appeared to be adapted to their form of life. Darwin absolutely needed to find an alternative explanation, which was his main motivation for elaborating a theory of natural selection.
We think that these cases also use fiction in scientific reasoning, exactly like thought experiments.
Scientific research and thought experiments can enrich our available knowledge on natural properties and functions
In the course of time, scientific research improved gradually our available knowledge on the nature in which we live. Tribes living several thousand years ago could not know the presence of cohabiting microorganisms, nor did they know that they lived on a planet with the shape of a ball representing only a minute part of the immense Universe.
Without going into details, we can conclude that thought experiments can stimulate scientific research into particular directions, whereas results from scientific research can stimulate humans to carry out novel thought experiments. The two approaches to understand nature are complementary.
Recent astrophysical research has shown that some stars in the Universe are surrounded (like our sun) by planets (, ). These are called exoplanets. Actual research investigates whether some exoplanets offer convenient living conditions. A relevant thought experiment that predicts the existence of living organisms on other planets raises the question whether intelligent organisms could travel as aliens to our planet Earth. It would be a very difficult task for our scientific researchers to explore whether some exoplanets showing living conditions are inhabited by living organisms. If we define living organisms, as we have the habit on our planet, to be able to reproduce and to undergo biological evolution, exploration of the existence of living organisms can be very difficult in view of the great distances between the Earth and the relevant exoplanets. However, if such scientific investigations should become possible, interesting additional questions are whether living organisms on exoplanets possess genes that are carried on nucleic acid molecules and whether all kinds of organisms on a given exoplanet possess an identical genetic code.
Acquired knowledge on evolution processes.
Available scientific insights lead to the conclusion that natural evolution generally proceeds extremely slowly. This renders these ongoing evolutionary processes at the levels of the Universe, of our planet and of its living populations hardly visible to us in our lifetime. This slowness ensures relatively good stability of living populations and of their living conditions. Available knowledge from studies on biological evolution, on geology and on astrophysics forms a basis for these conclusions. Some natural scientists see these processes providing remarkable stability as pointing to a self-organization by nature.
Scientific and religious views on Creation processes
So far, scientific attempts to create a simple living organism ex nihilo have failed. In contrast, our available knowledge on biological evolution processes indicates that all living organisms present on our planet must have evolved stepwise in the past from one or more primitive microorganisms. One can consider this process as continuous permanent creation by nature’s self-organization. Thanks to self-organisation the Creator did not have to bring into existence each individual species, but he may have laid down principles that allow the components of the universe to become ever more diverse in the course of time.
Christian faith in general makes widespread use of thought experiments . In the cultural history of mankind, individual deities were often assumed to act as creators in favor of humanity. This led to the concept of Polytheism. However, more recently a single God became seen in the concept of Monotheism to act in all these events. We appreciate the Christian view of the Trinity  in which God is “Three in One and One in Three”, i.e., the Father, the Son, and the Holy Spirit. The Son is Jesus, the founder of the Christian religion, who provides to human believers useful stimulations for their life activities. In contrast, God Father and the Holy Spirit are not living organisms but rather conceptual entities. In this concept, the Trinity acts not only on Earth, but it is the Divinity for the entire Universe.
Thought experiments can be seen as the results of human reflections that are based on available knowledge and on ideas how the studied objects might contribute to the process under study. Thought experiments were already carried out a few thousand years ago. In contrast, serious scientific investigations have a shorter history and have become intense only in the last few hundred years. At present, solid scientific knowledge can beneficially contribute to thought experiments and thought experiments can stimulate to carry out scientific research projects. The two approaches are complementary and they can help obtain deeper insights into various natural properties and functions.
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Address of the authors:
Prof. em. Werner Arber, Biozentrum, University of Basel, Klingelbergstrasse 50/70, CH-4056 Basel, Switzerland | E-mail: Werner.Arber@unibas.ch
Prof. Marcel Weber, Dépt. de Philosophie, Université de Genève, 2, rue de Candolle, CH-1211 Genève, Switzerland | E-mail: Marcel.Weber@unige.ch
Professor Marcel Weber first studied Molecular Biology at the University of Basel and thereafter Philosophy, also at the University of Basel. He is now a philosopher with a solid background in the life sciences and has a professorship at the University of Geneva.