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Thought Experiments: Determining Their Meaning

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Abstract

This paper considers thought experiment as a special scientific tool that mediates between theory and experiment by mental simulation. To clarify the meaning of thought experiment, as required in teaching science, we followed the relevant episodes throughout the history of science paying attention to the epistemological status of the performed activity. A definition of thought experiment is suggested and its meaning is analyzed using two-dimensional conceptual variation. This method allows one to represent thought experiment in comparison with the congenerous conceptual constructs also defined. A similar approach is used to classify the uses of thought experiments, mainly for the purpose of science curriculum.

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Notes

  1. An anonymous reviewer argued that the term was already used by Örsted in 1811 (Örsted, H. C.: 1811, Naturvidenskabelige Skrifter, 3, 151–190), as discussed in Witt-Hansen, J.: 1976, H. C. Örsted, ‘Immanuel Kant, and the Thought Experiment’, Danish Yearbook of Philosophy 13, 48–65, and in D. Cohnitz of the University of Düsseldorf in his forthcoming article. This fact, however, cannot diminish the leading role of Mach in introducing this construct into active use, particularly in education.

  2. Somewhat similar to scientists, historians of science often do not mention TEs as a special means of furthering scientific claims. For example, Moss (1993), in her comprehensive study of Galileo, referred to his famous TEs as “dialectical arguments”, “eloquent examples”, “analogies”, “experiments”, but never—“thought experiments”. Similarly, among the philosophers of science, Giere (1984) in his course of philosophy of science, surveys the major methods of scientific reasoning and never mentioned “thought experiment”. In his analysis of the modern science (1999), “thought experiment” does not appear. He speaks about prototypes and things, and models, as “providing guidelines for the relevant similarity judgments” (ibid.: 123).

  3. In these words Newton introduced the concepts of time and space in his Principia (1999: 408). Posteriorly, after Einstein, we know that Newton was wrong in this subject.

  4. Koyré was considering the experiments with falling bodies. Vacuums became available to researchers later in the 17th century due to Otto von Guericke and Robert Boyle. Koyré noted the imperfect nature of the instruments available to Galileo, the absence of a reliable clock, etc.

  5. In fact, this was a revolutionary swing from the empirical knowledge about nature, expressed in objective terms and accumulated in the old civilizations (Sumerian, Babylonian, and Egyptian), to the scientific knowledge as practiced in the Classical Greece.

  6. Another opinion is that the break with the medieval science is due to Leonardo da Vinci (Draper 1890: 233), at least, in the continental Europe. This view is not too convincing since Leonardo held the medieval theoretical views (Randall 1957: 207–218). His strong empirical affiliation was not sufficient to produce a revolutionary change in science.

  7. Although the notion of “The scientific revolution” introduced by Koyré (1943) is presently a commonplace, there is a counter view among historians who state the evolution in science: “There was no such thing as the Scientific Revolution” (Shapin 1996: 1).

  8. Real and thought experiments are deeply interwoven. Shamos (1959: 46, 287, 318) in his compendium of Great Experiments in Physics included Newton’s TEs addressing absolute and relative movements, Maxwell’s TEs concerning electromotive force, and Einstein’s TE of a magnet and conductor, all in the rubric of “experiments” without any note.

  9. Thus, Brown ascribes to the Platonic type of TE the ability to construct new knowledge as in Galileo’s TE of falling bodies; in contrast, Norton refutes this idea: “pure thought cannot conjure up new knowledge” (Norton 2004b: 9).

  10. For different reasons, in many of his important statements Galileo did not acknowledge their authors, the brilliant minds of the medieval and renaissance periods. This behavior could be understood as breaking with the popular scholastic tradition heavily based on the citation of an authority. In the following years, seeking simplicity of a linear presentation, a whole collection of ideas were ascribed to Galileo (e.g., Cohen 1950/1993; Crombey 1959; Gliozzi 1962; Dugas 1986; Koyré 1968; Dijksterhius 1986).

  11. In another TE that Galileo introduced in his early work De Motu Antiquiora he in a similar way pointed at the logical error in the Aristotelian law of falling bodies, this time in the context of the bodies sinking in liquids of different densities. The same TE could be found in Buridan’s Questions on Physics, Book VII, Qu. 7 (ed. Paris 1509) (Moody 1994: 188–189).

  12. And indeed they do not. Within the Newtonian theory, the acceleration with which two masses approach each other directly depends on their masses. However, close to the Earth and for the masses much smaller than that of the Earth, in a very good approximation, they all fall with the same acceleration (Galileo’s law). Ignoring the approximate nature of Galileo’s law in the modern teaching of physics presents a conceptually deficient instruction.

  13. This was a remarkable recourse to experiment by Galileo who showed quite an opposite attitude to experiment in other cases, preferring convincing reasoning free of impeding factors which may mask the phenomenon. We read in Dialogo (Galilei 1632: 145): “Did you [Salviati] make an experiment? (Salviati represented Galileo)—No, I do not need it, as without any experience I can affirm that it is so, because it cannot be otherwise” and: “I without experiment am certain that the effect will follow as I tell you, because it is necessary that it should”. And the same Galileo: “…the fact that all human reasoning must be placed second to direct experience. Hence, they will philosophize better who give assent to propositions that depend upon manifest observations, than they who persist in opinions repugnant to the senses and supported only by probable reasons.” (Galilei 1613: 118). The case of Galileo may definitely confuse those trying to classify him as a rationalist or empiricist. The shortest summary was given by Koyré (1968: 43): “The new science is for him [Galileo] an experimental proof of Platonism”. This complexity causes a problem for “the poor teacher of elementary physics” who seeks a simple picture for his students (Cohen 1950/1993).

  14. See note 12.

  15. Natural experiments”, or analyses of actual phenomena in terms of a certain theory, as if arranged for testing certain theory, were among the major tools of Aristotelian physics. In our present day natural experiments continue to play a central role for “historical sciences” such as astrophysics, paleontology, geology, etc. (e.g., Diamond 1999: 424).

  16. By considering a chain of spheres suspended on a double edge, Stevin showed that the gliding force on the load placed on the inclined plane is in inverse proportion to the length of the inclined plane (e.g., Mach 1896/1976: 32–41).

  17. EPR—Einstein, Podolsky, and Rosen (1935) pointed to the nature of quantum description. Einstein provided the non-formal description of this paradox featuring the quantum description of reality in his Autobiographical notes (1949: 83–87).

  18. Brown approached this view when wrote in another place (2004): “The thought experiment establishes a phenomenon; the explanation comes later”. The new theory provides such explanation.

  19. This claim matches McAllister’s (1996) view that the evidential significance of TEs depends on the particular historical environment where history is the history of scientific paradigms. As Brown (2004) put it: “A particular thought experiment might be rightly used one way in one historical situation and wrongly used in another”. Our reservation is only regarding wrongly, in favor of differently.

  20. Newton’s bucket experiment refers to the appearance of the water surface in a suspended bucket. The dependence of the water surface’s profile on the relative motion of the water and the bucket was considered in four possible relative situations: (1) water is at rest but the bucket is rotating, (2) both are rotating, (3) water is rotating but the bucket is at rest, (4) both are at rest.

  21. In fact, Bishop Berkeley, not acknowledged by Mach, provided the same critique much earlier: “clearly we cannot know the absolute motion of any body” (Berkeley 1721/1965: 270). Newton ignored the possible influence of the Earth and distant celestial bodies on the water in the bucket.

  22. Even if failed in its original intention this TE may be beneficial in science class. In the attractive context of scientific debate Leibniz introduced another important quantity—mechanical energy: vis viva—a prototype of kinetic energy and vis mortum—a prototype of elastic potential energy (Dugas 1986: 221). The true expression for kinetic energy requires ½ and the quantity m·h, missed the factor g. These deficiencies, however, reflected the limits of the applied qualitative analysis.

  23. Norton (2004) exemplified erroneous application of special relativity in the TE of “rod and slot.”

  24. Full of irony, this case possesses a great pedagogical potential, showing to the learner that the greatest mind are not immune against misconceptions of the kind students may do on our days.

  25. Another representative example of the importance of concept definition concerns the concept of weight. It has been shown that its definition is essential for phenomena explanation and may influence phenomena understanding (e.g. Galili and Kaplan 1997; Galili 2001; Galili and Lehavi 2006).

  26. An anonymous reviewer pointed out that Mach was at that time promoting his notion of Physikalische Denkaufgaben (literally, physical thought problems) in the teaching journal Zeitschrift für den Physikalischen und Chemischen Unterricht which he had co-edited with B. Schwal since 1887.

  27. The choice of axis is not unique and may cause different classifications (e.g. experiment versus observation).

  28. In the course of IM the researcher may render a theoretical model, replacing a theory. This would change the activity to TM.

  29. The feature of variation of the roles played by a particular TE can encourage the genuine understanding of the considered theory. Learning through variation was stated to be an effective teaching strategy (Schecker and Niedderer 1996; Marton andTsui 2004).

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    Igal Galili

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Galili, I. Thought Experiments: Determining Their Meaning. Sci & Educ 18, 1–23 (2009). https://doi.org/10.1007/s11191-007-9124-4

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